Quantum key distribution over a metropolitan network using an integrated photonics based prototype

TL;DR

This work addresses the practical deployment of quantum key distribution by developing a PIC-based, time-bin BB84 QKD system with decoy states, packaged in standard 19-inch racks for metropolitan networks. The transmitter and receiver are integrated on photonic chips, with a dispersion-tolerant design that eliminates the need for dispersion compensating fibers at distances below 100 km, and operates at $1.25\ \mathrm{GHz}$. Field demonstration in Geneva shows stable key exchange over a metropolitan link with multi-day continuity, and extended measurements reaching $105.4\ \mathrm{km}$ when using Stirling cooling to suppress detector dark counts, highlighting the role of dispersion and detector noise in limiting distance. Overall, the study demonstrates the maturity and practicality of chip-based QKD for real-world telecom integration, emphasizing manufacturability, scalability, and autonomous operation.

Abstract

An industrial-scale adoption of Quantum Key Distribution (QKD) requires the development of practical, stable, resilient and cost-effective hardware that can be manufactured at large scales. In this work we present a high-speed (1.25GHz), field-deployable QKD prototype based on integrated photonics, that is consolidated into standard 19-inch rack compatible units. Through integrated photonics, the system prioritizes autonomous long-term stability in metropolitan settings. The architecture is further simplified by removing the need for chromatic dispersion compensation over metropolitan distances (below 100km). We demonstrate continuous key exchange over more than 4 km of metropolitan optical fiber, where the prototype maintained stable, uninterrupted operation across a measurement spanning more than 12 day-night cycles without manual intervention.

Figures (5)

• Figure 1: Schematic of the QKD system between a transmitter, Alice, and a receiver, Bob. Optically, Alice's setup includes a laser source, a polarization controller and a photonic integrated chip (PIC). The quantum channel (QC) transmits the quantum states to Bob that consists of a PIC and single-photon detectors (Z and X-basis measurements). Both parties are supported by printed circuit boards (PCB), personal computers (PC) and Field-Programmable Gate Arrays (FPGA) for control and processing, and utilize a service channel (SC) for clock synchronization basis reconciliation, error correction, and privacy amplification
• Figure 2: Pictures and schematics of the integrated transmitter. a) Schematic of the photonic integrated circuit of Alice showing the optical signal path. HT – Heaters; EOPS – electro-optic phase shifters; MZI – Mach-Zehnder interferometer; IM – intensity modulator; AA – absorption attenuator; PD – photodetectors. b) 90-degree fiber array used for optical coupling with PIC. c) PIC interposer PCB glued to SPI interface PCB.
• Figure 3: a) Simulated pulse broadening due to chromatic dispersion effects, based on measured laser parameters [except the calculated Fourier-limited scenario]. The 400 ps horizontal line indicates the time-bin width. The labels () indicate the function used for each curve. The frequency linewidth was measured using the same method as the ring filter in \ref{['section:integrated_transmitter']}. b) Time-resolved intensity profile of the signal pulses measured with a SNSPD (50 jitter) and a TDC (ID100 in high-resolution mode (1 resolution))
• Figure 4: Schematic of the receiver's photonic integrated circuit, showing the optical signal path. HT - heater, OUT X$_n$ - output connected to X basis detector, OUT Z - output connected to Z basis detector. 40/60 and 50/50 refer to the splitting ratio of the interferometer's input and output coupler respectively.
• Figure 5: Stability of the SKR and QBER$_Z$ over a 282h (around 12 days) measurement of a point-to-point key exchange in the deployed fiber link.