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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Polarization recovery and auto-compensation in quantum key distribution network

Lijun Ma, Xu Hai, Xiao Hong Tang

Published Aug 31, 2006• 20 citations
Publisher

Abstract

A Quantum Key Distribution (QKD) network can allow multi-user communication via secure key. Moreover, by actively switching communication nodes, one can achieve high key transmission rate for the selected nodes. However, the polarization properties of different fiber path are different and these properties also randomly drift over time. Therefore, polarization recovery after the switching and auto-compensation during key transmission are critical for the QKD network. In this work, we use programmable polarization controllers to implement polarization recovery and auto-compensation in the QKD network. We will also discuss its time limitation and future improvement.

DOI
10.1117/12.679575

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A Quantum Key Distribution (QKD) network can allow multi-user communication via secure key. Moreover, by actively switching communication nodes, one can achieve high key transmission rate for the selected nodes. However, the polarization properties of different fiber path are different and these properties also randomly drift over time. Therefore, polarization recovery after the switching and auto-compensation during key transmission are critical for the QKD network. In this work, we use programmable polarization controllers to implement polarization recovery and auto-compensation in the QKD network. We will also discuss its time limitation and future improvement.
In quantum key distribution (QKD) systems using polarization entanglement, channel polarization perturbations significantly compromise both efficiency and security. To address the slow response and instability of current entanglement-based polarization compensation techniques for entanglement-based quantum key distribution, we propose a polarization compensation scheme based on an adaptive gradient descent algorithm. The system's performance is tested over a 10 km single-sided fiber channel. Results show that this scheme maintains long-term stability of the polarization state under both random and periodic perturbations. The average compensation time is 248 ms, with an entanglement source average visibility of 83.43% and an average quantum error rate of 8.28%. This method effectively mitigates polarization perturbation in entanglement-based quantum key distribution (QKD). These results are pivotal for the development of long-distance, multi-node quantum communication networks.
Abstract The information-theoretic unconditional security offered by quantum key distribution has spurred the development of larger quantum communication networks. However, as these networks grow so does the strong need to reduce complexity and overheads. Polarization-based entanglement distribution networks are a promising approach due to their scalability and no need for trusted nodes. Nevertheless, they are only viable if the birefringence of all-optical distribution fibres in the network is compensated to preserve the polarization-based quantum state. The brute force approach would require a few hundred fibre polarization controllers for even a moderately sized network. Instead, we propose and investigate four different realizations of polarization compensation schemes that can be used in quantum networks. We compare them based on the type of reference signals, complexity, effort, level of disruption to network operations and performance on a four-user quantum network.
We previously demonstrated a high speed, point to point, quantum key distribution (QKD) system with polarization coding over a fiber link, in which the resulting cryptographic keys were used for one-time pad encryption of real time video signals. In this work, we extend the technology to a three-node active QKD network - one Alice and two Bobs. A QKD network allows multiple users to generate and share secure quantum keys. In comparison with a passive QKD network, nodes in an active network can actively select a destination as a communication partner and therefore, its sifted-key rate can remain at a speed almost as high as that in the point-to-point QKD. We demonstrate our three-node QKD network in the context of a QKD secured real-time video surveillance system. In principle, the technologies for the three-node network are extendable to multi-node networks easily. In this paper, we report our experiments, including the techniques for timing alignment and polarization recovery during switching, and discuss the network architecture and its expandability to multi-node networks.
A complete fiber-based polarization encoding quantum key distribution (QKD) system based on the BB84 protocol has been developed at National Institute of Standard and Technology (NIST). The system can be operated at a sifted key rate of more than 4 Mbit/s over optical fiber of length 1 km and mean photon number 0.1. The quantum channel uses 850 nm photons from attenuated high speed VCSELs and the classical channel uses 1550 nm light from normal commercial coarse wavelength division multiplexing devices. Sifted-key rates and quantum error rates at different transmission rates are measured as a function of distance (fiber length). A polarization auto-compensation module has been developed and utilized to recover the polarization state and to compensate for temporal drift. An automatic timing alignment device has also been developed to quickly handle the initial configuration of quantum channels so that detection events fall into the correct timing window. These automated functions make the system more practical for integration into existing optical local area networks.
Quantum key distribution is information-theoretically secure and a promising candidate for security against quantum computers. However, experimental implementations might deviate from assumptions made in the security proofs and leave the system vulnerable. Measurement device-independent QKD (MDI-QKD) scheme closes all the loopholes on the detection side, which is the most susceptible part of QKD. Therefore, the state preparation part of QKD systems becomes the only source of imperfections that needs to be considered in the security proofs of MDI-QKD. We examine our polarization encoding module and how imperfection in its components can affect state preparation. Our experiment demonstrates the feasibility of generating a secure key with imperfect states prepared by off-the-shelf devices. Another practical challenge that we address in implementing fibre-based MDI-QKD is polarization compensation. Polarization variations due to temperature fluctuations and fibre birefringence are inevitable and require polarization stabilization. Before this work, polarization compensation schemes for QKD either reduced the key sharing cycle or required extra equipment, making them less scalable or commercially viable and preventing the wide adaptation of polarization encoding. We propose and implement a novel polarization compensation scheme in the MDI-QKD systems that avoids the abovementioned drawbacks by using part of discarded detections. Our scheme evaluates the polarization drift in real-time based on single measurements corresponding to decoy intensities. We successfully implemented this active polarization compensation for four hours over 40 km of buffered fibre spool without thermal insulation or vibrational isolation. This thesis is a decisive step toward making polarization encoding MDI-QKD practical and secure, paving the way to realizing a polarization encoding MDI-QKD network. Future networks could be hybrid (ground and satellite links), and polarization encoding is a suitable candidate if an efficient stabilization scheme for fibre links is present. To this aim, our novel compensation scheme solely relies on recycling detections and retains a simple structure for a user node so that expanding the network could be feasible in terms of cost and practicality. Also, it does not reduce the key-sharing cycle, which is essential in networks where the detection system is shared, and each user has limited access time.
Polarizations of single-photon pulses have been controlled with long-term stability of more than 10 hours by using an active feedback technique for auto-compensation of unpredictable polarization scrambling in long-distance fiber. Experimental tests of long-term operations in 50, 75 and 100 km fibers demonstrated that such a single-photon polarization control supported stable polarization encoding in long-distance fibers to facilitate stable one-way fiber system for polarization-encoded quantum key distribution, providing quantum bit error rates below the absolute security threshold.