Quantum Optics Seminar by Ondrej Cernotik – Niels Bohr Institute - University of Copenhagen

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Quantum Optics Seminar by Ondrej Cernotik

Gaussian states and Gaussian transformations represent an interesting counterpart to two-level photonic systems in the field of quantum information processing. On the theoretical side, Gaussian states are easily described using first and second moments of the quadrature operators; from the experimental point of view, Gaussian operations can be implemented using linear optics and optical parametric amplifiers. The biggest advantage compared to two-level photonic systems, is deterministic generation of entangled states in parametric amplifiers and highly efficient homodyne detection. In this presentation, we propose new protocols for manipulation of entanglement of Gaussian states. 

Firstly, we study entanglement concentration of split single-mode squeezed vacuum states by photon subtraction enhanced by local coherent displacements [1]. These states can be obtained by mixing a single-mode squeezed vacuum state with vacuum on a beam splitter and are, therefore, generated more easily than two-mode squeezed vacuum states. We show that performing local coherent displacements prior to photon subtraction can lead to an enhancement of the output entanglement. This is seen in weak-squeezing approximation where destructive quantum interference of dominant Fock states occurs, while for arbitrarily squeezed input states, we analyze a realistic scenario, including limited transmittance of tap-off beam splitters and limited efficiency of heralding detectors.

Next, motivated by results obtained for bipartite Gaussian states [2], we study symmetrization of multipartite Gaussian states by local Gaussian operations. Namely, we analyze strategies based on addition of correlated noise and on quantum non-demolition interaction. We use fidelity of assisted quantum teleportation [3] as a figure of merit to characterize entanglement of the state before and after the symmetrization procedure. Analyzing the teleportation protocol and considering more general transformations of multipartite Gaussian states, we show that the fidelity can be improved significantly.

References: 

[1] O. Cernotik and J. Fiurasek, Phys. Rev. A 86, 052339 (2012).
[2] J. Fiurasek, Phys. Rev. A 86, 032317 (2012).
[3] P. van Loock and S. L. Braunstein, Phys. Rev. Lett. 84, 3482 (2000).