4.5 Article

Capacities of Gaussian Quantum Channels With Passive Environment Assistance

Journal

IEEE TRANSACTIONS ON INFORMATION THEORY
Volume 68, Issue 1, Pages 339-358

Publisher

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
DOI: 10.1109/TIT.2021.3122150

Keywords

Hilbert space; Entropy; Quantum communication; Quantum channels; Receivers; Encoding; Eigenvalues and eigenfunctions; Quantum Gaussian channels; quantum Gaussian capacity; super-activation

Funding

  1. Horizon-2020 Programme of the European Commission through the Future and Emerging Technologies (FET)-Open Grant Agreement QUAntum Readout TEchniques and Technologies (QUARTET) [862644]
  2. Spanish Ministerio de Economia y Competitividad (MINECO) [FIS2016-86681-P, PID2019-107609GB-I00/AEI/10.13039/501100011033]
  3. Fondo Europeo de Desarrollo Regional (FEDER) Funds
  4. Generalitat de Catalunya [2017-SGR-1127]

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Passive environment-assisted communication is investigated in terms of information transmission capabilities. Gaussian unitaries acting on Bosonic systems are considered for both quantum and classical communication. Coding theorems are proved, and an uncertainty-type relation between the classical capacities of the sender and the helper is derived, providing lower bounds on the classical information transmission rate.
Passive environment-assisted communication takes place via a quantum channel modeled as a unitary interaction between the information carrying system and an environment, where the latter is controlled by a passive helper, who can set its initial state such as to assist sender and receiver, but not help actively by adjusting her behaviour depending on the message. Here we investigate the information transmission capabilities in this framework by considering Gaussian unitaries acting on Bosonic systems. We consider both quantum communication and classical communication with helper, as well as classical communication with free classical coordination between sender and helper (conferencing encoders). Concerning quantum communication, we prove general coding theorems with and without energy constraints, yielding multi-letter (regularized) expressions. In the search for cases where the capacity formula is computable, we look for Gaussian unitaries that are universally degradable or anti-degradable. However, we show that no Gaussian unitary yields either a degradable or anti-degradable channel for all environment states. On the other hand, restricting to Gaussian environment states, results in universally degradable unitaries, for which we thus can give single-letter quantum capacity formulas. Concerning classical communication, we prove a general coding theorem for the classical capacity under an energy constraint, given by a multi-letter expression. Furthermore, we derive an uncertainty-type relation between the classical capacities of the sender and the helper, helped respectively by the other party, showing a lower bound on the sum of the two capacities. Then, this is used to lower bound the classical information transmission rate in the scenario of classical communication between sender and helper.

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