IEEE International Conference on Space Optical Systems and Applications (ICSOS)
Preliminary results of a space-qualified polarization tracking system for QKD space systems using a genetic algorithm
Angel Jimenez-Girela, Daniel Merino-Perez, P. García Parejo, A. Crespo-Ramos, E. Gonzalez Del Campo, Delia Rodríguez de Llera González, A. Álvarez‐Herrero
We present a closed-loop polarization tracking system (PTS) designed to compensate polarization perturbations in real time for satellite-based QKD links. The system employs a liquid crystal polarization control device (PCD) and a genetic algorithm that optimizes the driving voltages to minimize the detected intensity, thereby restoring the input polarization state. Experimental validation under static and dynamic perturbations demonstrates the capability of the PTS to maintain high polarization extinction ratios and low polarization-induced QBER. These results confirm the feasibility of implementing closed-loop, real-time polarization correction in dynamic free-space quantum communication scenarios.
We present a closed-loop polarization tracking system (PTS) designed to compensate polarization perturbations in real time for satellite-based QKD links. The system employs a liquid crystal polarization control device (PCD) and a genetic algorithm that optimizes the driving voltages to minimize the detected intensity, thereby restoring the input polarization state. Experimental validation under static and dynamic perturbations demonstrates the capability of the PTS to maintain high polarization extinction ratios and low polarization-induced QBER. These results confirm the feasibility of implementing closed-loop, real-time polarization correction in dynamic free-space quantum communication scenarios.
In polarization-encoded satellite-to-ground quantum communication, the on-board transmitter and a receiver at the optical ground station encode and decode linearly polarized quantum bits for execution of quantum key distribution (QKD) protocol. Dynamic polarization tracking and compensation is a key technique in realizing such a space based QKD system. In this work, we report the design and development of motorized waveplate based active polarization compensation system. Integrated testing and validation with Decoy-state enabled BB84 QKD testbed over ~5m free space channel and quantum link polarization extinction ratio (PER) ≥ 100:1 has been carried out successfully. Satellite motion was emulated as a continuous 1°/s polarization basis rotation at the Quantum Tx output. When active polarization compensation system was enabled, sazimuth (Az.) deviation was measured with ±1.5° accuracy. During this experiment, overall signal QBER was consistently maintained at <3.5% with secure key generation rate of ~1350kbps, proving the suitability of this scheme for realistic satellite-to-ground QKD scenarios.
In this paper, we present a device, designed to be used in a space payload, capable of correcting any arbitrary polarization state. The device consists of a set of three liquid-crystal variable-retarder (LCVR) cells, in which by selecting the appropriate values of the voltage to be applied in each of them, it can correct the undesirable effects. In particular, the degradation effects of the polarization states, such as ellipticity and rotation with respect to the reference system, are compensated. Among many other potential applications, this device has been conceived to be used in quantum-key-distribution (QKD) intersatellite link or satellite-ground polarization-coding protocols as a polarization-correction device (PCD). To demonstrate that these effects are corrected, we have carried out an experiment in which we have tested how the BB84 protocol states are compensated after being strongly perturbed, achieving polarization extinction ratio (PER) values around 39 dB. Our solution offers the advantage of being small and mechanism free, being the ideal solution to be implemented in small satellites. These satellites are the core to be used in future constellations required for a global quantum communications network.
Polarization instability is a critical challenge for polarization-entangled satellite quantum key distribution (QKD), where atmospheric effects and platform motion continuously distort photon polarization. To maintain entanglement fidelity, these transformations must be accurately identified and compensated prior to detection. In this work, a compact and fast polarization-compensation approach based on liquid-crystal (LC) variable retarders is presented, using a co-propagating classical reference signal (beacon) for real-time polarization tracking. An LC-based polarimeter is implemented, and its performance is evaluated using both direct and Fourier-based Stokes parameter reconstruction. Experimental results indicate that accurate polarization estimation can be achieved with a limited number of measurements, enabling a favorable trade-off between speed and precision. The impact of liquid-crystal switching dynamics is also analyzed, highlighting the importance of selecting appropriate operating conditions for real-time applications. In addition, the effect of polarimetric inaccuracies on QKD performance is assessed through simulations of an entanglement-based protocol. The results show that only a moderate increase in quantum-bit error rate is introduced, while remaining compatible with secure key distribution. These findings demonstrate that LC-based polarization control represents an efficient and practical solution for real-time compensation in satellite QKD systems.
Abstract Polarization drift in fiber and free-space optical links is a major factor in the dynamic increase of bit error rate in polarization-coded quantum key distribution (QKD) systems. A dynamic polarization compensation method applicable to both links is a challenge. Here we propose a universally applicable real-time polarization compensation method, that the Muller parameters of the optical links are first detected using a polarization detector, and then the optimal parameters of the controller are obtained by gradient descent algorithm. Simulation results indicate advantages over current methods, with fewer waveplates, faster speed, and wider applicability for various optical links. In equivalent experiments of both satellite and fiber optical links, the average polarization extinction ratio of 27.9 dB and 32.2 dB are respectively achieved. The successful implementation of our method will contribute to the real-time polarization design of fiber and free-space QKD systems, while also contributing to the design of laser-based polarization systems.
Polarization instability is a critical challenge for polarization-entangled satellite quantum key distribution (QKD), where atmospheric effects and platform motion continuously distort photon polarization. To maintain entanglement fidelity, these transformations must be accurately identified and compensated prior to detection. In this work, a compact and fast polarization-compensation approach based on liquid-crystal (LC) variable retarders is presented, using a co-propagating classical reference signal (beacon) for real-time polarization tracking. An LC-based polarimeter is implemented, and its performance is evaluated using both direct and Fourier-based Stokes parameter reconstruction. Experimental results indicate that accurate polarization estimation can be achieved with a limited number of measurements, enabling a favorable trade-off between speed and precision. The impact of liquid-crystal switching dynamics is also analyzed, highlighting the importance of selecting appropriate operating conditions for real-time applications. In addition, the effect of polarimetric inaccuracies on QKD performance is assessed through simulations of an entanglement-based protocol. The results show that only a moderate increase in quantum-bit error rate is introduced, while remaining compatible with secure key distribution. These findings demonstrate that LC-based polarization control represents an efficient and practical solution for real-time compensation in satellite QKD systems.
Satellite networks and quantum communications offer complementary opportunities for enhanced operations. Quantum communications provide security for the transmissions between satellites and ground stations; while the free-space link of satellite networks provide the potential of long distance transmission of quantum bits (qubit) for space communications. However, with the promising advantages of the two approaches, challenges remain to fully develop quantum-based satellite communications such as robust and reliable information detection which is difficult to achieve due to the movement of satellites. In this paper, a tracking algorithm is proposed for polarization-encoded quantum satellite communications where polarization states are used to determine the bit transfer between the transmitter and receiver. The polarization tracking is essential for the decoding of a qubit and the quantum key distribution (QKD). A practical channel model for free-space quantum communications is adopted in this paper. With the estimated polarization, a novel dynamic polarization compensation scheme is also proposed. The results show that our methods can accurately estimate the polarization, providing much lower quantum bit error rate (QBER) by compensation, as compared with the direct qubit detection without polarization tracking and compensation scheme.