Characterisation of Temporal and Spatial Phase in Free-Space Phase-Based Quantum Key Distribution

Quantum key distribution (QKD) has been demonstrated in free space over long horizontal and vertical channels using the polarisation of light as an encoding degree of freedom. The optical phase of light, however, promises better performance for QKD than polarisation, due to the recent introduction of a new phase-based protocol called ‘twin-field’ (TF) QKD. The implementation of TF-QKD on a free-space channel requires precise temporal and spatial phase stabilisation, which has not yet been simultaneously tested for this protocol. We experimentally performed this test using a hybrid interferometer with free-space and fibre-based arms. Our results demonstrate real-time phase stabilisation of the system, enabling the application of TF-QKD over hybrid quantum networks that include both free-space and fibre-based channels.