Improvement of continuous-variable quantum key distribution systems by using optical preamplifiers

TL;DR

This paper investigates how optical preamplifiers can improve continuous-variable quantum key distribution systems by compensating for detector inefficiencies, demonstrating that optimal amplifiers can restore security and enhance performance.

Contribution

It provides a detailed analysis of the impact of optical preamplifiers on the security and efficiency of CV-QKD systems, including new modified secret key rate formulas.

Findings

Optimal amplifiers fully compensate detector imperfections.

Imperfect amplifiers can still enhance system performance.

Security proofs confirm the viability of using preamplifiers.

Abstract

Continuous-variable quantum key distribution protocols, based on Gaussian modulation of the quadratures of coherent states, have been implemented in recent experiments. A present limitation of such systems is the finite efficiency of the detectors, which can in principle be compensated for by the use of classical optical preamplifiers. Here we study this possibility in detail, by deriving the modified secret key generation rates when an optical parametric amplifier is placed at the output of the quantum channel. After presenting a general set of security proofs, we show that the use of preamplifiers does compensate for all the imperfections of the detectors when the amplifier is optimal in terms of gain and noise. Imperfect amplifiers can also enhance the system performance, under conditions which are generally satisfied in practice.