Quantum Cryptography Approaching the Classical Limit

TL;DR

This paper demonstrates that continuous-variable quantum cryptography remains secure even with extremely high preparation noise, showing robustness up to microwave wavelengths, provided channel losses are under 50%.

Contribution

It proves security resilience against large preparation noise and extends quantum cryptography security to microwave wavelengths.

Findings

Security is unaffected by preparation noise up to 10,000 times vacuum variance.

Security persists with channel losses below 50%.

Quantum cryptography is feasible at microwave wavelengths.

Abstract

We consider the security of continuous-variable quantum cryptography as we approach the classical-limit, i.e., when the unknown preparation noise at the sender's station becomes significantly noisy or thermal (even by as much as 10,000 times the variance of the vacuum mode). We show that, provided the channel transmission losses do not exceed 50%, the security of quantum cryptography is not dependent on the channel transmission, and is therefore, incredibly robust against significant amounts of excess preparation noise. We extend these results to consider for the first time quantum cryptography at wavelengths considerably longer than optical and find that regions of security still exist all the way down to the microwave.