Communication with Unreliable Entanglement Assistance

TL;DR

This paper introduces a new model for unreliable entanglement assistance in quantum communication, where the system adapts transmission strategies based on the success of entanglement generation, and derives capacity formulas for such scenarios.

Contribution

It develops regularized formulas for classical and quantum capacities with unreliable entanglement assistance, and analyzes optimal coding strategies for noiseless and noisy channels.

Findings

Time division is optimal for noiseless channels.

Unreliable entanglement assistance can still enhance transmission rates.

Suboptimality of time division in noisy channels.

Abstract

Entanglement resources can increase transmission rates substantially. Unfortunately, entanglement is a fragile resource that is quickly degraded by decoherence effects. In order to generate entanglement for optical communication, the transmitter and the receiver first prepare entangled spin-photon pairs locally, and then the photon at the transmitter is sent to the receiver through an optical fiber or free space. Without feedback, the transmitter does not know whether the entangled photon has reached the receiver. The present work introduces a new model of unreliable entanglement assistance, whereby the communication system operates whether entanglement assistance is present or not. While the sender is ignorant, the receiver knows whether the entanglement generation was successful. In the case of a failure, the receiver decodes less information. In this manner, the effective transmission rate is adapted according to the assistance status. Regularized formulas are derived for the classical and quantum capacity regions with unreliable entanglement assistance, characterizing the tradeoff between the unassisted rate and the excess rate that can be obtained from entanglement assistance. It is further established that time division between entanglement-assisted and unassisted coding strategies is optimal for the noiseless qubit channel, but can be strictly suboptimal for a noisy channel.