Long-range Quantum Cryptography: Amplified Quantum Key Distribution (AQKD)

TL;DR

This paper introduces Amplified Quantum Key Distribution (AQKD), a new protocol that significantly extends the range and rate of quantum key distribution over optical fibers with amplifiers, using a novel post-processing method.

Contribution

The paper presents AQKD, a protocol compatible with optical amplification that achieves higher secret key rates and longer transmission distances than traditional BB84 QKD.

Findings

AQKD provides three times the secret key rate of BB84 at 100km.

AQKD achieves over tenfold gain in key rate at 140km.

AQKD doubles the maximum inter-city fiber transmission distance to 300km.

Abstract

Recently, Takeoka, Guha and Wilde have established a new (loose) upper bound on the secret key capacity of a pure-loss bosonic channel that is several times larger than an error-free BB84 QKD channel with same transmittance. This suggests that new QKD protocols may exist with higher secret key rates and effective ranges larger than the metro-area limits of BB84 QKD over optical fiber. In this paper we illustrate such a possibility with a new protocol called Amplified Quantum Key Distribution (AQKD), which will be compatible with the optical amplification typically encountered on inter-city fiber spans. We analyze one instantiation of AQKD, in which the transmitter enclave contains an optical amplifier (OA), with security against passive optical tapping. For this scenario AQKD is shown to: give three times the secret key rate of BB84 at spans up to 100km, increasing to more than a ten-fold gain at 140km; and double the maximum single-span optical fiber transmission range to inter-city distances (300km). Essential to these performance gains is a new information-theoretic protocol post-processing stage called generalized advantage distillation (GAD). AQKD could be implemented with commercial off-the-shelf (COTS) hardware as an augmentation of present-day BB84 QKD. AQKD avoids the expense, complexity and resource duplication of "trusted relay" approaches to QKD range extension. We expect AQKD to be compatible with fiber spans containing intermediate OAs, which will facilitate its deployment as an overlay on existing fiber networks.