Studying Free-Space Transmission Statistics and Improving Free-Space QKD in the Turbulent Atmosphere

TL;DR

This paper analyzes atmospheric turbulence effects on free-space quantum key distribution (QKD), demonstrating how turbulence statistics can be used to enhance key rates, especially in satellite-based QKD, by employing a signal-to-noise ratio filter.

Contribution

It provides a detailed analysis of turbulence-induced fluctuations in free-space QKD and introduces a novel SNR filter technique to improve secure key generation, including satellite scenarios.

Findings

Turbulence has minimal impact on decoy state QKD security in typical regimes.

Using SNR filtering increased secret key rate by 25.2%.

Simulations show SNRF's potential in satellite QKD to enable more key exchanges.

Abstract

The statistical fluctuations in free-space links in the turbulent atmosphere are important for the distribution of quantum signals. To that end, we first study statistics generated by the turbulent atmosphere in an entanglement based free-space quantum key distribution (QKD) system. Using the insights gained from this analysis, we study the effect of link fluctuations on the security and key generation rate of decoy state QKD concluding that it has minimal effect in the typical operating regimes. We then investigate the novel idea of using these turbulent fluctuations to our advantage in QKD experiments. We implement a signal-to-noise ratio filter (SNRF) in our QKD system which rejects measurements during periods of low transmission efficiency, where the measured quantum bit error rate (QBER) is temporarily elevated. Using this, we increase the total secret key generated by the system from 78,009 bits to 97,678 bits, representing an increase of 25.2% in the final secure key rate, generated from the same raw signals. Lastly, we present simulations of a QKD exchange with an orbiting LEO satellite and show that an SNRF will be extremely useful in such a situation, allowing many more passes to extract a secret key than would otherwise be possible.