The capacity of transmitting atomic qubit with light

Stepan N. Balybin, Polina R. Sharapova, Olga V. Tikhonova

В. Н. Горбачев, Andrew I. Trubilko

The interaction between two broadband modes of a parametric source of light and two atoms located at different positions has been considered. As follows from the obtained Bell inequalities for light and atoms, quantum correlations of modes, which are caused by the nonlocality of the state of the source, are completely transmitted into atoms. The system under consideration can be used as a quantum channel where information is encoded in nonorthogonal states by modulating the intensity of a pump wave. The channel capacity has been calculated, which is determined by the Helstrom limit.

Mai-Lin Liang, Bing Yuan, Jianan Zhang

R. Clark Jones

The chief emphasis of the paper is given to the calculation of the information capacity of a beam of light for on-off signal modulation. A preliminary and approximate calculation in Sec. 2 is based on Gaussian modulation. On-off modulation permits an exact calculation of the information capacity; the calculation is based on the Poisson statistics of nondegenerate ensembles of Bose entities.Most of the results are given in terms of the information capacity per transmitted photon, measured in bits per photon, and called the information efficiency It. When there is no ambient light, and when the probability p that the gate is open is equal to one-half, the maximum possible information efficiency is just one bit per transmitted photon; when, however, the probability p of an open gate is less than one-half, the information efficiency It may be higher, with the upper bound log2(1/p).The calculation of the information efficiency in Sec. 3 supposes that no light reaches the receiver except that from the gated source. In Sec. 4 ambient light also is permitted to reach the receiver; extensive numerical results were obtained with a high-speed digital computer and are presented in tables and graphs.The term light is used throughout the paper instead of radiation, in order to emphasize that the results hold only for nondegenerate beams of radiation. The output of laser oscillators is therefore excluded.

G. Gordon, G. Kurizki

Abstract A general quantum noisy channel is analyzed, wherein the transmitted qubits may experience symmetry‐breaking decoherence, along with memory effects. We examine generalized Bell‐basis and find that the optimal basis is not the fully entangled one, but a combination of factorized and partially‐entangled states in the presence of memory, asymmetry and the state‐bias of the noise. Capacity‐maximization is shown to be achievable by combining temporal shaping of the transmitted qubits and optimal basis selection.

Norbert Lütkenhaus, Hauke Häseler, Nathan Killoran