Classical and Quantum Factors of Channels.

Charles H. Bennett, Peter W. Shor, John A. Smolin +1 more

Prior entanglement between sender and receiver, which exactly doubles the classical capacity of a noiseless quantum channel, can increase the classical capacity of some noisy quantum channels by an arbitrarily large constant factor depending on the channel, relative to the best known classical capacity achievable without entanglement. The enhancement factor is greatest for very noisy channels, with positive classical capacity but zero quantum capacity. We obtain exact expressions for the entanglement-assisted capacity of depolarizing and erasure channels in $d$ dimensions.

Peter P. Rohde

Sarah Brandsen, Avijit Mandal, Henry D. Pfister

Belief propagation (BP) is a classical algorithm that approximates the marginal distribution associated with a factor graph by passing messages between adjacent nodes in the graph. It gained popularity in the 1990’s as a powerful decoding algorithm for LDPC codes. In 2016, Renes introduced a belief propagation with quantum messages (BPQM) and described how it could be used to decode classical codes defined by tree factor graphs that are sent over the classical-quantum pure-state channel. In this work, we propose an extension of BPQM to general binary-input symmetric classical-quantum (BSCQ) channels based on the implementation of a symmetric "paired measurement". While this new paired-measurement BPQM (PMBPQM) approach is suboptimal in general, it provides a concrete BPQM decoder that can be implemented with local operations. Finally, we demonstrate that density evolution can be used to analyze the performance of PMBPQM on tree factor graphs. As an application, we compute noise thresholds of some LDPC codes with BPQM decoding for a class of BSCQ channels.

Ákos Uzonyi, Máté Galambos

We report on the successful combination of a quantum channel operating at 810 nm and a classical channel at 1310 nm in a free-space laboratory experiment. The use of commercially available small form-factor pluggable (SFP) transceivers enabled the creation of the classical channel. Rigorous filtering ensured that the classical channel had no impact on the quantum channel, even when the classical channel was active during the alignment and calibration phases. Entanglement distribution was verified using the Clauser-Horne-Shimony-Holt (CHSH) inequality, and the measured value of |S|=2.58 remained unaffected regardless of whether the classical channel was active. These results demonstrate the feasibility of integrating quantum and classical communication channels in free space, paving the way for practical hybrid quantum-classical networks.