R. Mark Stevenson

Twin-field quantum key distribution (TF-QKD) has become a leading protocol to bring quantum communications to the national scale. The protocol requires the establishment of a shared phase and frequency reference between distant parties, which is commonly achieved by using an external reference laser in an optical injection locking (OIL) architecture. In this work, we analyze the side channels in OIL-based TF-QKD that may arise from adversarial manipulation of the various degrees of freedom of this untrusted reference beam. We experimentally demonstrate two realistic attack scenarios: fast intensity modulation of the reference laser, and additional signals embedded in the reference light exploiting wavelengths undetectable by conventional monitoring techniques. These attacks can allow a potential eavesdropper to deterministically increase the mean photon number of the sources, or circumvent the decoy-state technique, respectively. To counter these vulnerabilities, we propose practical and highly effective countermeasures that reinforce the security of TF-QKD systems without significant additional complexity or performance degradation.

We investigate the feasibility of satellite-to-ground quantum key distribution (QKD) at multi-GHz clock rates, where secure key generation time is constrained, due to LEO satellite overpasses (≈ 300 s). Higher repetition rates present an immediate route to increased secure key rate (SKR), though performance is limited by detector jitter, coupling losses, and atmospheric turbulence, depending on the receiver architecture. We combine finite-key modelling, detailed detector timing characterization, adaptive optics (AO) modelling, and real-time free-space QKD experiments at 1 GHz to evaluate practical receiver configurations based on multi-mode fiber-coupled avalanche photodiodes (MMF-APDs) and single-mode fiber-coupled superconducting nano-wire single-photon detectors (SMF-SNSPDs), at projected rates > 1 GHz. The results indicate clear operating regimes: MMF-APDs maximize secure key rates below ∼ 1 GHz, predominantly due to their low coupling loss. However, low-jitter SMF-SNSPDs are required to obtain positive key rates at > 1GHz clock rates (estimated optimal operating point near ∼ 7.55 GHz), though AO correction is required for effective single-mode coupling under turbulence. The experimental and projected results provide realistic design targets for future high-rate satellite QKD systems, while highlighting the technological requirements needed to achieve end-to-end operation beyond the GHz regime.

We report a novel technique of extracting randomness from the phase of optical pulses coming from a gain-switched Distributed Feedback (DFB) laser cavity beyond the phase diffusion limit, with a demonstration at 10 GHz.