Jean-Baptiste BéguinJean-Baptiste Béguin

A thesis submitted April 24, 2014 for the degree of Doctor of Philosophy and defended May 15, 2015.

The PhD School of Science
Faculty of Science, Niels Bohr Institute, Theoretical Quantum Optics, University of Copenhagen, Denmark

Principal supervisor:  Eugene S. Polzik

Additional supervisors: 
Jürgen Appel
Jörg H. Müller

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Quantum interfaces between light and the collective degrees of freedom of an ensemble of identical atoms have been proposed as a valuable and promising alternative to cavity quantum electrodynamics enhanced interaction with single particles, Hammerer et al. (2010). Many features of the quantum world (e. g. multipartite entanglement, squeezed states), which are central to the future developments of Quantum Information Science and Metrology, can be explored with mesoscopic collective states of atoms.

An efficient quantum interface needs a high optical depth for the atomic ensemble and a measurement sensitivity limited by both the intrinsic quantum noise of light and the quantum projection noise of atoms. This was achieved in the past in a free space optical dipole trap ensemble of Nat _ 106 atoms, which triggered the operation of a collective Ramsey atomic clock assisted by entanglement Appel et al. (2009b); Louchet-Chauvet et al. (2010). We have characterized and prepared non-classical collective spin-squeezed states of atoms in this setup, with optical quantum non demolition measurement, Kiesel et al. (2012). We then pursued the goal of generating other non-classical collective states of atoms with non-gaussian statistics, conditioned on discrete heralding optical measurement, Christensen et al. (2014).

In the main part of this thesis, we propose an alternative to free space atomic ensembles to prepare quantum collective states. We build and explore a new interface based on the degrees of freedom between the evanescent fields of an optical nanofiber and fewer atoms Nat _ 103. We experimentally show an improvement of more than 2 orders of magnitude in the single-atom coupling strength and we demonstrate a simple method to implement an optical non-destructive measurement of the atomic state populations, which allowed to achieve −14 dB atom number squeezing, in an one-dimensional optical nanofiber lattice trap, Béguin et al. (2014). This shows the ability to explore spin-squeezing and quantum state tomography of non-classical states with negative Wigner functions, using a nanofiber. Finally, we report preliminary observations of collective atomic Bragg scattering in this extreme onedimensional geometry, in view to realize a switchable atomic mirror, Chang et al. (2012).

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