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Physical Review Applied

Polarization-correction device with liquid crystals for quantum-key-distribution satellite systems

A. Jimenez-Girela, D. Merino-Pérez, A. Campos-Jara, J. Socas Negrín, P. García Parejo, A. Álvarez‐Herrero

Published May 21, 2025• 6 citations
Publisher

Abstract

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.

DOI
10.1103/8plr-m6n8

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•Jan 1, 2021•0 citations
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.
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.
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.
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.
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.
Real global-scale quantum communications and quantum key distribution systems cannot be implemented by the current fiber and free-space links. These links have high attenuation, low polarization-preserving capability or extreme sensitivity to the environment. A potential solution to the problem is the space-earth quantum channels. These channels have no absorption since the signal states are propagated in empty space, however a small fraction of these channels is in the atmosphere, which causes slight depolarizing effect. Furthermore, the relative motion of the ground station and the satellite causes a rotation in the polarization of the quantum states. In the current approaches to compensate for these types of polarization errors, high computational costs and extra physical apparatuses are required. Here we introduce a novel approach which breaks with the traditional views of currently developed quantum-error correction schemes. The proposed solution can be applied to fix the polarization errors which are critical in space-earth quantum communication systems. The channel coding scheme provides capacity-achieving communication over slightly depolarizing space-earth channels.
Quantum key distribution (QKD) is a secure key-transmitting method based on quantum mechanics to deliver private keys for encryption. Satellite QKD is a promising technique that overcomes the limited transmission distance in optical-fibre-based systems. To enable polarisation encoded QKD, polarisation reference-frame mismatches between the satellite and a receiving optical ground station (OGS) have to be compensated. We developed and implemented a compensation method for a BB84 protocol designed QKD receiver by an additional set of three waveplates (HWP, QWP, QWP). In simulations and laboratory demonstrations, we show the robustness under conditions that we expect on our future satellite downlink mission. As a result, we expect good compensation quality for a wide range of signal to noise ratios and satellite rotation frequencies by requiring only a few signal photons of 2000 of two non-orthogonal polarisation states, respectively, to measure the reference-frame mismatch used for one compensation sequence.