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arXiv:2411.08358 · EPJ Quantum Technology

10 GHz robust polarization modulation towards high-speed satellite-based quantum communication

Ze-Xu Wang, Huaxing Xu, Ju Li, Huicun Yu, Jinquan Huang, Hui Han, Chang-Lei Wang, Ping Zhang, Feifei Yin, Kun Xu, Bo Liu, Yitang Dai

Published Apr 11, 2025• 11 citations
arXivHTMLPDF

Abstract

In practical satellite-based quantum key distribution (QKD) systems, the preparation and transmission of polarization-encoding photons suffer from complex environmental effects and high channel loss. Consequently, the hinge to enhancing the secure key rate (SKR) lies in achieving robust, low-error, and high-speed polarization modulation. Although the schemes that enable self-compensation demonstrate remarkable robustness, their modulation speed is limited to around 2 GHz to prevent the interaction between the electrical signal and the reverse optical pulses. Here, we utilize the non-reciprocity of the lithium niobate modulators and eliminate the modulation on the reverse optical pulses. This characteristic is widely available in the radio-frequency band, allowing the modulation speed to no longer be limited by the self-compensating optics and enabling further increases. The measured average intrinsic quantum bit error rate of the four polarization states at 10 GHz system repetition frequency is as low as 0.53% over 10 min without any compensation. The simulation results show that our scheme can maintain a SKR of about 5 kbps even at the extreme communication distances between the satellite and the earth. Our work can be efficiently applied in high-speed, high-loss satellite-based quantum communication scenarios.

DOI
10.1140/epjqt/s40507-025-00349-y

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In practical satellite-based quantum key distribution (QKD) systems, the preparation and transmission of polarization-encoding photons suffer from complex environmental effects and high channel loss. Consequently, the hinge to enhancing the secure key rate (SKR) lies in achieving robust, low-error, and high-speed polarization modulation. Although the schemes that enable self-compensation demonstrate remarkable robustness, their modulation speed is limited to around 2 GHz to prevent the interaction between the electrical signal and the reverse optical pulses. Here, we utilize the non-reciprocity of the lithium niobate modulators and eliminate the modulation on the reverse optical pulses. This characteristic is widely available in the radio-frequency band, allowing the modulation speed to no longer be limited by the self-compensating optics and enabling further increases. The measured average intrinsic quantum bit error rate of the four polarization states at 10 GHz system repetition frequency is as low as 0.53% over 10 min without any compensation. The simulation results show that our scheme can maintain a SKR of about 5 kbps even at the extreme communication distances between the satellite and the earth. Our work can be efficiently applied in high-speed, high-loss satellite-based quantum communication scenarios.
Satellite-based quantum key distribution (QKD) has made significant strides in the past few years, and an international space race toward practical quantum networks has begun. Being one major encoding method, highly robust, high-performance and miniaturized polarization modulation is critical to meet the needs of future satellite applications. Here, we employ the Sagnac-based method to realize inherent-stable polarization modulation and propose a novel optical design to achieve a state-of-the-art repetition rate of 2 GHz. This value approaches the theoretical repetition frequency limit with commercially-available phase modulators. Furthermore, the polarization encoder is miniaturized into a robust 6.0 cm × 6.5 cm module and two commercially mature phase modulators. A 90-min consecutive test was successfully performed, verifying the robustness of this scheme with an average quantum bit error rate as low as 0.42%. The presented intrinsically stable high-speed polarization modulation provides an effective solution for future satellite applications.
In polarization-encoding quantum key distribution (QKD) systems, achieving high-speed, robust, and low-error polarization modulation helps to increase the secure key rate (SKR). Moreover, the advanced QKD protocols place higher demands on the system’s photon preparation capability, which implies providing additional polarization states for practical implementation. Here, we redesign the Sagnac interferometer and utilize the non-reciprocity of the lithium niobate modulator to eliminate the reverse modulation that limits the modulation speed. The proposed scheme enables high-speed, robust, and low-error modulation with arbitrary polarization states. The average bit error rate (BER) for the 10 min measurement is 0.2332% with a system repetition frequency of 16.5 GHz.
Polarization modulation plays a key role in polarization-encoding quantum key distribution (QKD). Here, we report a new, to the best of our knowledge, polarization modulation scheme based on an inherently stable Sagnac interferometer. The presented scheme is free of polarization mode dispersion and calibration as well as insensitive to environmental influences. Successful experiments at a repetition frequency of 1.25 GHz have been conducted to demonstrate the feasibility and stability of the scheme. The measured average quantum bit-error rate of the four polarization states is as low as 0.27% for 80 consecutive minutes without any adjustment. This high-speed intrinsically stable polarization modulation can be widely applied to many polarization-encoding QKD systems, such as BB84, MDI, etc.
Quantum key distribution (QKD) allows distant parties to exchange cryptographic keys with unconditional security by encoding information on the degrees of freedom of photons. Polarization encoding has been extensively used for QKD along free-space, optical fiber, and satellite links. However, the polarization encoders used in such implementations are unstable, expensive, and complex and can even exhibit side channels that undermine the security of the protocol. Here we propose a self-compensating polarization encoder based on a lithium niobate phase modulator inside a Sagnac interferometer and implement it using only commercial off-the-shelf (COTS) components. Our polarization encoder combines a simple design and high stability reaching an intrinsic quantum bit error rate as low as 0.2%. Since realization is possible from the 800 to the 1550 nm band using COTS devices, our polarization modulator is a promising solution for free-space, fiber, and satellite-based QKD.
Free-space quantum key exchanges between ground stations and low earth orbiting satellites will be characterized by high link losses, typically of the order of 30 dB or higher. These losses, together with the need to transmit weak Poissonian laser pulses containing on average substantially less than 0.1 photons per transmitted bit to preserve security, will result in exceedingly low channel efficiencies, typically of the order of 10-5. In order to achieve even a relatively modest secure key bit rate of 100 kbps, it will therefore often be necessary to key the transmitter at rates in excess of 10 Gbps. In this paper we outline several different methods of achieving such fast polarization keying including the use of dual drive Mach-Zehnder intensity modulators on lithium niobate in a hybrid fibre-guided modulator structures. We then propose a total integrated polarization keying structure in LiNbO3. We describe the fabrication and testing of such ultra-broadband polarization keyers suitable for use in high loss, short-wave free-space quantum key distribution systems employing silicon photon counters. We also indicate how these devices may be incorporated into quantum key satellite courier payloads and ground station terminals.
Quantum key distribution (QKD) leverages the principles of quantum mechanics to exchange a secret key between two parties. Despite its promising features, QKD also faces several practical challenges such as transmission loss, noise in quantum channels and finite key size effects. Addressing these issues is crucial for the large-scale deployment of QKD in fiber and satellite networks. In this paper, we present a 1550 nm QKD system realizing the efficient-BB84 protocol and based on the iPOGNAC scheme. The system achieved repetition rates up to 1.5~GHz and showed an intrinsic QBER of ∼0.4%\sim 0.4\%∼0.4%. The system was first tested on a laboratory fiber link and then on an intermodal link in the field, consisting of both deployed fiber and a 620 m free-space channel. The experiment was performed in daylight conditions, exploiting the Qubit4Sync synchronization protocol. With this trial, we achieved a new benchmark for free-space BB84 QKD systems by generating a sustained secret key rate (SKR) above 1~Mb/s for 1 hour. Finally, exploiting a recently discovered finite-size bound, we achieved a secure key rate of about 10 Mb/s at low losses (5 dB), and around 6.5~kb/s in the high-loss (38.5 dB), low block length (N=104N=10^4N=104) regime. The latter results demonstrate the system's suitability for highly lossy and time-constrained scenarios such as QKD from low Earth orbit satellites.