期刊
QUANTUM
卷 5, 期 -, 页码 -出版社
VEREIN FORDERUNG OPEN ACCESS PUBLIZIERENS QUANTENWISSENSCHAF
DOI: 10.22331/q-2021-12-23-608
关键词
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资金
- MIUR via PRIN [2017SRNBRK]
- Spanish MINECO [FIS2016-80681-P]
- AEI/FEDER funds
- Generalitat de Catalunya [CIRIT 2017-SGR-1127]
- European Union [845255]
- Baidu-UAB collaborative project Learning of Quantum Hidden Markov Models
- Marie Curie Actions (MSCA) [845255] Funding Source: Marie Curie Actions (MSCA)
The article explores the transmission of classical information using quantum Gaussian states on phase-noise channels with finite decoherence time, emphasizing the advantages of photon-number encodings and squeezed-coherent encodings, as well as the sub-optimality of using part of the energy to establish a reference frame. The results demonstrate the superiority of non-classical encoding methods in a communication setting.
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. The Holevo capacity of these channels is always attained with photon-number encodings, challenging with current technology. Hence for coherent-state encodings the optimal rate depends only on the total-energy distribution and we provide upper and lower bounds for all m, the latter attainable at low energies with on/off modulation and photodetection. We generalize this lower bound to squeezed-coherent encodings, exhibiting for the first time to our knowledge an unconditional advantage with respect to any coherent encoding form 1 and a considerable advantage with respect to its direct coherent counterpart 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 use of part of the energy to establish a reference frame is sub-optimal even at large energies. Our results represent a key departure from the case of phase-covariant Gaussian channels and constitute a proof-of-principle of the advantages of using non-classical, squeezed light, in a motivated communication setting.
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