Seminars – Niels Bohr Institutet - Københavns Universitet

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About the Center for Quantum Optics > Quantum Optics Lab > Seminars

Our group regularly has guests who often present some of their research to us in seminars. Additionaly we ourselves share certain topics with each other in informal seminars. Below is a list of both upcoming and previously held seminars.


Upcoming seminars:
None scheduled at present.

Previously held seminars:


24th of August 2011 13:15 in Quantop Lounge

Naoto NAMEKATA, (Nihon University, Tokyo) talk on "Recent Photon Detection Technology Breaks New Ground in Optical Quantum Communication and Processing".

In the past decade, we have developed photon detectors at a
telecommunication wavelength. Performances of the photon detectors
have considerably progressed, and they now enable us to realize
quantum communications, especially the fiber-optic quantum key
distribution. Recently, the photon detector has been able to obtain
the photon number resolution with high detection efficiency.
Therefore, optical quantum state engineering is up to multi-photon
level. In this talk, I will review the recent progress of the photon
detection technologies and show the some experimental researchs using
them, i.e. the quantum key distribution, the Schrodinger's kitten
state generation, and the plasmonics in quantum regeme.


4th of May 2010, 15:15 in Auditorium A.

Ling Miao, Physical Review Letters, talked on "Physical Review Letters: Its Review Process and Publications, Its Role in Physics Research, and Its Future.".

For researchers working in physical sciences, Physical Review Letters is one of the most desirable venues for publication of their work. What makes the journal highly desirable? And, how should it meet the challenges posed by an increasingly competitive and more "impact"-oriented scientific publishing culture?
In this talk, I will discuss these questions with you by looking at PRL's publication philosophy, its review process and publications, and what it means to the physics community and to physics research. Your thoughts and views as well as suggestions will be most welcome.

4th of May 2010, 10:15 in Auditorium A.

Ulrik Lund Andersen, DTU, talked on "Quantum Plasmonics".

The emergent field of quantum plasmonics has received enormous interest due to its unique promises for the way light can be localized beyond the diffraction limit and manipulated at the nanoscale. One particular interesting application of quantum technologies based on surface plasmons is quantum information as it allows for scalability and coherent coupling to single emitters.
In this talk I will discuss recent advances in the excitation and propagation of non-classical surface plasmons. In particular, I will address a recent experiment on the faithful transmission of squeezed states of light through plasmonic waveguides, and report on the recent progress on the excitation of single surface plasmons with Nitrogen-Vacancy centers in diamond

1st of April 2010, 13:00 in Auditorium A

Wolfgang Tittel, Institute for Quantum Information Science, University of Calgary, talked on "Integrated quantum memory for quantum communication ".

Quantum memory constitutes a key element for quantum repeaters, which promise overcoming the distance barrier of quantum communication. Impressive experimental and theoretical progress has been reported over the past few years and gives hope that a workable quantum memory can eventually be built.
We will present our latest results of storage of light in a Ti:Tm:LiNbO_3 waveguide cooled to 3 Kelvin using a photon echo quantum memory protocol. In particular, we will preset storage of sub ns pulses, simultaneous storage of more than hundred modes, and discuss progress towards storage of entangled photons.

12th of March 2010, 11:00 in Auditorium M.

Tolga Bagci, NBI, talked on "Cavity Optomechanics Towards Quantum Regime".

In this talk, I will try to give a general view of the field, Cavity Optomechanics , which deals with the interaction of mechanical resonators with light, confined in high-finesse cavities. Combined with state-of-the-art techniques , the field is promisingly approaching a regime where quantum radiation pressure effects might be dominant.
The talk will also cover a relevant topic,namely radiation pressure cooling, that might herald the experimental demonstration of reaching the ground state of a nanomechanical resonator. Finally, I will relate the topic to our experiment where we propose using a two-color scheme for both probing and cooling of the mechanical object.

4th of February 2010, 15:00 in Auditorium A.

Gretchen Campbell, NIST, Gaithersburg Md, talked on "Optical clock in a 87Sr optical lattice".


2nd of February 2010, 10:15 in Auditorium A

Arno Rauschenbeutel, Johannes Gutenberg-Universität, Mainz, talked on "Nanofiber Photonics and Quantum Optics".

Recently, optical nanofibers with diameters smaller than the wavelength of the guided light have attracted considerable interest in the field of quantum optics due to their high potential for efficiently interfacing light and matter. In my talk I will report on two experiments using such nanofiber-optical interfaces. In the first experiment, we perform ultra-sensitive spectroscopic measurements on 3,4,9,10-perylene-tetracarboxylic dianhydride molecules (PTCDA) deposited on the fiber surface at ambient conditions. We use the guided mode of the nanofiber both for excitation of the molecules and for fluorescence collection and we show that surface coverages as small as 1 ‰ of a compact monolayer still give rise to absorption and fluorescence spectra with a good signal to noise ratio. In the second experiment, we trap about 2000 cold neutral cesium atoms close to the surface of an optical nanofiber using the optical dipole force exerted by the evanescent field of the nanofiber guided light. The atoms are probed with a weak resonant field which is sent through the nanofiber and which interfaces with the atoms via the evanescent field. Remarkably, the atomic ensemble almost entirely absorbs this probe field, yielding an optical depth of up to 18. This opens the route towards non-linear optics and quantum communication applications with fiber-coupled atomic ensembles.

1st of February 2010, 14:00 in Auditorium A.

Daniel Oblak, NBI, defended his PhD thesis on "Quantum State Engineering in Cold Caesium Atoms".

Quantum mechanics is characterised by several strange features, which include quantum uncertainty, quantum measurements, and entanglement. This thesis involves all three of these. The quantum uncertainty poses a fundamental standard quantum limit (SQL) in applications where quantum systems are used to gauge some quantity. A prime example is that of atomic frequency standards, which with unprecedented precision measure an atomic quantum state. Quantum measurements in the form of Quantum Non-Demolition (QND) measurements can be engineered so as to overcome the SQL by redistributing quantum uncertainty amongst different variables of the system. Such squeezed spin-states rely on inter-atomic correlation, which goes by the name of entanglement.
In this work we present a detailed description of how we have implemented a QND measurement with laser pulses in a Mach-Zehnder Interferometer (MZI) and demonstrate that we can engineer a squeezed state in a cold trapped ensemble of Cs atoms. We verify that the squeezing is useful for improving the precision of atomic clocks. Along the way, we also investigate several remarkable features of the interaction, by which atoms and light-particles (photons) exchange phase-shifts.

After the questioning by the opponents, approximately at 4 pm, a reception will be held in the institute cantine.

1st of February 2010, 10:15 in Auditorium A.

Pierre Lemonde, L'Observatoire astronomique de Paris-Meudon-Nancay, talked on "Optical frequency metrology at Laboratoire national de métrologie et d'essais - Système de Références Temps-Espace (LNE-SYRTE)".

Research on optical frequency metrology at LNE-SYRTE presently follow three different directions. First, on optical atomic clocks, I will present recent results on two projects towards high performance optical lattice clocks with Sr and Hg atoms. In particular, I will focus on the optimization of the clocks frequency stability and associated experimental developments- non-destructive detection scheme and ultra-stable laser. Second, on long distance clock comparison, I will describe a high performance coherent optical fiber link and preliminary results towards a free-space coherent optical link. Finally, I will show recent results on the generation of microwave signals with stability close to 10-16 at 1s averaging time by downconversion of an optical source using a fiber femto-second frequency comb.

22nd of January 2010, 15:00 in the Auditorium M.

Nikolaj Korolev, NBI, defended his master's thesis on "Spontaneous emission in light-atom interactions for atomic ensembles".

In my thesis I have reviewed the Faraday interaction between a coherent light field and an atomic ensemble. The Faraday interaction is an important tool in the growing field of quantum information, which is widely used to perform quantum memory protocols. At the Niels Bohr Institute, Eugene Polzik's group has had succes with performing the direct mapping protocol based on the Faraday interaction. However so far there has not been a satisfactory description of the spontaneous emission that the system undergoes. In this work there is included the full level structure of the atoms and obtained a complete description of the decoherence from spontaneous emission.

22nd of January 2010, 12:15 in the Auditorium M.

Jonas M. Petersen, NBI, defended his master's thesis on "An ion crystal quantum repeater".

An important goal of quantum information science is reliable quantum communication. This is a serious challenge given the rapid decoherence of quantum systems. A possible way forward is to use so-called quantum repeaters to distribute entanglement between the sender and the receiver and a promising scheme is the DLCZ repeater (1) which uses ensembles of atoms for the entanglement distribution. Ion Coulomb crystals (2) are attractive ensembles for the DLCZ repeater since they have the potential to be good quantum memories and they couple well to light when put in an optical cavity.
In this master's thesis defence we present a detailed calculation of the quantum repeater's so-called write-in interaction in an ion crystal in a standing wave cavity. The combination of the standing wave and the thermal movement of the individual ions is shown to destroy the write-in. A possible solution to the problem is given in the form of moving the whole crystal during the interaction with light. Then, we calculate the so-called read-out interaction and again try to solve the problem of the standing wave by fast displacement of the crystal.

11th January 2010, 13:15 in the Quantop lounge.

Amine Laghaout, Royal Institute of Technology (KTH), will talk on "Feasibility of Bell tests with the W state".

Entanglement of multi-qubit systems is central to quantum information and communication. It can also be of interest from a theoretical perspective in furthering tests of non-locality. An experimental scheme for such tests will be presented using the tripartite W state, whereby a single photon is entangled with two vacuum modes. The challenge, however, is that the characterization of such Fock-state qubits requires measurements in non-commuting bases. A theoretical method for performing such polyvalent projective measurements in will be worked out. Ultimately, the goal of the analysis is to assess whether Bell tests with the W are feasible given the actual performance of photodetection devices.

13th of November 2009, 10:15 in Auditorium M.

Anna Grodecka-Grad, University Paderborn, will talk on "Decoherence channels in the optical manipulation of charge and spin qubits in quantum dots".

Optically driven spin control schemes in quantum dot systems exploiting spin dependent charge evolution are considered as promising candidates for quantum computers. We show that even in the absence of direct spin-reservoir coupling, the spin state of a confined carrier in a quantum dot can undergo dephasing. The indirect dephasing process is studied in detail for specific optical spin control protocols using off- resonant trion excitations in doped semiconductor quantum dots. The microscopic description of the interaction between charges and their phonon reservoir is presented together with its non-Markovian nature. Moreover, the decoherence channels resulting from radiative decay of the trion and imperfections of an adiabatic evolution are discussed. The parameters which allow for coherent control of the spin with a single qubit gate error as low as 10^-4 are indicated.

17th of September 2009, 14:15 in Auditorium M.

Axel Griesmaier, University of Stuttgart, will talk on "Investigation of dipolar Bose Einstein condensates".

Chromium atoms carry a very large magnetic moment of 6 Bohr magnetons. This leads to significant magnetic forces between the atoms in a chromium Bose-Einstein condensate. Such dipole-dipole interactions in quantum gases have recently gained much interest due to their long-range and anisotropic character which greatly differs from the well known s-wave scattering. Though being still rather weak on an absolute scale, these forces can become the dominant interaction when a Feshbach resonance is used to reduce the contact interaction to zero. A condensate in this purely dipolar regime shows very specific behavior. The dynamic and static properties of such a condensate, for instance, depend on the geometry of the trap. I will present our recent experimental results like the stability diagram of a dipolar BEC and the dynamics of the dipolar collapse of a condensate and discuss some further interesting properties of gases with dipolar interactions.

17th of September 2009, 13:15 in the Auditorium M.

Jonas Schou Neergaard-Nielsen, National Institute of Information and Communications Technology, Tokyo , will talk on "Entanglement distillation from Gaussian states".

Entanglement distillation is the process of increasing the entanglement between two parties at the expense of the rate of entangled states using only local operations and classical communication. It is essential for combating decoherence in long-distance quantum communication. For discrete variable systems, distillation has been around for several years. For continuous variables (CV), the matter is complicated by the well-known fact that Gaussian operations alone is insufficient to distill entanglement from Gaussian states. Recently, Hage et al. [Nature Physics 4, 915 (2008)], and Dong et al. [Nature Physics 4, 919 (2008)] succeeded in distilling entanglement from CV entangled states that had been subjected to non-Gaussian noise (phase diffusion and time-dependent attenuation).
Their schemes make use of Gaussian operations such as homodyne detection only. Because of that they are not applicable to linear loss, which is perhaps the most generic type of decoherence and which preserves the Gaussianity of the input state. To address that missing part, we have demonstrated distillation from an initial Gaussian entangled state by using non-Gaussian operations, namely photon subtraction. By probabilistically subtracting a photon at the side of either one or both of the parties (Alice and Bob), we were able to increase an initial weak entanglement as quantified by the logarithmic negativity. Furthermore, in the case of two-photon subtraction, the two-mode squeezing level of the output state was also improved, thereby immediately increasing the state's usefulness for teleportation. The state reconstruction was done by full two-mode homodyne tomography as well as a simplified two-mode tomography method.

9th of September 2009, 14:00 in the Quantop lounge.

Jelmer Renema, University of Leiden, will talk on "Magnetometry with room-temperature Caesium atoms".


7th of September 2009, 13:15 in the Auditorium A.

Alexei Trifonov, Harvard University, talked on "Quantum Entanglement between Optical Photon and Solid-State Spin Qubit".

Nonlocal entanglement is a fundamental phenomenon in quantum mechanics. It has been extensively investigated in systems of optical photons and with isolated ions and atoms. Recently it emerged as a powerful resource for realization of various ap- plications in quantum information science. Here, we demonstrate a non-local entanglement between a single optical photon and a solid-state qubit associated with a single electronic spin of Nitrogen Vacancy impurity in diamond. Our experiment demonstrates a high degree of control over solid state qubits in optical domain and provide a fundamental building block towards realization of quantum optical networks based on long-lived electronic and nuclear spin memory in solid-state.

20th of August 2009, 11:15 in the Quantop Lounge.

Andreas Næsby Rasmussen, Danish Technical University, talked on "The influence of pure dephasing on a two-level quantum dot coupled to a microcavity".

I will talk a bit about the work I was part of at DTU Fotonik which dealt with the dynamics of a single two-level quantum dot coupled coupled to a microcavity. This is a typical candidate for a Single Photon Source. The emission spectra from an excitation of this QD-cavity will be discussed and it turns out that when coupling to a phonon bath is included in the model, a surprising shift in the dynamics and thus in the emission spectra occurs. This was published in [1]. I will try to talk a bit about what have been suggested as an explanation for this behaviour as well as whether a consensus is starting to form.
[1] PRA 78, 045802 (2008)

10th of July 2009, 13:15, in the Auditorium M.

Nir Bar-Gill, Weizmann Institute of Science, talked on "Many-Body Excitations and their Decay in a BEC".

The study of excitations and decay in Bose-Einstein Condensates provides insight into fundamental phenomena in many-body quantum systems. We use Bragg spectroscopy as a sensitive tool for creating and probing these excitations and their dynamics. In this talk I will present experimental and theoretical work on quantum interference effects, which appear for low momentum excitations, and short-time non-exponential decay of excitations for a BEC loaded into a 1D opticallattice.
In a Bose-Einstein condensate, the excitation of a Bogoliubov phonon with low momentum is strongly suppressed due to destructive interference between two indistinguishable excitation pathways. Here we show that scattering of this sound excitation into a double-momentum mode is strongly enhanced due to constructive interference. We further show that due to parity considerations, this effect is extended to higher-order excitations.
Relaxation and decoherence of excited quantum states, caused by their coupling to the environment, is a ubiquitous phenomenon. For a BEC loaded into a 1D optical lattice, we observe strong enhancement of the decay compared to the Golden Rule (long-time limit) results. This enhancement of decay increases with the lattice depth. It indicates that the description of decay in this system must take into account short-time, non-exponential contributions

29th of June 2009, 13:00 in the Auditorium M.

Peter Lodahl, DTU Photonics, talked on "Efficient Coupling of a Quantum Dot to a Photonic Crystal Waveguide".

The interest in all-solid state implementations of quantum photonic devices for quantum information technology has increased tremendously in recent years. The basis for many of these devices is the efficient interaction between a single emitter and a single optical mode. Much work has centred on quantum dots embedded in cavities. The efficient coupling between a single quantum dot and the optical cavity mode allows channelling single photons to the cavity mode with near-unity efficiency. However, one serious limitation of this approach is that the bandwidth is rather modest. Furthermore, out coupling in general implies high optical losses, which significantly limits the overall efficiency of the single-photon source. An alternative technique is to couple a quantum emitter to a propagating mode by implementing a waveguide in a photonic crystal. In this case, the efficient coupling is mediated by the pronounced slow-down of light propagation near the edge of the photonic crystal waveguide mode. Furthermore, due to the 2D photonic bandgap of the structures, coupling of the quantum dot to radiation modes is efficiently inhibited [1]. The combined action of these two effects leads to a very high b-factor of the single-photon source. The b-factor expresses the fraction of photons emitted to the photonic crystal waveguide relative to the total number of photons emitted, and is the figure of merit for a single-photon source. We will discuss here the experimental realization of a photonic crystal waveguide single-photon source [2]. By time-resolved spectroscopy we extract the b-factor of the device, which approaches 90%. By probing quantum dots at different emission wavelengths we extract an unprecedented large bandwidth of 20 nm, which is a major advantage of the technology compared to the case of a cavity.
[1] B. Julsgaard, J. Johansen, S. Stobbe, T. Stolberg-Rohr, T. Sünner, M. Kamp, A. Forchel, and P. Lodahl, Appl. Phys. Lett. 93, 094102 (2008).
[2] T. Lund-Hansen, S. Stobbe, B. Julsgaard, H. Thyrrestrup, T. Sünner, M. Kamp, A. Forchel, and P. Lodahl, Phys. Rev. Lett. 101, 113903 (2008).

16ht of June 2009, 13:00 in the Auditorium M.

Geza Toth, Ikerbasque and U. of the Basque Country, talked on "Generation of macroscopic singlet states in atomic ensembles".

We study squeezing of the spin uncertainties by quantum non-demolition (QND) measurement in non-polarized spin ensembles. Unlike the case of polarized ensembles, the QND measurements can be performed with negligible back-action and produce squeezing even in the limit of strong measurement. The generated spin states approach many-body singlet states, and contain a macroscopic number of entangled particles, even when individual spin is large. We show that the state prepared this way can be detected as entangled by measuring the uncertainties of the collective spin components. We estimate the achievable squeezing of the spin uncertainties for achievable experimental parameters.
We introduce the Gaussian treatment of unpolarized spin states.
[1] G. Toth and M.W. Mitchell, arXiv:0901.4110.

10th of June 2009, 13:15 in the Auditorium M.

J. M. Petersen , Quantop NBI, gave a midterm-talk on "An ion crystal quantum repeater".

The further development of quantum information science is dependent on reliable quantum communication. To send and recieve quantum states over reasonably long distances is an immense challenge given the rapid decoherence of quantum systems. A possible way forward is to use so-called ”quantum repeaters” which – instead of transmitting the actual quantum state – establish entanglement between the sender and the reciever. When entanglement is established it is then possible to do quantum communication with the transmission of only classical information.
In this colloquium we shall detail the workings of such a quantum repeater scheme and describe the hitherto most promising proposal for an actual realisation – the DLZC repeater (1) which is implemented in ensembles of freely moving neutral atoms. Finally, we explore one specific system that might be an improvement on the DLZC repeater: Using an ion crystal. These crystals are made of several thousand ions that are cooled by lasers and trapped in an electric trap where they form periodic structures. The control of the geometry of such a crystal is very good and the lifetime exceptionally long. A further advantage is that the crystal is located in a high finesse cavity which allows for strong coupling to light. The experimental work is done in Aarhus in the group lead by M. Drewsen.
The intended audience is fellow students of physics familiar with the undergraduate curriculum - in particular optics and quantum mechanics. No knowledge of quantum information science is assumed.
(1) L.-M. Duan, M. D. Lukin, J. I. Cirac, and P. Zoller: Long-distance quantum communication with atomic ensembles and linear optics, Nature 414, 413 (2001).

29th of May 2009, 13:00 in the Auditorium 3 at HCØ.

William D. Philips, NIST, gave a Niels Bohr Lecture.

An atomic-gas Bose-Einstein Condensate, placed in the periodic light-shift potential of an optical standing wave, exhibits many features that are similar to the familiar problem of electrons moving in the periodic potential of a solid-state crystal lattice. Among the differences are that the lattice potential can be turned on and off or accelerated through space. Experiments that are not easily done with solids are often straightforward with optical lattices, sometimes with surprising results. Among the new possibilities are the accurate simulation of important models of condensed matter physics that are exceedingly difficult to solve mathematically.

About the Speaker:
William D. Phillips has been a leading figure in experimental atomic physics for many years. He is most well known for his pioneering studies of laser cooling, for which he was awarded the 1997 Nobel prize in physics, and in recent years he has continued to make important contributions to the field of ultra cold atoms and Bose- Einstein condensation.In addition to his impressive scientific achievements William Phillips is famous for being an excellent speaker.

28th of April 2009, 14:15 in the Auditorium A.

David Mermin, Cornell University, talked on "Spooky Actions at a Distance? A Colorful Gedanken Demonstration for Non-scientists".

In the spirit of Faraday, I shall give a lecture demonstration, designed to convey to a member of the general public the peculiar behavior demonstrated by Danny Greenberger, Mike Horne, and Anton Zeilinger's version of the Einstein-Podolsky-Rosen experiment. Because I lack the talents of Faraday, my lecture demonstration will be a gedanken demonstration, with cartoon pieces of apparatus. Because the audience is likely to consist of non-non-scientists, I may address an occasional remark to experts.

7th of April 2009, 13:15 in the Auditorium M.

David Mermin, Cornell University, talked on "Compatibility of State Assignments".

If the quantum state of a system is not an objective property of that system, but an encapsulation of our knowledge of the system (as Heisenberg explicitly maintained) then different people with different degrees of knowledge about one and the same system, ought to assign different states to it. This raises the question of whether there are any constraints on the states that different people can assign to one and the same system. As far as I know the only person to explicitly raise and answer this question in the 20th century was Rudolf Peierls.
I shall review Peierls's answer, argue that he was wrong, argue that Peierls would have agreed with me that he was wrong, suggest what I believe is the right answer, and mention subsequent arguments that I am wrong.

25th of February 2009, 13:15 in the Auditorium A.

David Mermin, Cornell University, gave an Niels Bohr lecture on "What has quantum mechanics to do with factoring".

Abstract: Quantum computer science will be introduced in the context of its most sensational algorithm: the highly efficient factoring routine discovered by Peter Shor. I will emphasize those features of Shor's procedure that puzzled, surprised, and charmed me in the course of my own efforts to better understand how it does its magic. The subject offers some offbeat glimpses of both quantum mechanics and computation.

About the speaker:
David Mermin retired officially in 2006 from his position as professor at Cornell University, USA, which he has held for more than 40 years. This has not prevented him from immersing himself in the subject of quantum computation, on which he has published the recent monograph "Quantum Computer Science" (Cambridge University Press, 2007). His work on quantum computation grew out of a longstanding interest in the foundations of quantum mechanics, to which he has made extensive contributions. His research interests cover a broad range of topics, including density-functional theory, low-dimensional systems, superfluidity and quasicrystals. Together with Neil Ashcroft he is the author of the classic textbook "Solid State Physics" from 1976, which is still being widely used. He is a frequent contributor to the "Reference Frame" columns in Physics Today. David Mermin is a member of the National Academy of Sciences and a recipient of the Julius Edgar Lilienfeld Prize. He is visiting the Niels Bohr International Academy February-May 2009.

21st of January 2009, 15:15 in the Auditorium D.

Lars S. Madsen, Quantop NBI, defended his Master's thesis on "quantum light-atom interactions in cesium vapor cells".

This talk will consists of both experimental and theoretical work done on quantum light-atom interactions in cesium vapor cells.
One of the current limitations when working with these interactions is the so called "light induced decoherence", which is an unknown decoherence mechanism proportional to both the number of photons and the number of atoms.
We will treat an advanced version of the well known Quantum Non Demolition (QND)/ dipole (a1) interaction by including the quadrupole (a2) terms. We compare with other theoretical work and show experimental results which verify the theory and show that half of the "light induced decoherence" can be explained as a coherent part of the interaction.

9th of January 2009, 10:15 in Auditorium A.

Kim Lefmann, Risø National Laboratory, talked on "Time dependent quantum magnetism: coherence in solid state physics".

Solid state physicists have decades of experience in detecting quantum correlations in the ground state of many-body systems. One particular powerful method is neutron scattering. I will present new ideas as to how we can add time-dependence to neutron scattering, to ultimately study quantum spin systems out of equilibrium. This would in particular be interesting close to a quantum phase transition, where the entanglement length in the system may diverge and where coherent oscillations of the order parameter has been predicted.The talk will start with a review of quantum magnetism and neutron scattering for non-experts.

18th of December 2008, 13:00 in the Auditorium A.

Dieter Meschede, University of Bonn, will talk on "Controlling multiple neutral atoms in a 1D optical lattice".

In this presentation I will document the art of experimenting with multiple neutral atoms in our laboratory. I will show that in a 1D optical lattice individual neutral atoms can be trapped, detected and manipulated with excellent precision. Furthermore, we have recently been able to realize full single site detection of atoms, i.e. in adjacent micropotentials separated by only 400 nm, that is well below the optical resolution limit. I will also report on an non conventional method for cooling atoms to the lowest quantum states, show preliminary results on experimental realizations of quantum walks and report observations of quantum jumps in a cavity-QED experiment.

17th of December 2008, 14:15 in Auditorium A.

Patrick J. Windpassinger, will defend his Ph.D. thesis on "Non-destructive quantum state measurements and Quantum noise squeezing".

A method for non-destructive probing of the clock state population of laser-cooled, dipole trapped Cs atoms at the standard quantum limit is presented.
The non-destructive probing allows us to follow the evolution of the population difference of the Cs-atom clock states when subjected to microwave fields in real time. This way, Rabi oscillations on the clock transition can be observed non-destructively over an extended period of time. We apply microwave spectroscopy techniques to characterize the evolution of the quantum state in the trap and especially focus on the effect of probe induced inhomogeneous dephasing and of probe induced spontaneous photon scattering on the atomic ensemble.
We push the population readout precision to the quantum mechanical limits and demonstrate that the measurement precision is limited by quantum noise. We demonstrate that the correlations between two consecutive, non--destructive measurements are non--classical and that therefore an entangled state of atoms has been created in the ensemble. The correlations allow us to infer a quantum noise reduction of 72%, i.e., -5.4dB of remaining noise and -3.5dB of spectroscopically relevant quantum noise squeezing.

15th of December 2008, 11:00 in the Auditorium A.

Florian Marquardt, University of Munich, will talk on "Optomechanics".

In this talk I will review recent progress in the physics of the interaction between radiation and mechanical motion.The paradigmatic system in this field of 'optomechanics' consists of an optical cavity with a movable mirror attached to a cantilever.I will discuss how the coupled dynamics of the light field inside the cavity and the cantilever motion gives rise to a series of interesting effects. On the level of classical dynamics, I will present the theory of nonlinear oscillations and the corresponding attractor diagram. Furthermore, it is possible to cool the cantilever by irradiating the cavity with a red-detuned laser beam. I will present the quantum theory of optomechanical cooling and discuss the prospects for reaching the ground state of the cantilever's center-of-mass motion. Among the interesting opportunities that open up in the quantum regime, I discuss the quantum nonlinear dynamics of an optomechanical system and the detection of quantum jumps.

4th of December 2008, 15:30 in Auditorium A.

Paulo A. Nussenzveig, University of Sao Paulo, will talk on "Experimental Generation of Bright Three-Color Tripartite Entanglement".

Quantum information technology will require quantum networks to send information from one place to another. If we consider present day candidates for pieces of quantum hardware, we immediately realize that each have strengths and weaknesses. One can thus envisage hybrid systems, combining strengths of different hardware, which can interact with light at different frequencies. We have been investigating the generation of multipartite multicolor entanglement between bright beams of light, emitted by an optical parametric oscillator. In this talk, I will describe our very recent experimental demonstration of three-color tripartite entanglement.

1st of December 2008, 14:00 in Auditorium A.

Andrew J. Hilliard, will defend his Ph.D. theis on "Collective Scattering in a Bose-Einstein Condensate".

My talk will describe the construction of a machine to generate Bose Einstein condensates in Rubidium 87 and the first experiments performed with this machine on superradiant Rayleigh scattering.
Bose Einstein condensates of Rb 87 are produced by evaporatively cooling atoms in a magnetic trap of the quadrupole-Ioffe configuration. The atoms are loaded into the magnetic trap in a region of ultra-high vacuum from a double Magneto-Optical trap set-up. The evaporative cooling is achieved by selectively driving radio-frequency transitions to untrapped magnetic substates. During the evaporation, the magnetic trap is relaxed so that density dependent heating does not substantially reduce the number of atoms in the condensate. With a duty cycle of about a minute, we produce pure, prolate condensates containing up to a few million atoms.
The application of an off-resonant beam of light along the long axis of the condensate leads to a form of collective Rayleigh scattering analogous to the superradiance that occurs in electronically inverted samples. One can think of this process as the amplification of quantum noise: photons are spontaneously scattered out of the pump beam, and due to the extended optical depth along the long axis of the BEC, the modes that propagate along this axis see the most gain. In the end-pumped geometry, the strongest superradiant mode is the one where photons are back-scattered by the atoms. The overlap of stationary and recoiling atoms recoil produces a density modulation - a Bragg grating - which amplifies the back-scattering.
We have performed a systematic study of the effects of pump detuning on the process while keeping the single particle scattering rate constant. In this way, we move between the case where the pump beam functions as a reservoir of photons to the situation where superradiance is clamped by a lack of photons in the pump beam. Our experimental results are strongly supported by simulations of the system based on 1D Maxwell Schrödinger equations. We demonstrate that the dynamics result from the structures that build up in the light and matter fields along the long axis of the condensate. In particular, we find that the emission of the first superradiant pulse may be understood in terms of the overlap of light and matter wave gratings. Finally, the random nature of the spontaneous scattering that initiates the collective scattering is manifest at later times in the distribution of arrival times and photon numbers of the first superradiant pulse.

28th of October 2008, 12:00 in the Quantop lounge.

Georg Bruun, NBI, will talk on "Probing spatial spin correlations of ultracold gases by quantum noise spectroscopy".

Spin noise spectroscopy with a single laser beam is demonstrated theoretically to provide a direct probe of the spatial correlations of cold fermionic gases. We show how the generic many-body phenomena of anti-bunching, pairing, antiferromagnetic, and algebraic spin liquid correlations can be revealed in the measured spin noise as a function of laser width, temperature, and frequency.

13th of October 2008, 12.30 in the Auditorium M.

Konrad Banaszek, will talk on "Realistic quantum-enhanced phase estimation".

It is widely recognized that the use of nonclassical states of light allows one to overcome the shot-noise limit in precision phase measurements. The enhancement comes from exploiting inherently quantum features, such as entanglement, that are sensitive to decoherence mechanisms present in realistic measurements. In this talk we present designs for quantum-enhanced phase measurements that are robust to experimental imperfections, analyzed in several different settings. The ultimate precision of such schemes exceeds the shot-noise limit, but is inevitably worse than the ultimate quantum mechanical Heisenberg limit for the decoherence-free scenario.

9th October 2008, 12:15 in the Auditorium M.

Eran Kot, Tel Aviv, will talk on "Scattering of a Single Atom on a Localized BEC in Optical Lattice".

Recent works on the scattering of atom beams by Bose-Einstein condensate in an optical lattice have predicted the appearance of total reflection due to Fano resonance with the condensate. These predictions where made using the Gross-Pitaevskii model, which is a mean field approximation. Such results are expected to become inaccurate in the limit of small number of particles in the condensate. We present a model based on the Bose-Hubbard Hamiltonian for the scattering of a single boson on a localized condensate, which avoids the mean field approximation. The resulting transmission does not converge to the classical results when the number of particles is increased, but shows other features. These features includes points of total reflection, and different asymptotics as the classical limit is approached.

2nd of October 2008, 10:15 in the Auditorium M.

Marco Casadei, University of Bologna, will talk on "Study on the one-dimensional asymmetric Hubbard model".

The phase diagram of the one-dimensional asymmetric Hubbard model is investigated with the DMRG approach. Special attention is put on researching the phase transition, foreseen from a bosonization study, from spin density to charge density wave for finite difference of the hopping coefficient and attractive interaction. The results are showing the absence of the transition, supporting the previous mean field analysis, in disagreement with the bosonization one

4th of September 2008, 10:15 in the Quantop lounge.

Jens Baltrusch, Institute for Quantum Information Processing, Ulm University, will talk on "Quantum computation with Wigner crystals of ions".


24th of June 2008, 13:15 in the Auditorium M.

Sergey I. Bozhevolnyi, Institute of Sensors, Signals and Electrotechnics (SENSE), University of Southern Denmark, Odense, will talk on "Plasmonics: enhancing and guiding fields at nanoscale".

The explosive progress in nanoscience has led to uncovering and exploring numerous physical phenomena occurring at nanoscale. One of the main research directions in nano-optics is the search for configurations that efficiently interconvert propagating (µm-sized) and strongly localized (nm-sized) optical fields resulting thereby in strongly enhanced local fields, which are indispensable for optical characterization, sensing and manipulation at nanoscale. After briefly reviewing various configurations used for creating enhanced optical fields, a novel route exploiting retardation-based resonances involving (slow) surface plasmons (SPs) supported by metal nanostructures is considered in detail [1-3]. It is argued that the suggested configuration can be advantageously used in order to realize strong and robust field enhancement effects.
Photonic components are superior to electronic ones in terms of operational bandwidth but suffer from the diffraction limit that constitutes a major problem on the way towards miniaturization and high density integration of optical circuits. The degree of light confinement in dielectric structures, including those based on the photonic band-gap effect, is fundamentally limited by the light wavelength in the dielectric used. The main approach to circumvent this problem is to take advantage of hybrid nature of SPs whose subwavelength confinement is achieved due to very short (nm-long) penetration of light in metals. After briefly reviewing various SP guiding configuration the results of our investigations of subwavelength photonic components utilizing SP modes propagating along channels cut into gold films are overviewed [4-6], demonstrating first examples of ultracompact plasmonic components that pave the way for a new class of integrated optical circuits [7].

1. T. Søndergaard and S. I. Bozhevolnyi, Slow-plasmon resonant nanostructures: Scattering and field enhancements, Phys. Rev. B 75, 073402 (2007).
2. S. I. Bozhevolnyi and T. Søndergaard, General properties of slow-plasmon resonant nanostructures: nano-antennas and resonators, Opt. Express 15, 10869 (2007).
3. G. D. Valle, T. Søndergaard, and S. I. Bozhevolnyi, Plasmon-polariton nano-strip resonators: from visible to infra-red, Opt. Express, 2008, vol.16, No.10, pp.6867-6876.
4. S. I. Bozhevolnyi, V. S. Volkov, E. Devaux, and T. W. Ebbesen, Channel plasmon-polariton guiding by subwavelength metal grooves, Phys. Rev. Lett. 95, 046802 (2005).
5. S. I. Bozhevolnyi, V. S. Volkov, E. Devaux, J.-Y. Laluet, and T. W. Ebbesen, Channel plasmon subwavelength waveguide components including interferometers and ring resonators, Nature 440, 508 (2006).
6. V. S. Volkov, S. I. Bozhevolnyi, E. Devaux, J.-Y. Laluet, and T. W. Ebbesen, Wavelength selective nanophotonic components utilizing channel plasmon polaritons, Nano Lett. 7, 880 (2007).
7. T. Ebbesen, C. Genet, and S. I. Bozhevolnyi, Surface-plasmon circuitry, Physics Today, May 2008, pp.44-50.

28th of June 2008, 10:15 in the Quantop Lounge.

Keiichi Adamatsu, Tohoku University, Sendai, will talk on "Measurement of optical Kerr nonlinearity at a single photon level in a photonic crystal fiber" and/or "Coherent state transfer from photon polarization to electron spins in a semiconductor".


20th of June 2008, 15:45 in the Auditorium A.

Peter Hannaford, Swinburne University of Technology, Melbourne, Australia, talked on "Magnetic Lattice for Ultracold Atoms and BECs".

Optical lattices produced by the interference of intersecting laser beams are widely used in many experiments to trap periodic arrays of ultracold atoms and BECs to study fundamental phenomena such as the superfluid to Mott insulator quantum phase transition.
An alternative approach for producing periodic lattices for ultracold atoms is to use the magnetic potentials of a periodic array of magnetic microtraps. Magnetic lattices produced from permanent magnetic microstructures are highly stable with low technical noise and low heating rates; they can have large barrier heights and large trap curvature leading to high trap frequencies; and they can be produced with a wide range of periods, down to about 1 μm. In order to be trapped in a magnetic lattice the ultracold atoms need to be prepared in low magnetic field-seeking states, allowing the use of radiofrequency evaporative cooling in situ and radiofrequency spectroscopy. Thus we consider magnetic lattices to be complementary to optical lattices, in much the same way as magnetic traps are complementary to optical dipole traps.

27th of June 2008, 13:15 in the Auditorium D.

Prof. Gershon Kurizki, Weizmann Institute of Science, Rehovot, Israel, talked on "Dynamic control of quantum entanglement: from discrete to continuous variables".

The challenges of manipulating internal and translational entangled states of quantum objects in noisy environments will be elucidated. New aspects of quantum measurements will be discussed.
In this lecture we study the dynamical control of an entangled multipartite system coupled to a bath. Such a system undergoes rapid disentanglement in many realistic scenarios due to local, symmetry-breaking differences in the particle-bath couplings. We show that locally controlled perturbations, addressing each particle individually, can impose a symmetry allowing the existence of decoherence-free multipartite entangled systems [1].
We next show that particles subject to fields that couple their internal and translational (momentum) states may undergo a crossover from randomization (diffusion) to strong localization (sharpening) of their momentum distribution. The predicted crossover should be manifest by a drastic change of the interference pattern as a function of the coupling fields [2].
We finally explore the feasibility of creating a translationally entangled state for massive particles, and its use for matter wave teleportation [3]. To this end, we study collisions as a tool for generating translational entanglement. In one-dimensional scattering, resonances are shown to determine the degree of postcollisional entanglement [4]. New results show that frequent probing protects/freezes such motional states.

[1] G. Gordon and G. Kurizki, Phys Rev Lett 97, 110503(2006).
[2] N. Bar-Gill and G. Kurizki, Phys. Rev. Lett. 97, 230402 (2006).
[3] L. Fisch and G. Kurizki, Europhys. Lett. 75, 847(2006).
[4] A. Tal and G. Kurizki, Phys. Rev. Lett. 94, 160503 (2005).

26th of June 2008, 13:15 in the Auditorium D.

Prof. Gershon Kurizki, Weizmann Institute of Science, Rehovot, Israel, talked on "Why and how can we control decoherence".

ABSTRACTA unified approach to quantum decoherence and its dynamic control will be presented. Applications of this approach to the control of quantum noise , relaxation and quantum thermodynamic processes will be discussed . In this lecture, we present the framework for universal dynamical control of two-level systems (TLS) or qubits experiencing amplitude or phase noise (AN or PN) due to coupling to a thermal bath. Completely analogous formulae are obtained for the dynamical control of the AN and PN decoherence rates, thus underscoring the unified nature of this universal formalism [1,2]. We next apply this universal control to create disturbances of thermal equilibrium between TLSs and a bath, by frequent, brief quantum non-demolition measurements of the TLS energy states. By making the measurements increasingly frequent, we encounter first the anti-Zeno regime and then the Zeno regime (namely where the TLSs' relaxation respectively speeds up and slows down). The corresponding entropy and temperature of both the system and the bath are then found to either decrease or increase depending only on the rate of observation, contrary to the standard thermodynamical rules that hold for memory-less (Markov) baths. From a practical viewpoint, these anomalies may offer the possibility of very fast control of heat and entropy in quantum systems, allowing cooling and state purification over an interval much shorter than the time needed for thermal equilibration or for a feedback control loop [3].

[1] G. Gordon, N. Erez and G. Kurizki, J. Phys. B-AMO 40, S75(2007).
[2] G. Gordon and G. Kurizki, Phys Rev Lett 97, 110503(2006).
[3] Noam Erez, Goren Gordon, Mathias Nest and Gershon Kurizki, Nature 452, 724-727(2008).

4th of June 2008, 15:15 in Auditorium 3 at HCØ.

Gerald Gabrielse, Leverett Professor of Physics, Harvard, gave a Niels Bohr lecture on "A One-Electron Quantum Cyclotron: New Measurements of the Electron Magnetic Moment and the Fine Structure Constant".

Not since 1987 has the electron magnetic moment and the fine structure constant been measured more accurately. Now, a one-electron quantum cyclotron has made possible much more accurate measurements of both. The dimensionless electron magnetic moment (often called the electron g value) is measured 15 times more accurately than in a celebrated measurement that stood for 20 years. The fine structure constant is measured 20 times more accurately than in any independent measurement.
A quantum non-demolition measurement reveals the quantum structure in the cyclotron motion of an electron suspended by itself for months at a time. Cavity-inhibited spontaneous emission and a one-particle self-excited oscillator (SEO) give the resolution needed to carry out quantum-jump spectroscopy of the lowest energy levels of the weakly-bound electron-apparatus system. The SEO is the classical measurement system for the quantum states of the electron cyclotron motion and spin.

14th of May 2008, 10:15 in the Quantop Lounge.

Maximilian Schlosshauer, University of Melbourne, talked on "Putting mechanics back into the quantum: Decoherence and dissipation".

Quantum-electromechanical systems are nanoscale mechanical resonators that have been widely studied and appreciated through both experiment and theory. Despite their macroscopic size and mechanical, ordinary-matter nature, these resonators can exhibit distinct quantum behavior that is of great interest and promise to an experimental exploration of questions in the foundations of quantum mechanics. First, I will sketch the feasibility and features of "cat states" involving macroscopically distinct positions. I will then present two models for decoherence and dissipation in nanomechanical resonators that are inspired by recent experimental evidence. The models also close an important gap in the set of "canonical" models of decoherence.
Quantum-electromechanical systems are nanoscale mechanical resonators that have been widely studied and appreciated through both experiment and theory. Despite their macroscopic size and mechanical, ordinary-matter nature, these resonators can exhibit distinct quantum behavior that is of great interest and promise to an experimental exploration of questions in the foundations of quantum mechanics. First, I will sketch the feasibility and features of "cat states" involving macroscopically distinct positions. I will then present two models for decoherence and dissipation in nanomechanical resonators that are inspired by recent experimental evidence. The models also close an important gap in the set of "canonical" models of decoherence.

12th of May 2008, 10:15 in the Quantop Lounge.

Alessandro Zenesini, University of Pisa, talked on "BEC's in 1-D optical lattices".


15th of Apr. 2008, 11:15 in Auditorium D.

Wojciech Wasilewski, University of Torùn, will talk on "Eksperimental characterization of down conversion based photon sources".

Photons are the best carriers of quantum information, in particular in quantum cryptography, communication or quantum computing. This is a motivation behind large effort on improving existing and devising new photon sources. There is however much room for developing methods capable of precise characterization of multimode state of single photons. In my talk I will present several new methods devised by us. In particular I will describe a method for measuring spectral density matrix of an ultra short single photon, which completely describes its temporal shape and coherence.

9th of April 2008, 15:15 in the Auditorum M.

Anne Louchet, Laboratoire Aimé Cotton, Orsay (France), talked on "Optical coherent driving of spin waves in Tm:YAG".

Most optical quantum storage protocols, either based on EIT, Raman scattering or photon echo, require that quantum information be transferred from photons to long-lived atomic superposition states. In rare earth ion-doped crystals, offering long optical and hyperfine coherence lifetime, quantum information could be stored in nuclear spin superposition states. For 25 years, the optical excitation of nuclear spins in rare earths has been thoroughly studied but confined to praseodymium (Pr) and europium (Eu) ions. However these systems must be driven by dye lasers that are very difficult to stabilize below 1 kHz, the typical width of the storage superposition states.
We present the first investigation of nuclear spin optical excitation in a thulium (Tm)-doped crystal. With a 1/2 nuclear spin, Tm exhibits a simple nuclear Zeeman structure, where the level spacing is easily controlled with an external magnetic field. Besides, the 793nm absorption wavelength of Tm lies in the range of semi-conductor laser sources, that are easy to stabilize below 1kHz. Therefore, a Tm-doped crystal appears to be a simple and versatile substitute to Pr- and Eu-doped crystals for quantum storage of light in a solid.
I will first describe the building of a Lambda-system in Tm:YAG. Then I will discuss the optical coherent driving of spin waves in Tm:YAG and the optical recovery of spin wave excitation, with the help of Coherent Raman Beats and Raman echoes. We measure the relevant nuclear superposition lifetime and inhomogeneous broadening

12th of Feb. 2008, 15:15 in the Quantop lounge.

Aurelien Dantan, Quantop, University of Aarhus, talked on "Spin squeezing with cavity-QED".

Atomic spin squeezing is associated with the reduction of the quantum fluctuations level of the collective spin of an atomic ensemble below that of a coherent state. After reviewing the input-output theory for calculating the atom-field quantum fluctuations under a cw excitation I will discuss possible schemes to produce such spin-squeezed states using the interaction with quantized cavity fields.

11th of Feb. 2008, 12:15 in the Quantop lounge.

Andrew Hilliard, Quantop, NBI, talked on "Superradiant scattering in a BEC".


15th January 2008, 13:15 in the Quantop lounge.

Oliver Morsch, INFM Universita di Pisa, talked on "Freezing by shaking: Dynamical control of matter-wave tunneling in periodic potentials".

The tunneling of particles through a classically forbidden barrier between two potential wells is a hallmark of quantum physics. A particle initially trapped on one side of the barrier can "decay" to the other side, and intuitively one would expect that decay to be accelerated if the system is disturbed. Surprisingly, it turns out that certain kinds of disturbances can actually reduce the tunneling and even suppress it completely. We have observed this dynamical suppression of tunneling in periodically "shaken" optical lattices filled with Bose-Einstein condensates. In my talk I shall discuss these findings and their theoretical background and also present recent results in which tunneling is first suppresssed by tilting the lattice and then resuscitated by shaking it, an effect which is analogous to photon-assisted tunneling in solid state physics.

11th January 2008, 12:15 in the Quantop Lounge.

Jacob Sherson, University of Mainz, talked on "towards single site addressability of ultracold atoms in optical lattices".

Abstract: I will describe the progress in the setup of a new experiment utilizing an ultra-high resolution imaging system to achieve first single site detection and later single site manipulation. The resolution will be 300nm, which is smaller than the lattice spacing of the infrared lattice. The aim of our project is to prepare and to study single one- and two dimensional quantum systems. Single site addressability will allow us to modify or perturb the system on a local scale and to observe the ensuing dynamics of the many-body system in real time. Quantum gates and entanglement between neighbouring atoms can, for example, be obtained by collisions in a spin-dependent lattice. We intend to load the ultracold atoms into the lattice from an all optical Bose-Einstein condensate generated in a dynamically compressed crossed optical dipole trap formed by a 50 W YAG laser.

16th November 2007, 14:15 in Auditorium A.

Anders Sørensen, Quantop, Niels Bohr Institute, talked on "Quantum Teleportation -- Why?".

The name teleportation catches the attention of both the scientific community as well as the general public. There is little doubt that much of this attention is motivated by the teleportation of human beings in various movies and TV-shows. Although one could imagine the teleportation protocol being used in this context, this is actually not the main motivation behind the efforts to realize teleportation. In general teleportation plays a very important role in the field of quantum information, which try to exploit the counter intuitive and often paradoxical effects of quantum mechanics for practical applications. I will try to explain what the teleportation is, and why we are trying to realize it.

14th November 2006, 15:15 in the Auditorium 3, HCØ.

Jeff Kimble, Caltech, talked on "TITLE OF TALK".


31st October 2007, 12:15 in the Quantop lounge.

Erika Andersson , talked on "Efficient realisation of generalised quantum measurements"

Generalised quantum measurements (POMs or POVMs), which go beyond standard von Neumann projective measurements, are an essential part of many protocols in quantum information science, including state identification, state estimation and quantum cryptography. It is therefore important to find efficient means of experimentally implementing these non-trivial measurement schemes. The "standard" realisation of a generalised measurement requires an ancillary quantum system, sometimes with many dimensions, and quantum operations coupling the original system plus the ancilla. Recently, a method of performing generalised measurements using repeated projective measurements was proposed [1]. It was shown that an arbitrary generalised quantum measurement could be realised by adding a single auxiliary state to the original system and performing successive projective measurements on this enlarged system. I will explain the method and give a possible experimental implementation using photons or atomic qubits. A potential drawback with the method is the number of iterations required to complete the measurement. We have therefore investigated a way to reduce the number of projective measurements required. This latter way of realising generalised quantum measurements could be used e.g. for probing the state of a cavity field by sending an atom through it and measuring the atom.

[1] Realization of positive-operator-valued measures by projective measurements without introducing ancillary dimensions, Guoming Wang and Mingsheng Ying, quant-ph/0608235.

10th August 2007, 15:00 in the Quantop lounge.

Peter Krüger, University of Heidelberg, talked on "superfluidity in two-dimensional systems".


10th August 2007, 14:00 in Auditorium A.

Kasper Jensen , Quantop, NBI, presented his masters thesis on "Classical and Quantum Noise of Atoms and Light".


11th June 2007, 16:00 in Auditorium A.

Christina Olausson , Quantop, NBI, presented her masters thesis on "A Rubidium Bose-Einstein Condensate. Toward the BEC-Light Quantum Interface".

The history of Bose-Einstein condensation dates back to the early 1920s, where a cooperation between Satyendranath Bose and Albert Einstein resulted in the development of Bose-Einstein statistics of noninteracting particles. For atoms the results predicted that at very low temperatures, a macroscopic fraction of the atoms would occupy the quantum mechanical energy ground state, if the phase-space density was larger than approximately unity.
The development and improvement of laser cooling and trapping techniques in the 1980s and early 1990s opened a new door to lower temperature, and when laser cooling and trapping was combined with magnetic trapping and evaporative cooling, the first Bose-Einstein condensates were finally created in 1995.
The talk will describe the apparatus, which is designed and built to trap, cool and condense a gas of rubidium atoms in the basement of the Niels Bohr Institute. The characteristics of the atomic cloud during the different stages of the cooling process will be presented, together with the signatures of Bose-Einstein condensation.
The main purpose of creating the condensate is to investigate the quantum interface between light and a Bose-Einstein condensate. One such experiment involves the investigation of superradiant scattering in a condensate. The concepts and preliminary results of the experiment will be introduced.

30th May 2007, 12:15 in the Quantop lounge.

Denis Vasilyev, V. A. FockPhysics Institute, St. Petersburg University, talked on "Quantum thick holograms".


2nd May 2007, 11:00 in the Quantop lounge.

Dmitriy V. Kupriyanov, Department of Theoretical Physics, State Polytechnic University, St. Petersberg, talked on "Quantum memory via coherent scattering of light by optically thick atomic medium".

The quantum interface between light and the spin subsystem of a polarized ensemble of atoms will be discussed. The physical nature of the interface is in a polarization-sensitive interaction of the field and matter subsystems in the process of off-resonant coherent forward scattering of light by spatially extended and optically thick atomic sample. For atoms, which spin angular momentum > 1, the various detection schemes for Raman-type coupling between the light and spin polarization components will be presented and discussed. The spectral analysis of the polarization-sensitive light-matter interaction shows how temporal spectral modes of quantum light couple to particular spatial spin modes distributed in the spatially extended atomic sample. In particular the discussed interaction mechanisms can be applied for quantum memory storage and retrieval schemes and for deterministic entanglement protocols. The proposed protocols are attractive due to their simplicity since they involve just a single pass of light through atoms without the need for elaborate pulse shaping or quantum feedback. As a practically relevant example we consider the interaction of a light pulse with hyperfine components of D1 line of 87Rb. The quality of the proposed protocols is verified via analytical and numerical analysis.

3rd May 2007, 15:15 in Auditorium A.

Rodolphe Le Targat, LNE-SYRTE, Observatoire de Paris, talked on "Optical Lattice Clocks with Sr Atoms".

Over the past few years, the extensive development of Optical Lattice Clocks (OLC)based on neutral atoms has given hope for an ultimate accuracy in the range of 10-17, surpassing the performances of the state-of-art atomic microwave fountains.
We report a new accuracy evaluation of the LNE-SYRTE OLC with fermionic 87Sratoms. The previous accuracy budget [1], in 2006, was limited by the effect of the first order sensitivity of the clock transition to the magnetic field. The interrogation is now achieved by preparing spin-polarized atoms alternately optically pumped in the mF = ± 9/2 Zeeman substates, which strongly decreases the uncertainty on the magnetic field effects.
This work was carried out in collaboration with the PTB Institute, the frequencymeasurements were made with their femtosecond frequency comb based on a fiber laser [2], and referenced to the three atomic fountains FO1, FO2 and FOM at LNE-SYRTE. The resulting accuracy of the 87Sr clock is in the low 10-15 range. The stability of the measurement at 1s is 5.10-14, with roughly equal contributions of the Sr clock and of the fountain. We also performed the first evaluation of an OLC based on the 88Sr bosonic isotope,using a static magnetic field to allow the transition [3]. The systematic effects budget and a comparison with the fermionic isotope will also be discussed.

1. R. Le Targat et al., Phys. Rev. Lett. 97, 130801 (2006).
2. P. Kubina et al., Optics Express, 13, 904-909 (2005).
3. Z. Barber et al., Phys. Rev. Lett. 96, 083002 (2006).

19th April 2007, 13:15 in Auditorium B.

Dirk Witthaut, Univeristy of Kaiserslautern, talked on "Quantum vs. classical dynamics of (small) Bose-Hubbard systems".


12th April 2007, 10:45 in the Quantop lounge.

Wolfgang Wiedemair, Univeristy of Heidelberg, talked on "Quantification of lung perfusion using 'Arterial Spin Labeling' techniques in magnetic resonance tomography".

Magnetic resonance imaging allows display of tissue perfusion without application of contrast agents using the 'Arterial Spin Labeling' technique (ASL). This technique detects a change in the distribution of magnetization in a tissue volume caused by inflowing magnetically labeled blood. At a main magnetic field strength of 1.5 T this difference amounts to approximately 2% of the overall signal. For the lung this yields a low signal-to-noise-ratio (SNR) of 3-5. In the scope of this project, a technique for improving SNR during free breathing was developed. The temporal variation of the signal distribution caused by respiratory motion is corrected by means of a 'Retrospective Respiratory Gating' procedure. Using this method it was possible to obtain a 3-fold increase in SNR in lung images. Absolute perfusion in different lung areas of volunteers was found to be in the range of (3,1-5,9) ml/(g min). The developed method allows for a diagnostically meaningful examination of the lung during free breathing.

4th April 2007, 12:15 in the Quantop lounge.

Johannes Weirich, NBI, talked on "The AdS/CFT correspondence - a string theory gauge theory duality".

String theory, even though invented as a model for describing the in- teractions of the most fundamental particles known today, was for a long time considered to be an abstract mathematical theory without connec- tion to phenomenological physics. On the other hand, the theory that in the best way describes the inter- actions of elementary particles, the standard model, has a number of well known problems, for example it does not include gravity. String theory does include gravity and it does not suffer from the prob- lems the standard model involves. But the results we have obtained from string theory could so far not reproduce the experimentally well confirmed predictions from the standard model.
In 1997 Juan Maldacena conjectured a relationship between string the- ory and a gauge theory, which was a big step towards describing the well known physics in terms of string theory and remedy the known problems of the existing theories. In this talk I will give an introduction to the duality of string theory and gauge theory, and explain the ideas and techniques how to map the theories onto each other.

21st March 2007, 15:15 in Auditorium 3 on HCØ.

Prof. Michel H. Devoret, Quantronics Labs at Yale University and College de France, gave a Niels Bohr lecture on "Quantum-mechanical radio-electrical circuits".

Could the bits of a computer be atom-like entities behaving quantum-mechanically? The miniaturization of transistors and Boolean gates down to single atoms or electrons has been envisioned as early as the 1980's, but it is only in the last decade that the superiority, for certain classes of problems, of the quantum computer over its conventional classical counterpart was understood theoretically. This discovery has spurred a flurry of activity aimed at implementing practically a "quantum machine" which would compute. In our own laboratory, we have followed the lead of superconducting integrated circuits, whose fabrication directly benefits from the micro- and nano-technology developed for semiconducting devices. The problem with solid-state implementations of "qubits" is their potentially strong coupling to unwanted degrees of freedom in the various materials of the circuit. Yet, we have shown experimentally that for a particular superconducting tunnel junction circuit - the so-called "quantronium"- electrical symmetries could be exploited to suppress, to a large extent, this undesirable coupling [1]. Interestingly, these circuits actually display quantum mechanics at the collective level of Kirchhoff's voltages and currents, not simply at the level of conduction electrons. Recently, we have performed a fast state-projection readout using a microwave pulse which excites the large junction of the circuit [2]. In the last few years, recent advances in Europe, Japan and the US have propelled the quantum mechanical coherence of superconducting circuits at a stage where genuine quantum information processing involving a register of several qubits can be engineered.

[1] D. Vion et al., Science 296 (2002) 286
[2] I. Siddiqi, R. Vijay, F. Pierre, C.M. Wilson, M. Metcalfe, C. Rigetti, L. Frunzio, and M.H. Devoret, Phys. Rev. Lett. 93 (2004) 207002.

16th March 2007, 12:00 in the Quantop lounge.

Christina Olausson, Quantop, NBI, talked on "Superradiant scattering of laser light in a Bose-Einstein condensate".

Superradiance is the collective emission of light from an ensemble of excited atoms and involves the spontaneous buildup of coherence in a macroscopic ensemble of atoms. In a sample with dimensions larger than the wavelength of the emitted light, the coherent scattering can lead to coherently recoiling atoms, if the emission time is small compared to the coherence time. The recoiling atoms interfere with atoms at rest, creating a matter wave grating, which amplifies the scattering process. The scattering process is strongly dependent on the geometry of the atomic sample; an enlongated Bose-Einstein condensate offers the right conditions and is ideal for investigating the superradiant emission.
The talk will introduce the principles of superradiant scattering and the recent experiments on the topic, and preparatory considerations related to future superradiance experiments with the groups rubidium BEC will be presented. The future experiments will mainly be concerned with the conditions for the onset of superradiant scattering.

27th February 2007, 15:15 in Auditorium A.

Mads Lykke Andersen, NBI, talked on "Single photon emitters using surface plasmons".

A field which receives a tremendous deal of attention is nano-scaled devices based on surface plasmons in sub-wavelength-sized metallic structures. Surface plasmons polaritons (SPPs) are electromagnetic excitations associated with charge density waves that propagate along the surface of a metallic conductor.
The extreme concentration of light in SPPs enables strongly enhanced coherent coupling to a single photon emitter (e.g. a quantum dot) if it is placed in the close vicinity of the metallic nano-structured wire. As a consequence of this coupling, an excited QD will with large probability, as high as 95%, decay by exciting a SPP mode in the nanowire. The strong coupling between QDs and SPPs has several applications in quantum information science. One such application is the generation of single photons on demand. The single photons are generated by out-coupling the SPP to a nearby and co-propagating dielectric waveguide.

28th February 2007, 10:00 in Auditorium A.

Mads Lykke Andersen, NBI, presented his Masters Thesis on "Entanglement purification for continuous variable photonic states".

In quantum communication a reliable entanglement connection between two distant parties, Alice and Bob, is needed. Photonic states provides as excellent source of entanglement as they are weakly interacting and travel fast, furthermore we know that continuous variable photonic states are easy to produce and manipulate experimentally.
When photonic states are transmitted through an optical fiber they are subjected to noise and decoherence effects, this process leads to a loss in entanglement. We will present a purification protocol that enables us to regain entanglement in continuous variable photonic state. The protocol is designed to work on two mode squeezed vacuum states that have had an photon subtracted from each mode, as this state show a strong correlation between what sign Alice and Bob will find on their respective measurement of quadrature phase.
The suggested protocol mimics a qubit purification protocol that was presented by D. Deutsch et al. in 1996, and is implemented by using beamsplitters and measurements of quadrature phases.

14th February 2007, 11:15 in Auditorium A.

Josh Dunn, JILA - University of Colorado, talked on "Modeling Atom-Photon Interactions: Applications to Cooling".

The main theme of this talk is the application of various theoretical methods to understand fundamental atomic cooling processes. Our understanding of these processes is made possible by a diverse array of physical concepts. In particular, I will explore a novel means of reducing phase space using Feshbach resonances found in interparticle collisions. I will discuss the use of Monte Carlo techniques for developing an accurate understanding of the dynamics of atoms with multilevel structure moving in complex three-dimensional laser fields. Next, I will address a cooling scheme involving three-level atoms interacting with two distinct laser fields. By utilizing the phenomenon of electromagnetically induced transparency, traditional free-space laser-cooling can be coherently engineered to vastly improve effectiveness. Finally, I will introduce some initial work on our theoretical understanding of non-Markovian dynamics in dense atomic gases, a field that has been recently experiencing many experimental advances.

25th January 2007, 11:15 in the Quantop Lounge

Ulrich Busk Hoff, Copenhagen University, talked on "Microwave Cavity QED: Electronic cavity lifetime measurements and observations of photon quantum jumps".

Cavity QED constitutes an ideal ground for experimental studies and manipulations of light-matter interaction. This colloquium reports on the latest achievements in microwave cavity QED in the strong coupling regime. Thanks to a recently developed type of niobium-sputtered superconducting mirrors, cavities with quality factors as high as 4.2×1010 have been obtained, corresponding to a photon storage time of the order 0.1s. Probing the cavity field by circular Rydberg atoms, it has been possible to monitor in real time the thermally induced cavity photon number at a temperature of 0.8K, using a Quantum Non-Demolition measurement scheme, based on dispersive atom-field interaction. Abrupt changes of the photon number are observed when a photon is randomly emitted or absorbed in the cavity, showing the alternation between the vacuum state |0ñ and the one-photon Fock state |1ñ. These are the first observations of photon quantum jumps, showing the birth, life, and death of single photons in the cavity.

23rd January 2007, 11:15 in Auditorium A

Franziska Kaminski, ETH Zürich, talked "Decay Rates of Dipoles in the Near-Field of Gold Nanoantennas - 3D FDTD Simulations".


22nd January 2007, 10:15 in Auditorium A

Martijn Wubs, Universitat Augsburg, talked on "What can a qubit tell about its environment after a Landau-Zener sweep?".

Recent experiments showed superconducting circuits that behave as the electronic analogue of cavity quantum electrodynamics: A qubit interacts with a quantum harmonic oscillator. Parameters such as the ``atomic transition frequency'' are highly tunable in circuit QED. Topic of this talk by a theorist is the manipulation of a qubit via Landau-Zener transitions, in particular by transitions that are caused by the coupling to a circuit-QED oscillator. It turns out that for zero temperature, nonadiabatic transition probabilities can be calculated exactly. It is discussed how the qubit and oscillator are entangled after a Landau-Zener sweep, and how single photons can be generated. The model is then generalized to a qubit interacting with arbitrarily many oscillators. Rather surprisingly, Landau-Zener transition probabilities of the qubit can still be calculated exactly in this case. Dissipative Landau-Zener transitions can thus be studied in qubits subject to relaxation, pure dephasing, or both. This general result is relevant for the `bad cavity limit' of (circuit) cavity QED, but applications to qubits in other areas of physics are promising as well.

17th January 2007, 11:00 in Auditorium A

Prof. Richard Jozsa, University of Bristol, talked on "Efficient classical simulation of quantum computations".

In quantum computation we usually seek quantum circuits that cannot be classically efficiently simulated. These provide the basis for quantum algorithms that potentially exceed classical computational power. Thus the identification of classes of quantum circuits that *can* be efficiently simulated, is an important tool for studying the relation between quantum and classical computation. We will describe some recent techniques for identifying such classes, leading to examples of surprising generality. Given any such class we may ask for the "least" extra quantum ingredient that reinstates full quantum computational power. We will give examples showing that this extra ingredient can be made to appear remarkably trivial. From this point of view quantum computational power is a tantalisingly delicate extension of its classical analogue.

11th January 2007, 11:00 in Quantop lounge

Yan Ying, Lund University, talked on "Experimental Demonstration of Tripartite Entanglement and Controlled Dense Coding for Continuous Variables" and on "Watt Level CW Frequency-Stabilized Nd:YAP/KTP Laser with Dual Wavelength Output".

Abstract 1:
A tripartite entangled state of optical field for continuous variables is experimentally produced using nondegenerate optical parametric amplifier (NOPA) and linear optics devices. The controlled dense coding among a sender (Alice), a receiver (Bob), and a controller (Claire) is demonstrated by exploiting the tripartite entanglement. The information transmission capacity of the quantum channel between Alice and Bob is controlled by Claire. The channel capacity accomplished under Claire's help is always larger than that without her help.

Abstract 2:
A continuous laser-diode end-pumped Nd:YAP/KTP laser with dual wavelength output at 0.54µm and 1.08µm has been achieved. The ring laser resonator consists of five mirrors and a piece of frequency-doubling crystal KTP is placed between two concave mirrors. After calculating the waist-size and the astigmatism of the fundamental wave between the concave mirrors, the concave mirrors with proper curvature radii are chosen and the incident angles of infrared light are less than 3 degree. The lower dopant concentration of the Nd:YAP (0.6 at.-%) is chosen. The output power of the second-harmonic wave and the fundamental wave are respectively 1.02 W and 700 mW, with the frequency stabilities ±484 kHz and ±240 kHz, respectively.

5th December 2006, 16:00 in Auditorium M

Rasmus Olsson, Quantop, Niels Bohr Institute, will present (in Danish) his thesis on "Eksperimentel påvisning af kvante-teleportation fra lys til atomer".


9th November 2006, 12:15 in the Quantop Lounge

Anne Ersbak Bang Nielsen, Aarhus University, will talk on "Generation of non-classical states from a continuous-wave optical parametric oscillator".


31st October 2006, 13:15 in the Quantop Lounge

Graciana Puentes, University of Leiden, will talk on "Light scattering with entangled photons".

In this talk I will present experimental results on mixed-state generation by optical scattering of polarization-entangled photons. For a large variety of scattering media, ranging from polymer suspensions to multi-mode fibers, we recognized two markedly different classes of twin-photon scattered states; namely, Werner-like states and sub-Werner-like states. Additionally, I will show how to use scattering processes for controlled maximally entangled mixed-state engineering.

26th October 2006, 16:15 in Auditorium A

Bo Melholt Nielsen, Niels Bohr Institute, talked on "Cavity quantum electrodynamics and some experimental applications".

The talk is a short introduction into the experimental investigation of quantum electrodynamics (QED) - with emphasis on the interactions between light and a single atom strongly coupled in an optical cavity. Two experiments studying this phenomena will be discussed. The first is the real-time cavity QED, where a single atom transiting the cavity is observed. The second is the atom-cavity microscope, where the trajectory of a single atom trapped inside the cavity is observed.

27th October 2006, 14:00 in HCØ 3rd. floor, Room D317

Professor Moshe Shapiro, The Weizmann Institute of Science and University of British Columbia, talked on "Quantum Theory of Slow Light".

Professor Moshe Shapiro is professor of chemical physics at the Weizmann Institute of Science and the Canada Research Chair professor in quantum control. He is the winner ofseveral prizes, including the Landau Prize and the Weizmann Prize. Author (together with P. Brumer) of the book: Principles of Quantum Control of Molecular Processes (John Wiley & Sons, 2003).

11th October 2006, 15:15 in HCØ Auditorium 2

Eugene Polzik, Danish National Research Counsil Centre Quantop, Niels Bohr Institute, gave a Niels Bohr lecture on "Quantum teleportation between light and matter".

Quantum teleportation is a transfer of a quantum state between two objects. It is performed using a quantum (entangling) channel and a classical communication channel. Teleportation is an important ingredient in distributed quantum networks, and can also serve as an elementary operation in quantum computers. Teleportation was first demonstrated in 1997-98 by Innsbruck and Caltech-Aarhus teams as a transfer of a quantum state of light onto another light beam. Here we demonstrate teleportation between objects of a different nature—light and matter, which respectively represent ‘flying’ and ‘stationary’ media. A quantum state of a few-photon pulse is teleported onto a macroscopic object (an atomic ensemble containing 1012 caesium atoms). The fidelity higher than any classical communication can possibly achieve has been achieved. Besides being of fundamental interest, teleportation using a macroscopic atomic ensemble is relevant for a practical implementation of a quantum repeater. An important factor for the implementation of quantum networks is the teleportation distance between transmitter and receiver; this is 0.5 metres in the present experiment. As our experiment uses propagating light to achieve the entanglement of light and atoms required for teleportation, the present approach should be scalable to longer distances.
Reference: Jacob F. Sherson, Hanna Krauter, Rasmus K. Olsson, Brian Julsgaard, Klemens Hammerer, Ignacio Cirac, and Eugene S. Polzik, Nature, 443, 557- 560, October 5, 2006

28th September 2006, 15:15 in Auditorium C

Bo Melholdt Nielsen, Niels Bohr Institute, talked on "Generation of a Schrödinger kitten state of light".

The talk is an account of an experimental realization of a highly non-classical quantum state of light, called the Schrödinger kitten state. The state is generated simply by subtracting photons from squeezed vacuum. The reconstruction is done by a conditional measurement of the light field quadratures and by applying the algorithm of maximum likelihood to post-selected data. The resulting Wigner function is clearly negative!

15th August 2006, 10:15 in Quantop Lounge

Mikkel Andersen, National Institute of Standards and Technology, Boulder, USA, talked on "Quantized Rotation of Atoms by Photons with Orbital Angular Momentum".

The transfer of spin (or internal) angular momentum from photons to matter has been well understood and studied for a long time. On the other hand, photons can also carry orbital angular momentum. We demonstrate the coherent transfer of orbital angular momentum from a first-order Laguerre-Gaussian field (©¤ per photon) to sodium atoms in a Bose-Einstein condensate (BEC) using stimulated Raman transitions. The resulting atomic state is a vortex state with angular momentum ©¤. We also create vortices with charge 2 by transferring to each atom the angular momentum of 2 photons. We demonstrate coherent superpositions of different rotational states, and show that the phase in these states is determined by the phase of the light used for their generation. Using this tool we rotate atoms in different trap geometries. In an asymmetric toroidal trap we observe a persistent flow with lifetime longer than the lifetime of the atoms in the trap; a clear demonstration of one of the super- fluid properties of BECs.

11th August 2006, 15:00 in Auditorium M

Jonatan Bohr Brask, Niels Bohr Institutet, talked on "Long-distance Communication with Atomic Quantum Memories".

For communication between quantum computers or for the purpose of quantum cryptography it is desirable to transmit signals which have a quantum nature - that is signals which can only be fully described by quantum mechanics - over long distances. All realistic channels are noisy, and if such a signal is sent by direct transmission, the quality degrades exponentially with the communication distance and all information encoded in the signal is eventually lost. If, on the other hand, the communicating parties share an entangled pair, then they may transmit quantum information losslessly from one to the other by means of quantum teleportation. Entanglement can be distributed among the parties by a so-called quantum repeater protocol. We have analysed probabilitic quantum repeater protocols based on three different quantum memories. The systems were modelled by harmonic oscillators and approached both analytically and numerically. Polynomial scaling of the error in generated entanglement were found for all three systems, and the achievable communication rates were obtained. Furthermore, a major source of errors was found to be unwanted excitations created during light-atom interaction. In the colloquium, I will give an introduction to the necessary concepts from quantum information theory, and proceed to describe our method of analysis and results achieved.

3rd August 2006, 11:15 in Auditorium C

Blair Blakie, Jack Dodd Centre for Photonics and Ultra-Cold Atoms, Dept. of Physics, University of Otago, New Zealand, talked on "Some Like it Hot".

In this talk I discuss a phase space technique based on the Wigner representation that provides an approximate description of dilute ultra-cold Bose gases at finite temperatures. As the quantum field evolution is represented using equations of motion for classical fields, this has become known as the "classical field method", although it does in general include quantum effects in a controlled degree of approximation. This technique provides a practical quantitative description of both equilibrium and dynamical properties of Bose gas systems. In this talk I will provide a phenomenological motivation for the theory and present results of recent applications of the theory to the shift in critical temperature of a trapped interacting Bose gas, and to the low temperature phase diagram of a trapped 2D Bose gas.

13th June 2006, 11:00 in the Quantop lounge

Jan Arlt, University of Hanover talked on "Cold atom experiments in Hannover".


13th June 2006, 14:00 in Auditorium A

Plamen G. Petrov, Niels Bohr Institute defended his Ph.D.-thesis on "Quantum noise limited light interferometry with cold trapped atoms".

We present a method of nondestructive characterization of cold and trapped caesium atomic samples which relies on optical phase-shift measurement in a shot-noiselimited Mach-Zehnder white-light interferometer. The phase shift imposed on an offresonant light due to dispersive interaction with atoms is monitored via a pulsed homodyne detection scheme, allowing for fast and nondestructive characterization of atomic samples. The estimated rate of real transition per atom for a single probe pulse is found to be lower than unity. The population fluctuations of the upper hyperfine level in the caesium electronic ground state is measured to scale linearly with the number of atoms, which is an evidence that the experimental apparatus has the sensitivity to track the projection noise of a coherent superposition state.

31st May 2006, 11:00 in Auditorium M

Michael Budde, University of Aarhus talked on "The Aarhus Bose-Einstein condensate: how and what now".

14 March 2006 the Quantum-gas group in Aarhus succeeded in making its first Bose-Einstein condensate of a dilute gas of Rubidium atoms. In this seminar I will explain how our Bose-Einstein condensate experiment works, why it took 3 years to build and what we plan to do with it.

17th May 2006, 13:15 in the Quantop lounge

Mason A. Porter, California Institute of Technology, talked on "Bose-Einstein Condensates in Optical Lattices and Superlattices".

Bose-Einstein condensates (BECs), formed at extremely low temperatures when particles in a dilute gas of bosons condense into the ground state, have generated considerable excitement in the atomic physics community, as they provide a novel, experimentally-controllable regime of fundamental physics. In this talk, I will discuss my research on the macroscopic dynamics of coherent structures in BECs loaded into optical lattice and superlattice potentials, for which I employ methods from dynamical systems and perturbation theory. Using Hamiltonian perturbation theory, I will give an analytical construction of wavefunctions (observed in recently reported experiments) whose spatial periodicity is an integer multiple of the lattice period. I will also discuss BECs in superlattice potentials and how to manipulate solitary waves controllably with appropriate temporal adjustments of such potentials. Time permitting, I will briefly discuss more recent work on parametric excitation of BECs and work in progress on BECs with inhomogeneous (space-dependent) scattering lengths.

3rd April 2006, 13:15

Sebastien Gleyzes, Ecole Normale Superieure, talked on "Cavity quantum electrodynamics: toward non local Schrödinger´s cat states".

I will present our CQED experiment with Rydberg atoms. I'll show how we can use it to generate quantum superpositions of mesoscopic states made up of a few tens of photons and prove their coherence. In order to study larger cat states we developed a new generation of mirrors that provide a photon lifetime of more than 10 ms in open geometry. We also designed a new set-up with two cavities. The preliminary tests are encouraging. This opens promising perspectives in the domain of quantum information, but also for the study of the quantum/classical limit of non-local quantum states.

29th March 2006, 12:15 in the Quantop lounge

Alexey V. Gorshkov, Harvard University, talked on "Optimal storage of photons in EIT media" .


15th March 2006, 14:15 in the Auditorium A

Frank Vewinger, University of Calgary, talked on "Control of coherent superpositions of degenerate states using adiabatic transfer" .

Quantum control of non-degenerate levels is meanwhile a routine process with the use of fs-laser pulses, leading to wave-packed formation. The controlled preparation and full characterization of superpositions of degenerate states, which is e.g. of interest in quantum control schemes, requires a very different approach. One implementation utilizes adiabatic transfer into the desired superposition of degenerate states using delayed pulses. I will discuss schemes for the preparation and complete characterization of superposition states of degenerate levels on the example of a stream of identically prepared Neon atoms in a beam. On the same model system I will discuss the direct observation of interference between two competing transfer pathways, leading to a population distribution in the final states that depends on the phase of the coupling lasers.

9th March 2006, 12:15 in the Quantop Lounge

Mark Saffman, University of Wisconsin, talked on "Quantum control of ground state and Rydberg atoms in microscopic optical traps - part 1" .

We present recent progress in loading and manipulation of neutral atoms in microscopic optical traps. This includes hyperfine qubit rotations at a rate of 1.4 MHz, 1 ms coherence time, and individual site addressing with crosstalk at the level of 0.001 . These results are a significant step towards quantum computing using optically trapped neutral atoms. We discuss work in progress aimed at observing strong, angle independent dipole-dipole interactions for fast two-qubit gates using microwave dressing of Rydberg states. We demonstrate two-photon coherent excitation of Rydberg levels by a 5s1/2 -> 5p3/2 -> nd5/2 or ns1/2 sequence, and show preliminary results on excitation suppression due to dipole-dipole interactions.

30th January 2006, 13:15 in the Auditorium A

Jörg Schmiedmayer, Physikalisches Institut, Universität Heidelberg, will talk on "Mesoscopic physics with ultra cold atoms".

Miniaturization and integration of atom-optical components on atom chips [1]potentially allow to manipulate matter waves on the quantum level if one can realize potentials with high spatial resolution (typical ~1 µm). These can be created either directly using high spatial resolution potentials from structures on the atom chip, which requires atom-surface distances of the same the order of a few µm, or by employing adiabatic radio frequency (RF) or micro wave (MW) potentials. In both cases this necessitates to have very smooth potentials very strong transversal confinement in the order of ~10kHz which leads to 1d traps with extreme aspect ratio up to >1000. We experimentally investigate manipulation of cold thermal atoms and BEC’s in this parameter range. Using lithographically fabricated atom chips previously observed large disorder potential are many orders of magnitude smaller [3]. This allows to create 1 dimensional Bose-Einstein condensates on the atom chip which can be used to study magnetic and electric disorder potentials with a resolution to better than 10-13 eV, competitive with the magnetic field sensitivity of SQIDS [4]. In these smooth atom chip traps we used RF induced adiabatic potentials to split a 1d condensate along its long axis. Bringing the two split clouds together we can measure the interference between the two ensembles and study the coherence in the atom manipulation process [5]. The prospects of these RF and MW induced potentials for quantum information processing will be discussed [6]. This work was supported by the European Union, contract numbers IST-2001-38863 (ACQP), HPRN-CT-2002-00304 (FASTNet), MRTN-CT-2003-50532 (AtomChips), and HPRI-CT-1999-00114 (LSF) and the Deutsche Forschungsgemeinschaft, contract number SCHM 1599/1-1 and DIP.
[1] For an overview see: Microscopic atom optics: from wires to an atom chip. Folman, R., Krüger, P., Schmiedmayer, J.,Denschlag, J. & Henkel,C., Adv. At. Mol. Opt. Phys. 48, 263 (2002).
[2] Groth, S. et al. Appl. Phys. Lett. 85, 2980 (2004).
[3] Krüger, P. et al., arXiv:cond-mat/0504686 (2005).
[4] Wildermuth S. et al. Nature 435, 440 (2005).
[5] Schumm Th. et al. Nature Physics AOP doi: 10.1038/nphys125 (Oct. 2005)
[6] Cirone M. et al. arXiv: quant-ph/0505194 (2005).

18th January 2006, 15:15 in the Auditorium D

Toke Lund-Hansen, NBI, talked on "Storage of single photons using electromagnetically induced transparency".

One of the goals of quantum information science is the realisations of a scalable quantum network capable of sending quantum information from one place to another. This can be used for the transmission of quantum information with applications such as distributed quantum computations and quantum key distribution (quantum cryptography). As information carriers in a quantum network photons are a good candidate as they can be transmitted over long distances in optical fibers with little decoherence. However, the probability of loosing the photons increases exponentially with distance not making the network scalable. To achieve scalability quantum repeaters can be used with quantum memories. To build a so called scalable quantum network with photons as the information carriers a quantum memories for photons is therefore needed. colloquium. A possible implementation of a quantum memory for light is the use of electromagnetically induced transparency (EIT) in atomic ensembles, which has been used for the storage of classical pulses of light. This has recently been demonstrated for single photons [1,2]. In this talk these experiments will be described along with the principles of EIT and single photon generation using atomic ensembles. References: [1] M.D. Eisaman et al. Electromagnetically induced transparency with tunable single-photon pulses. Nature 438, 837-841 (2005). [2] T. Chaneliere et al. Storage and retrieval of single photons transmitted between remote quantum memories. Nature 438, 833-836 (2005).
Supervisor: Anders Sørensen

21st December 2005, 11:15 in the Quantop Lounge

Matthias Christandl, Cambridge University, talked on "Spectra and Representations" or "Bit commitment is impossible" or "Perfect State Transfer in Spin Chains": "Generation of non-classical photon pairs with quantum memory".

recently, I have been looking at the questions of how the spectrum of a bipartite quantum states relates to the spectra of its reduced density matrices. There is a nice connection to the representation theory of the symmetric group. This is rather a mathematical result, but the tools we use are quite useful in quantum information theory in general ( see quant-ph/0409016 and quant-ph/0511029 ).
Bit commitment is impossible but how good can one commit to a string of bits? The answer turns out not to follow trivially from the no-go for bit commitment but instead depends on the security criteria imposed. This talk belongs to the field of cryptography - and highlights the importance of the (ongoing) discussion of the use of security criteria in cryptography.
Imagine a chain of m spin 1/2 particles, with fixed (i.e. time-independent) interaction. We give examples of couplings that allow for the transfer of the state of the first particle in the chain to the last particle in the chain. Such a chain might be useful as a 'quantum wire' connecting different parts of a quantum computer. (quant-ph/0309131 and quant-ph/0405029)

20th December 2005, 10:15 in Auditorium A

Thomas Fernholz, University of Amsterdam, will talk on: "Generation of non-classical photon pairs with quantum memory".

Quantum memories are an important ingredient for long-distance quantum communication schemes. I report on experiments towards the realization of a quantum node following the DLCZ scheme (1). A Raman process is used to create non-classical, collective states in an atomic gas, which can be converted into light by coherent anti-Stokes Raman scattering (CARS). Upon detection of spontaneously scattered Stokes photons, correlated Anti-Stokes Photons can be retrieved after a controllable period of time. I will present our setup and experimental results obtained with hot Rubidium vapor. In particular, I will show data on momentum correlations and the influence of buffer gas. (1) L.-M. Duan, M. D. Lukin, J. I. Cirac, P. Zoller, “Long-distance quantum communication with atomic ensembles and linear optics” Nature 414, 413 (2001)

16th December 2005, 11:15 in Auditorium A

Roy J. Glauber, Harvard University, will talk on: "A Century of Light Quanta" (Nobel Lecture 2005).


16th December 2005, 14:15 in Aud. 3, H.C. Ørsted Institute

John L. Hall, University of Colorado - JILA and NIST, will talk on: "Optical Atomic Clocks: Defining and Measuring Optical Frequencies" (Nobel Lecture 2005).

The idea of using the natural oscillation period of an ensemble of Cs atoms as our fundamental definition for the unit of time was internationally accepted in 1960. Lasers and optical technology appeared to offer great possibilities for improvement (and presumed replacement) beginning in ~1970. These standards work incredibly well (15 digits). Instead of a new optical source whose wavelength would be used to express the new definition of length, the community rallied around a Metre redefinition strategy based on an adopted value for the speed of light, 299 792 458 m/s exactly. Metrology scientists observed that this reduced the number of base units by eliminating the Metre from the SI. Broad-vision scientists noticed that assumptions about spatial isotropy, temporal constancy and several other issues were now incorporated into the metrology structure. Eventually, if Physics continues as it has in the past, as unending stages - like the Russian Matrouchka doll model, our children will likely have some delicious new observations to enjoy and rationalize.
A new and transformative capability of our present epoch is the effective and direct optical comb method. High performance clocks can probably also be based on vibrational transitions in which the frequency-determining atomic mass arises at least in part from the Strong Interactions. Technical success with clocks will provide many digits for the comparison of time-keeping by different physical systems. At this point in our developing knowledge of Science we have no clue about calculating the basic numbers of physics: why does the ratio of Electric to Magnetic couplings turn out to be 137.035 999 76(50)? Does this change in time? Is the ratio of E/M and Strong Interactions time invariant? If Yes, Why? If No, Why not? And why exactly do we want two distinct decay times for the K mesons? Did I mention Dark Matter and Dark Energy?
Starting in the year 2000, and celebrated by this year's Nobel Prize, we now have a very brightly illuminated corridor along one particular road in the full domain of Physical Science. On this street we will not be looking for something known but merely misplaced, car keys in the usual joke, but rather some unsuspected anomaly. The good news about such fundamental research is that we should be able to publish a permanently-cited paper even if the answers all turn out to be conventional - - "this highway of atomic physics is in good order, and is now known with 15 digits precision." But who knows what will happen?

11th November 2005, 11:15 in the Quantop lounge

Jaromir Fiurasek, talked on: "Conditional generation of arbitrary single-mode quantum states of light by repeated photon subtractions".

We propose a scheme for the conditional generation of arbitrary finite superpositions of (squeezed) Fock states in a single mode of a traveling optical field. The suggested setup requires only a source of squeezed states, beam splitters, strong coherent beams, and photodetectors with single-photon sensitivity. The method does not require photodetectors with a high efficiency nor with a single-photon resolution.

17th November 2005, 15:15 in Aud. 3, H.C. Ørsted Institute

Jeff Kimble, Caltech, talked on: "The Quantum Optics Circus - Flying Photons, Acrobatic Atoms, and Entangled Ensembles".

This year Nobel Prize in physics is in the field of Quantum Optics. The NBI lecture on November 17 is given by one of the main players in the field whose experiments throughout the last 25 year have been at the forefront of Quantum Optics research.

Since its inception more than 40 years ago, Quantum Optics has made remarkable advances in the exploration of the quantum character of light, including the microscopic control of single atoms and photons. Indeed, laser operation has been has been pushed to the conceptual limit with the realization of a laser that operates with one acrobatic atom. Single, flying photons can now be generated deterministically at the push of a button. Ensembles of atoms can be projected into an entangled quantum state by the "click" of a photodetector. Beyond their fundamental significance, such advances are helping to lay the foundations for the new science of Quantum Information, including for the realization of complex quantum networks.

23rd September 2005, 11:00 in Auditorium D

Danko Bosanac, Ruđer Bošković Institute, talked on: "Was there any choice in setting foundations of modern physics".

Stepping stones of modern physics are: black-body radiation law, photoelectric effect, wave-particle dualism, quantization of fields,... Was something missed along the path of these developments that could have had a profound impact on the modern view of Nature. What would that view look like if the determinism of Newton mechanics was abandoned when it was the right opportunity? One possible development will be reviewed with some of its consequences.

14th September 2005, 11:15 in Auditorium A

Vladimir Buzek, Slovak Academy of Sciences, talked on: "Dynamics of open systems from a perspective of quantum information theory".

In my talk I will analyze how information encoded in a quantum system is "diluted" in reservoirs. Using a simple collision-like model. I will show that the original information is diluted in quantum correlations that are established during the interaction between the quantum system and particles of the reservoir. I will analyze conditions of reversibility.

31st August 2005, 11:15 in the Quantop lounge

Hannah Krauter, Humboldt University, talked on: "Raman lasing in a microsphere resonator".

The Whispering gallery modes (WGM) of microsphere resonators experience very high quality factors, inducing the storage of light for up to microseconds. Such a microcavity also leads to a strong spatial confinement of light. Due to those characteristics stimulated Raman scattering(SRS) can be observed in silica spheres, despite the extremely low m gain coefficient of SRS in fused silica glass (g ˜ 10-13W). Very low thresholds can be achieved for a microsphereRaman laser in this system. The lowest observed threshold in this experiment was 3,5 µW pump power coupled into the resonator. This value is to the best knowledge of the author the lowest measured value for this kind of laser. Nearfield measurements of the laser modes were performed with a scanning optical nearfield microscope(SNOM). That way it could be proven that at low pump powers the system is lasing in a single mode.The influence of the thermal bistability of the resonance frequencies on the Raman laser was investigated and a strong dependency of the laser power on the direction of the variation of the pump frequency was observed. Furthermore a strong impact of a scatterer in the evanescent field of the modes represented by the tip of the near field probe was detected. Depending on the position of the probe the laser power was reduced or the lasing broke down completely. The phenomenon could be traced back to a reduction of the quality factor due to scattering losses of the modes,caused by the probe. This feature makes the microsphere Raman laser fit for sensing applications.

29th August 2005, 14:15 in Auditorium A

Georg Bison, University of Fribourg, talked on: "A laser-optical magnetometer for the mapping of the human cardio-magnetic field".

Magnetic fields produced by biological organisms contain valuable information on the underlying physiological processes and their pathologies. Currently, super-conducting detectors (SQUID) cooled far below room temperature are required to measure these generally weak bio-magnetic signals. Our group at Fribourg University has developed a sensitive laser magnetometer based on optical pumping of cesium atoms that makes it possible to map the magnetic field produced by the beating human heart. The device uses an optically detected magnetic resonance technique to obtain the required sensitivity (100 fT / Hz1/2) and bandwidth (30 Hz). A gradiometer formed by two identical sensors greatly reduces the influence of external stray magnetic fields. The magnetometer operates at room temperature and therefore opens the way to an affordable and convenient monitoring of biomagnetic fields in research and medical diagnostics. In first experiments with healthy volunteers we have generated spatially resolved movies of the QRS-dynamics of the cardiomagnetic cycle. Future improvements will concentrate on higher-order gradiometers for improved stray-field suppression, on multi-channel systems for faster mapping, on on-line data processing, and a clinical validation.

12th August 2005, 15:00 in Auditorium A

Martin W Sørensen, Niels Bohr Institute, defended his masters thesis on: "3-Dimensional Theory Describing Light Matter Interactions".

22 July 2005, 11:00 in Auditorium A

Jeff Lundeen, University of Toronto, talked on: "Exploring Quantum Measurement in Photonic Systems".

In this talk, I will describe experiments that demonstrate four different kinds of quantum measurement: Weak measurements, Which-way measurements, Interaction-free measurements and Super-resolving phase measurements. Weak measurements are designed to measure a quantum system without collapsing it and are thus ideal for examining systems that are subsequently post-selected. We weakly measured two systems in which normal measurement would have destroyed the very thing under investigation. The first of these systems is Young’s double-slit experiment. We examined the destruction of interference by a which-way measurement and show that it can always be explained by an experimentally measurable momentum-transfer. Interaction-free measurement indirectly detects the presence of an object without disturbing it. We performed an experiment in which two interaction-free measurements are intertwined to give results that seem to demonstrate a deep contradiction between quantum physics and classical reasoning. Lastly, I will describe a super-resolving phase measurement that ideally reaches the Heisenberg limit by using a multiphoton entangled state.

10th June 2005, 11:00 in Auditorium D

Keiichi Edamatsu, Tohoku University, talked on: "Generation of entangled photons via biexcitons in a Semiconductor".

Entanglement is one of the key features to quantum info-communication technology. Although spontaneous parametric down-conversion has been used frequently so far to generate highly entangled pairs of photons, entangled photon sources using semiconductors are desired for practical applications. In practice, it would be possible to generate entangled photons from biexcitons in semiconductors. Recently, we reported the first experimental evidence of the generation of entangled photons via biexcitons in a semiconductor (CuCl) crystal [1]. In my talk, I will present our most recent experimental result that exhibits a higher degree of entanglement and violation of Bell's inequality.
[1] K. Edamatsu et al., Nature 431, 167 (2004)

9th May 2005, 10:00 in Auditorium A

Akira Furusawa, University of Tokyo talked on: "Quantum Teleportation and its Applications".

Basic studies on quantum information science are being made with quantum optics intensively. Continuous-variable (CV) approach attracts much interest because of relative easiness to realize the experiments. Unconditional quantum teleportation have been realized in the first time with this approach[1], and various experimental successes of quantum teleportation and other protocols have been reported. In this talk, we will show the progress of this CV approach especially on high-fidelity quantum teleportation beyond the no-cloning limit, entanglement swapping[2], a quantum teleportation network[3] and telecloning[4].

19th April 2005, 15:15 in Quantop Lounge

Patrick Windpassinger, talked on: "A setup for electric trapping of neutral atoms".


18th April 2005, 11:00 in Auditorium D

Niels Kjærgaard, University of Otago, talked on: "Cold Collisions of two Ultracold Atomic Clouds".

This talk presents results from experiments on the collision of two doubly spin-polarized atomic rubidium clouds attained individually in a magnetic double-well potential and evaporatively cooled to temperatures just above the phase transition for Bose-Einstein condensation. Collisions at selectable low energy occur when the trapping potential is continuously modified to a single-well configuration. The resulting scattering process can be regarded as cold collisions of counter-propagating ultracold pulsed beams. Angular resolved detection of the magnitude of the scattering amplitude can obtained by using laser absorption imaging of the scattered atoms. In the energy interval covered, only so- called s and d partial waves contribute to the elastic scattering cross section, p-waves being forbidden from the requirement of a totally symmetric wave function for identical bosonic particles. The angular distribution of scattered particles evolves from s-wave halos at low collision energies to d-wave patterns at higher energies via quantum mechanically interfering s+d scattering states. From the observed interference patterns the scattering phase shifts for the two channels can be extracted.

5th April 2005, 11:00 in Quantop Lounge

D.E. Chang, Harvard University, talked on: "Cavity Quantum Electrodynamics with Surface Plasmons".

We describe a technique that enables strong, coherent coupling between individual optical emitters and electromagnetic excitations in conducting nanowires. The strong coupling is achieved via excitation of optical plasmons that are localized to sub-wavelength dimensions. We show that under realistic conditions optical emission can be almost entirely directed into the propagating, localized plasmon modes via a mechanism analogous to cavity quantum electrodynamics. We describe two applications of this technique involving coherent coupling between separated quantum bits in the optical domain on a microchip as well as e±cient generation of single photons on demand. Finally, realistic sources of imperfections are analyzed.

10th March 2005, 11:00 in Quantop Lounge

Anders Sørensen talked on: "Fault tolerant quantum communication with solid state photon emitters".

We describe a novel method for long distance quantum communication in realistic, lossy photonic channels. The method uses single emitters of light as intermediate nodes in the channel. One electronic spin and one nuclear spin coupled via the contact hyperfine interaction in each emitter, provide quantum memory and enable active error purification. We show that the fixed, minimal physical resources associated with these two degrees of freedom suffice to correct arbitrary errors, making our protocol robust to all realistic sources of decoherence. The method is particularly well suited for implementation using recently-developed solid-state nano-photonic devices.

16th February 2005, 11:00 in Quantop Lounge

Jörg Helge Müller talked on: "Continuous QND measurement and conditional spin-squeezing in Alkali atoms - Preprint by Geremia, Stocton, and Mabuchi".

26th November 2004, 13:15 in Auditorium C

Christian Hettich talked on: "Single photon generation".

1st November 2004, 13:15 in Auditorium A

Jonathan Goldwin, University of Colorado, talked on: "Quantum Degeneracy and Interactions in the 87Rb-40K Bose-Fermi Mixture"

Dilute gas Bose-Fermi mixtures offer unique and flexible model systems for studies of quantum mechanical many-body ensembles. Knowledge of the cross-species scattering length aBF is essential in determining the static and dynamic properties of such mixtures. In this talk I will describe how we produce a quantum degenerate atomic Bose-Fermi mixture with 87Rb and 40K atoms. We have used cross-dimensional rethermalization with non-degenerate samples to measure the magnitude of aBF for this system. In these measurements we have introduced a technique that eliminates the large systematic uncertainty in atom number, leading to a relatively high-precision determination of |aBF|. Furthermore we have recently observed a number of inter-species Feshbach resonances. These measurements refine our knowledge of the background scattering properties of the system, while making possible a wide range of new experiments relying on the unique tunability of the interactions that can now be achieved.

29th October 2004, 13:15 in the QUANTOP lounge

Carlos L. Garrido Alzar talked on: "Interaction of 3-level atoms and quantized fields in the Heisenberg-Langevin formulation"

21st September 2004, 11:00 in the QUANTOP lounge

Prof. Hans-A. Bachor, ARC Centre of Excellence for Quantum-Atom Optics, Canberra, talked on: "The quantum laser pointer and other ideas for spatial quantum optics"

Light is ideally suited to make very sensitive measurements, such as absorption, changes in the phase or the polarisation. Another application is the direction, or position of a laser beam - for example used in optical galvanometers. Like all others, this measurement is limited by quantum noise. This talk will present experiments where a the position of a laser beam is determined better than the quantum noise limit - we call it a quantum laser pointer. The detailed of this experiment will be discussed. In addition optimum schemes for such spatial measurements will be presented. This work can be seen as the beginning of a wider field of quantum imaging.

20th September 2004, 13:15 in the QUANTOP lounge

Thomas Aichele, Humboldt University, Berlin, talked on: "Generation of visible single photons on demand from single quantum dots"

Efficient generation of single photons is an important task for modern quantum technology such as quantum computation and quantum cryptography (e.g. Waks et al., Nature 420 p. 762). Here experiments on the photoluminescence (PL) of InP and CdSe single quantum dots in a cryogenic micro-PL setup are described. Intensity correlation measurements on different individual spectral lines show that they act as individual single photon emitters. Cross correlation measurements additionaly reaveal the existence and dynamics of multi-photon cascades in the quantum dots. An efficient separation of the photons from this cascade, similar to classical multiplexing, is described and enables their simultaneous use in quantum information processing. A simple BB84 quantum cryptography experiment was set up to test the functionality of this multiplexing system.

7th September 2004, 12:15 in the QUANTOP lounge

Jacob Sherson talked on: "A Gaussian state analysis of continuous entanglement generation"