Novel High-Speed Polarization Source for Decoy-State BB84 Quantum Key Distribution over Free Space and Satellite Links

TL;DR

This paper introduces a high-speed, stable polarization source for decoy-state BB84 quantum key distribution over free space and satellite links, enabling long-distance secure communication with high repetition rates.

Contribution

The authors develop a novel telecom optical polarization and amplitude modulator based on a Mach-Zehnder interferometer and sum-frequency generation, achieving high clock rate and stability for QKD.

Findings

Achieved 76 MHz pulse repetition rate with phase randomization.

Demonstrated system stability for up to 160 minutes.

Confirmed high loss channel performance with 57 dB loss simulations.

Abstract

To implement the BB84 decoy-state quantum key distribution (QKD) protocol over a lossy ground-satellite quantum uplink requires a source that has high repetition rate of short laser pulses, long term stability, and no phase correlations between pulses. We present a new type of telecom optical polarization and amplitude modulator, based on a balanced Mach-Zehnder interferometer configuration, coupled to a polarization-preserving sum-frequency generation (SFG) optical setup, generating 532 nm photons with modulated polarization and amplitude states. The weak coherent pulses produced by SFG meet the challenging requirements for long range QKD, featuring a high clock rate of 76 MHz, pico-second pulse width, phase randomization, and 98% polarization visibility for all states. Successful QKD has been demonstrated using this apparatus with full system stability up to 160 minutes and channel losses as high 57 dB [Phys. Rev. A, Vol. 84, p.062326]. We present the design and simulation of the hardware through the Mueller matrix and Stokes vector relations, together with an experimental implementation working in the telecom wavelength band. We show the utility of the complete system by performing high loss QKD simulations, and confirm that our modulator fulfills the expected performance.