SAMOP 2021 – scientific programme

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QI: Fachverband Quanteninformation

QI 11: Quantum Communication

QI 11.5: Talk

Thursday, September 23, 2021, 15:30–15:45, H4

Squeezing-enhanced communication without a phase reference — Marco Fanizza¹, •Matteo Rosati², Michalis Skotiniotis², John Calsamiglia², and Vittorio Giovannetti¹ — ¹NEST, Scuola Normale Superiore and Istituto Nanoscienze-CNR, I-56126 Pisa, Italy — ²Física Teòrica: Informació i Fenòmens Quàntics, Departament de Física, Universitat Autònoma de Barcelona, 08193 Bellaterra (Barcelona) Spain

We study the problem of transmitting classical information using quantum Gaussian states on a family of phase-noise channels with a finite decoherence time, such that the phase-reference is lost after m consecutive uses of the transmission line. This problem is relevant for long-distance communication in free space and optical fiber, where phase noise is typically considered as a limiting factor. We show that the optimal Gaussian encoding is generated by a Haar-random passive interferometer acting on pure product states. We upper- and lower-bound the optimal coherent-state rate and exhibit a lower bound to the squeezed-coherent rate that, for the first time to our knowledge, surpasses any coherent encoding for m=1 and provides a considerable advantage with respect to the coherent-state lower bound for m>1. This advantage is robust with respect to moderate attenuation, and persists in a regime where Fock encodings with up to two-photon states are also suboptimal. Finally, we show that the advantage carries over to the private capacity of the channel and that the use of part of the energy to establish a reference frame is sub-optimal even at large energies.