Deployed quantum key distribution network: further, longer and more users

TL;DR

This work demonstrates a deployed entanglement-based QKD network capable of long-duration, autonomous operation over metropolitan fiber, addressing core challenges in synchronization, phase stabilization, and automation. By leveraging a broadband energy-time entangled photon source and wavelength multiplexing across 18 ITU channels, the system delivers secure key exchange over $50$–$100$ km with no human intervention, including continuous 325-hour operation at 50 km and a 30-hour 100-km link. The approach uses a unified post-processing software that tracks QBERs, timing drift, and losses to dynamically optimize key generation, while stabilizing Z- and X-basis measurements through pump-power control and non-local interferometer phase locking. The results show SKRs reaching $7.069$ kbps at 50 km and up to $175$ bps at 100 km, with potential to support up to $36$ users across $18$ channels, underscoring the practicality of large-scale, autonomous quantum networks without trusted nodes and paving the way for inter-city and satellite-connected quantum links.

Abstract

Entanglement-based quantum links are the backbone of future quantum internet networks, enabling secure communication between distant cities. Realizing such networks requires addressing multiple practical challenges in long-distance quantum key distribution : time synchronisation, interferometer stabilisation and automation. Here, we report several advances. First, we maintained an operational QKD link continuously for 325hours over 50km between two remotes locations, demonstrating the feasibility of long-duration key generation. We further extended secure key distribution up to a 100km operational link connecting the University of Nice to a ground-based optical station, a setup compatible with future quantum satellite connections. Finally, by employing wavelength demultiplexing to separate photons of entangled pairs, we performed QKD across multiple ITU channels, achieving secure key exchange via the BBM92 protocol and time-energy observables.

Figures (12)

• Figure 1: Satellite image of the 4-nodes Côte d’Azur quantum network. For the 100 km demonstration, the entangled photon pair source (EPPS) is located at Inria in Sophia Antipolis while Alice and Bob nodes are located at the University Côte d'Azur campus in downtown Nice, 51.9 km from the source, and at GéoAzur in Caussols (Observatoire de la Côte d’Azur), 48.2 km from the source, respectively. For the 50 km demonstration, the entangled photon pair source (EPPS) is located at INPHYNI in Nice while Alice and Bob nodes are located at the University Côte d'Azur campus in downtown Nice, 19.2 km from the source, and at Inria in Sophia-Antipolis, 32.7 km from the source, respectively.
• Figure 2: Measured Spontaneous Parametric Down-Conversion (SPDC) emission spectrum at the output of the nonlinear crystal, centered at 1560.20 nm. The solid blue curve shows the experimental spectrum, while the colored bands (not to scale) indicate the DWDM channels routing to Alice and Bob. Each user can measure quantum correlations inside channels of similar color.
• Figure 3: Schematic representation of the QKD link. The source (in green) is composed of a 780 nm CW laser, pumping a PPLN crystal, generating energy-time entangled photon pairs via Spontaneous Parametric Down-Conversion. On Alice's (pink) and Bob's (blue) stations, a Dense Wavelength Division Multiplexing (DWDM) allows to select correlated $100$ GHz channels $20$ & $22$, then a 50/50 beam splitter is used to passively select between Z or X basis. The former is composed of two paths, leading to two different detection times to project onto the short ($|\,s\rangle$) and long ($|\,l\rangle$) states, while the latter uses actively stabilized Michelson interferometers to project on $|\,-\rangle$=$\frac{1}{\sqrt{2}}$($|\,s\rangle$-$|\,l\rangle$) and $|\,+\rangle$=$\frac{1}{\sqrt{2}}$($|\,s\rangle$+$|\,l\rangle$) states.
• Figure 4: Diagram of the interconnections between the different post-processing steps (blue) required to secure the keys (purple), to address the experimental issues to handle (red), and to perform the stabilization procedures accordingly (green).
• Figure 5: Time trace of the single photon detection rate at Alice's Z basis detector with polarization stabilization (blue curve) and without polarization stabilization (orange curve). The detection rate for the polarization compensated case is maintained stable for more than 17 h.