Experimental validation of a reversal operator-based QKD polarization control method

The ability to efficiently align the transmitter and receiver of a polarization-based Quantum Key Distribution (QKD) system at initialization and during the exchange of qubits is critical for its correct operation, otherwise resulting in a reduction of the secret key rate. We address this issue by implementing a deterministic polarization compensation method based on the reversal operator of polarization variations. The working principle of this reversal operator is based on Quantum Bit Error Rate (QBER) measurements and their mapping on the Poincaré sphere, enabling fast tracking and compensation of polarization misalignments. Our experimental implementation demonstrates that this method allows for an accurate alignment of the polarization coding and measurement basis using only three QBER measurements. The results confirm that the reversal operator method is exceptionally time-efficient when compared to the widely used compensation methods, such as gradient-based algorithms, requiring at least 5× fewer QBER measurements.