Operational entanglement-based quantum key distribution over 50 km of real-field optical fibres

TL;DR

This paper tackles the challenge of implementing a practical, entanglement-based QKD link in a real metropolitan telecom network. It demonstrates a fully automated system using energy-time entangled photons across 50 km of deployed fibers, with clock synchronization embedded in the quantum signal and real-time post-processing. The network achieves a raw key rate of 40 kbps per ITU channel pair and a final secret key rate of 6.5 kbps per pair, with QBER under 7%, over extended uninterrupted operation and support for multiplexing up to 40 channels. This work marks the first fully operational entanglement-based metropolitan QKD network in real field, highlighting a scalable path to practical quantum-secure communications.

Abstract

We present a real field quantum key distribution link based on energy-time entanglement. Three nodes are connected over the city of Nice by means of optical fibers with a total distance of 50\,km. We have implemented a high-quality source of energy-time entangled photon pairs and actively stabilized analysers to project the quantum states, associated with an innovative remote synchronization method of the end stations' clocks which does not require any dedicated channel. The system is compatible with the ITU 100\,GHz standard telecom-grid, through which a raw key rate of 40\,kbps per pair of channels is obtained. A post-treatment software performs all the necessary post-processing procedures enabling to establish secret keys in real time. All of those embedded systems and achieved performance make this network the first fully operational entanglement based metropolitan quantum network to be implemented in real field.

Figures (4)

• Figure 1: Experimental setup. The source, located at the central station, creates energy time bipartite entanglement shared between Alice and Bob (end stations) using DWDM (ITU channels 20 and 22). Each analyser (Alice and Bob) sends randomly the photons in either the Z or the X basis using a $50/50$ beam splitter. The X basis is set up using two, stabilized Michelson interferometers with the same delay, one at Alice's station, the other at Bob's. The Z basis consists of a $50/50$ beam-splitter on Bob's side with a short and a long path to the detectors while Alice only has one detector, short and long paths being created electronically. Everything is fibered, with standard SMF-28 fibers for all components except for the connection between the pump laser and the PPLN waveguide ensured by a polarization maintaining fiber.
• Figure 2: Spectrum of the photon pairs generated from the nonlinear crystal inside of the photon pair source.
• Figure 3: Simulated SKR (blue) and QBERz (red) as a function of $\mu$ : the number of photon pairs outputing from the source per time wondow ($128\,$ps).
• Figure 4: Secret Key Rate (top) and QBER in both basis (bottom) as a function of time for a 30h uninterrupted run.