Utility of noiseless linear amplification and attenuation in single-rail discrete-variable quantum communications

TL;DR

This paper explores how noiseless linear amplification and attenuation, along with optimized measurements, can significantly improve quantum communication protocols like teleportation and superdense coding under channel losses.

Contribution

It demonstrates that simple, experimentally feasible operations like NA and NLA, combined with optimized measurements, can substantially enhance quantum communication performance.

Findings

NLA/NA and optimized POVMs increase teleportation fidelity by up to 78%.

They can double the quantum advantage in superdense coding in certain regimes.

Optimal POVMs effectively reduce to NA or NLA, simplifying implementation.

Abstract

Quantum communication offers many applications, with teleportation and superdense coding being two of the most fundamental. In these protocols, pre-shared entanglement enables either the faithful transfer of quantum states or the transmission of more information than is possible classically. However, channel losses degrade the shared states, reducing teleportation fidelity and the information advantage in superdense coding. Here, we investigate how to mitigate these effects by optimising the measurements applied by the communicating parties. We formulate the problem as an optimisation over general positive operator-valued measurements (POVMs) and compare the results with physically realisable noiseless attenuation (NA) and noiseless linear amplification (NLA) circuits. For teleportation, NLA/NA and optimised POVMs improve the average fidelity by up to 78% while maintaining feasible success probabilities. For superdense coding, they enhance the quantum advantage over the classical channel capacity by more than 100% in some regimes and shift the break-even point, thereby extending the tolerable range of losses. Notably, the optimal POVMs effectively reduce to NA or NLA, showing that simple, experimentally accessible operations already capture the essential performance gains.