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arXiv:2507.01205

Passive polarization and phase stabilization scheme for Twin-Field QKD

Nascimento, Christiano M. S., Felipe Calliari, Guilherme P. Temporão

Published Jul 1, 2025• 0 citations
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Abstract

Twin-Field Quantum Key Distribution requires first-order interference between coherent states sent by Alice and Bob in a mid-station Charlie. In order to obtain stable operation and maximum interferometric visibility, not only phase stabilization but also polarization control is required, especially in optical-fiber setups. In this paper, we propose an experimental setup that simultaneously provides passive stabilization of phase and polarization fluctuations by combining a Sagnac-like interferometer with a "Plug-and-Play" configuration employing Faraday mirrors. Experimental results show a net interferometric visibility maintained around 95.3% during 72 hours of continuous operation, with a standard deviation of 0.47%. The setup can be straightforwardly adapted to a multi-user scenario employing either a star network, a bus topology, or a combination of both.

DOI
10.48550/arxiv.2507.01205

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•Nov 25, 1996•292 citations
Twin-Field Quantum Key Distribution requires first-order interference between coherent states sent by Alice and Bob in a mid-station Charlie. In order to obtain stable operation and maximum interferometric visibility, not only phase stabilization but also polarization control is required, especially in optical-fiber setups. In this paper, we propose an experimental setup that simultaneously provides passive stabilization of phase and polarization fluctuations by combining a Sagnac-like interferometer with a "Plug-and-Play" configuration employing Faraday mirrors. Experimental results show a net interferometric visibility maintained around 95.3% during 72 hours of continuous operation, with a standard deviation of 0.47%. The setup can be straightforwardly adapted to a multi-user scenario employing either a star network, a bus topology, or a combination of both.
Twin-field quantum key distribution (TF-QKD) relies on first-order interference between coherent optical fields combined at an intermediate station, which requires simultaneous phase and polarization stabilization. We present a Sagnac-like interferometric architecture arranged in a hybrid star–bus network topology that enables any-to-any connectivity while providing passive stabilization of phase and polarization fluctuations for multiple wavelengths. The proposed configuration, which integrates Faraday mirrors in a Plug-and-Play scheme, operates without active feedback. Experimental results in a 110 km-long installed fiber link between the cities of Linköping and Nyköping, Sweden, show 99.2% visibility maintained over 14 h of continuous operation with 0.33% standard deviation. The main limitation of this configuration—Rayleigh backscattering—was circumvented by time filtering.
We present the experimental implementation of a three-time-bin phase-encoded Twin-Field Quantum Key Distribution (TF-QKD) protocol using a Sagnac-based star-topology plug-and-play architecture. The proposed encoding method leverages the relative phases of three consecutive time bins to encode two bits per signal. The Sagnac loop configuration enables self-compensation for both phase and polarisation drifts, eliminating the need for active stabilisation. However, field deployments are subject to rapid phase fluctuations caused by external vibrations, which can degrade interference visibility. We used the first time bin for real-time phase-fluctuation monitoring. Although this monitoring reduces the effective key generation rate, the system achieved a secure key rate of approximately 1.5e-5 bits per pulse, with a corresponding visibility of up to 87% over a 50 km asymmetric optical fibre channel. These results demonstrate the practicality, stability, and scalability of the proposed three-time-bin TF-QKD protocol for real-world quantum communication networks.
Twin-field (TF) quantum key distribution (QKD) offers a promising approach to long-distance QKD networks due to its superior performance over large channel losses. Due to specialized hardware requirements, nearly all long-distance TFQKD demonstrations have only two users exchanging keys, rather than a network with three or more users. In this work, we experimentally demonstrate a proof-of-principle three-user-pair Sagnac TFQKD network spanning 127-km using single-photon avalanche detectors without any active phase stabilization or postcompensation. We implement efficient procedures for maintaining polarization stability and circumventing Rayleigh backscattering noise to achieve a stable Sagnac interference visibility of 93±193\pm193±1% over one hour. A secure key rate of 1.398×10−51.398\times10^{-5}1.398×10−5 bits per pulse is achieved over an asymmetric communication channel with 102-km fiber and 45-dB overall loss. To our knowledge, this is the first TFQKD network without active phase stabilization or postcompensation achieved over long fibers. Our results represent a highly practical and cost-effective approach to long-distance QKD networks.
Meet Alice, Bob, and David. Establishing secure encryption keys between any two of these users is important for a long-range communication network, but with dubious Charlie also around, it becomes tricky. Twin-field quantum key distribution (TFQKD), while promising for this application, has been limited so far to two users, because of its requirement for phase stability of optical signals. In this work, the authors demonstrate a TFQKD network by connecting three users in a Sagnac fiber ring, a configuration with inherent phase stability. This experiment suggests that the Sagnac TFQKD system is an effective, practical approach to implementing a long-range secure communication network.
We present a time-multiplexed interferometer based on Faraday mirrors, and apply it to quantum key distribution. The interfering pulses follow exactly the same spatial path, ensuring very high stability and self balancing. Use of Faraday mirrors compensates automatically any birefringence effects and polarization dependent losses in the transmitting fiber. First experimental results show a fringe visibility of 0.9984 for a 23km-long interferometer, based on installed telecom fibers. In so-called private-key cryptographic systems, secure transmission through unprotected channels rely on the exchange of secret keys, known only to the sender, Alice, and the receiver, Bob. Quantum cryptography (QC) relies on the properties of quantum mechanics to obtain a provably secure key distribution [1, 2, 3, 4]. Most exisiting implementations rely on either the polarization [2, 5, 6] or the phase [7, 8, 9, 10] of very weak pulses of light as information carrier. To date, the longest transmission spans were obtained in optical fibers, at a wavelength of 1300 nm [6, 9, 10]. The main difficulty with polarization-based systems is the need to keep stable polarizations over distances of tens of kilometers, in standard telecom cables. Indeed, due to the birefringence of the fibers and the effect of the environment, the output polarization fluctuates randomly. Recent experiments [6]