Experimentally optimizing QKD rates via nonlocal dispersion compensation

Abstract

Quantum key distribution (QKD) enables unconditionally secure communication guaranteed by the laws of physics. The last decades have seen tremendous efforts in making this technology feasible under real-life conditions, with implementations bridging ever longer distances and creating ever higher secure key rates. Readily deployed glass fiber connections are a natural choice for distributing the single photons necessary for QKD both in intra- and intercity links. Any fiber-based implementation however experiences chromatic dispersion which deteriorates temporal detection precision. This ultimately limits maximum distance and achievable key rate of such QKD systems. In this work, we address this limitation to both maximum distance and key rate and present an effective and easy-to-implement method to overcome chromatic dispersion effects. By exploiting entangled photons’ frequency correlations, we make use of nonlocal dispersion compensation to improve the photons’ temporal correlations. Our experiment is the first implementation utilizing the inherently quantum-mechanical effect of nonlocal dispersion compensation for QKD in this way. We experimentally show an increase in key rate from 6.1 to 228.3 bits/s over 6.46 km of telecom fiber. Our approach is extendable to arbitrary fiber lengths and dispersion values, resulting in substantially increased key rates and even enabling QKD in the first place where strong dispersion would otherwise frustrate key extraction at all.

Continuous entanglement distribution over a transnational 248 km fiber link

“…To prohibit this, we deployed a single passive dispersion compensation module (DCM) with an equal and opposite CD of −1.8 ns acting on the photon traveling to St. Pölten only. Such nonlocal dispersion compensation 30 – 32 harnesses the intrinsic quantum properties of entangled photon pairs to narrow their g ( 2) distribution by acting on one photon of a pair only. The residual CD-induced temporal spread after compensation was masked by SNSPD jitter, TTM jitter, and GPS clock drift, which we identify as the remaining contributions to the overall timing uncertainty (see Methods section).…”

“…This can effectively be seen as a decrease in temporal measurement precision, which smears out the correlation function between Alice and Bob, thus increasing the QBER and rendering live tracking impossible in our high-loss setting. In our experiment, we benefit from the fact that entanglement-based QKD allows for non-local dispersion compensation 30 – 32 . This means that the total CD effect of both fiber links can be reduced to zero by use of just one DCM.…”

Deploying an Inter‐European Quantum Network

“…[ 14,15 ] It is worth noticing that fiber communications over thousands of kilometers are possible only thanks to the trusted‐node scenario (quantum states are measured and then subsequently re‐encoded). [ 16–20 ] In this way, it is possible to extend the maximum haul of a point‐to‐point link and to allow connection of multiple users. [ 21 ]…”

“…[14,15] It is worth noticing that fiber communications over thousands of kilometers are possible only thanks to the trusted-node scenario (quantum states are measured and then subsequently re-encoded). [16][17][18][19][20] In this way, it is possible to extend the maximum haul of a point-to-point link and to allow connection of multiple users. [21] However, the long-term goal of a unified quantum network across the entire world is hampered by practical difficulties (i.e., different fiber infrastructures, different telecom operators, etc.)…”

Experimental entanglement generation for quantum key distribution beyond 1 Gbit/s

“…High spectral brightness, i.e. the rate of photon pairs created per wavelength, enables efficient wavelength division multiplexing (WDM) of signals [9,10], diminishes dispersion effects [11] and will be necessary to couple to quantum memories in the future [12]. Collection efficiency is the probability of a photon created in the source being detected.…”