Laboratory Demonstration of Phase-Time Encoding Quantum Key Distribution System with Multimode Delay Interferometer
We report on the adaptation of a commercially available fiber-based quantum key distribution (QKD) system with phase-time encoding to free-space operation. The proposed approach minimizes architectural modifications by replacing the single-mode receiving interferometer with a free-space multi-mode delay interferometer, enabling efficient detection of spatially distorted quantum states typical for atmospheric channels. Several interferometer configurations are experimentally investigated, and a multi-mode Michelson interferometer with polarization routing is identified as the optimal solution in terms of interference visibility, loss, and alignment robustness. The impact of chromatic and modal dispersion on phase-time interference is analyzed and shown to be manageable under realistic system parameters. A laboratory QKD experiment demonstrates stable operation with a quantum bit error rate of about 4.5% and a secret key rate of hundreds of bits per second, confirming the feasibility of extending fiber-based QKD platforms to free-space links.