Squeezing (and) phase diffusion: recent theoretical and experimental results
Friday, June 28th 2019, 03:00 PM, Emeleus Lecture Theatre
Speaker: S. Olivares (University of Milan)
The phase of an optical signal represents a building block for quantum enhanced metrology and communication. However, the major obstacle to fully exploit the possible advantages the optical phase is the phase noise due to phase diffusion. Here we consider two scenarios. In the first one , we address binary phase-shift-keyed communication based on Gaussian states and prove that squeezing can enhance optical state discrimination at fixed channel energy. By investigating the ultimate bounds to discrimination (the Helstrom bound), we show that displaced-squeezed states (DDS) can be better discriminated than coherent states (CS) if the squeezing fraction (the ratio between the squeezed photons and the total energy of the signal) is below a given threshold. When phase noise affects the propagation of the signals, the performance of the DDS with a homodyne receiver can beat the Helstrom bound for CS. Furthermore, we find that as the phase noise increases, the protocol exploiting DDS and homodyning becomes nearly-optimal, i.e. approaches the corresponding Helstrom bound. Finally, we consider imperfections in the preparation of the seed signal (the state before the encoding). As one may expect, the presence of a not pure seed state increases the discrimination error probability; nevertheless, we still find that squeezing may improve state discrimination with respect to the protocol based on CS. In the second scenario , we show that, thanks to an active stabilisation scheme of our optical parametric amplifier (OPO) cavity and to a novel technique for pump phase stabilisation, we are able to explore experimentally the use the OPO to counteract phase noise, and to demonstrate reduction of phase diffusion for coherent signals. The high degree of control of the setup allows us not only to seed the quantum amplifier but also to simulate the effect of the phase noise. We theoretically and experimentally show that there is a threshold value on the phase-noise, above which the OPO can be exploited to “squeeze” phase noise. The threshold depends on the energy of the input coherent state, and on the relevant parameters of the OPO, i.e. gain and input/output and crystal loss rates.
 G. Chesi, S. Olivares and M. G. A. Paris, Squeezing-enhanced phase-shift-keyed binary communication in noisy channels, Phys. Rev. A 97, 032315 (2018).
 S. Cialdi, E. Suerra, S. Olivares, S. Capra and M. G. A. Paris, Squeezing phase diffusion, arXiv:1905.13158.
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