Quantum key distribution over a 2 km free-space channel with a high secure key rate

As free-space quantum key distribution (QKD) becomes a critical technology for mobile platforms like drones and satellites, it faces significant challenges from atmospheric turbulence. This study investigates whether an active, high-speed beam-stabilization system can mitigate turbulence-induced beam wander and improve the secure key rate in a practical 2 km outdoor free-space link.

The researchers developed a decoy-state BB84 QKD system operating at a 100 MHz repetition rate. The transmitter uses eight synchronized semiconductor lasers to generate signal and decoy pulses. To overcome atmospheric turbulence, the receiver incorporates an active stabilization framework consisting of fast-steering mirrors (FSMs) and position-sensitive detectors (PSDs). This system monitors the beam position and provides real-time feedback to correct for rapid beam wander, with a closed-loop control bandwidth exceeding 500 Hz. The team also implemented a hybrid error-correction protocol using Winnow and LDPC codes to process the raw data into a secure key.

• The active stabilization system significantly improved performance, increasing the average secure key rate from 103 kbps to 164.8 kbps.
• The stabilization loop reduced the standard deviation of the key generation rate by approximately 50%, indicating much higher link reliability.
• The system maintained a quantum bit error rate (QBER) of approximately 3.3% during the 2 km transmission.
• Numerical simulations based on the experimental data suggest that this architecture could scale to support secure key distribution over distances up to 30 km under clear-air conditions.

This work provides a practical, scalable framework for deploying high-rate quantum communication in realistic, turbulent environments. By demonstrating that active beam tracking is essential for maintaining stable coupling efficiency, the study offers a viable path for integrating mobile quantum nodes—such as vehicles or aircraft—into future global quantum networks.