Signal detection in multiaccess quantum channels

ARMEN E. ALLAHVERDYAN, D. B. Saakyan

Julio I. Concha, H. Vincent Poor

Shared communication channels are subject to multiple-access interference. Transmitter and receiver design techniques that explicitly deal with this interference have been shown to improve substantially the performance of communication systems over radio-frequency and other "classical" channels. Quantum multiple-access communication channels, on the other hand, have received comparatively little attention. In this paper, an input-output model for multiple-access quantum channels relevant to optical communications is proposed. The model accounts for multiaccess interference, signal attenuation, and random noise, and can be used in the analysis and design of communication systems. Using a result from optimization, a perturbation method is developed to find the minimum achievable error probability in small-interference channels. It is shown that the quantum measurement that minimizes the error probability in a no-interference channel is robust in the presence of small multiaccess interference. The results are illustrated with numerical examples, which show that optimal quantum detectors can significantly outperform conventional detectors even for moderate levels of crosstalk.

Łukasz Czekaj, Paweł Horodecki

We are studying classical capacities of quantum memoryless multiaccess channels in geometric terms and we are revealing a break of additivity of the Holevo-like capacity. This effect is a purely quantum mechanical one, since, as we point out, the capacity regions of all classical memoryless multiaccess channels are additive. It is the first such effect revealed in the field of classical information transmission via quantum channels.

ARMEN E. ALLAHVERDYAN, David B. Saakian

Ivan B. Djordjević

Aditi Sen, Ujjwal Sen

For quantum states of two subsystems, highly entangled states have a higher capacity of transmitting classical as well as quantum information, and vice versa. We show that this is no more the case in general: Quantum capacities of multiaccess channels, motivated by communication in quantum networks, do not have any relation with genuine multiparty entanglement measures. Importantly, the statement is demonstrated for arbitrary multipartite entanglement measures. Along with revealing the structural richness of multiaccess channels, this gives us a tool to classify multiparty quantum states from the perspective of its usefulness in quantum networks, which cannot be visualized by any genuine multiparty entanglement measure.