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Quantum Electronics and Laser Science Conference

Experimental quantum key distribution with active phase randomization

Yi Zhao, Bing Qi, Hoi‐Kwong Lo

Published May 1, 2007• 9 citations
Publisher

Abstract

Phase-randomization is an important assumption in many security proofs of practical quantum key distribution (QKD) systems. Here, we present the first experimental QKD with reliable active phase-randomization. A polarization-insensitive phase-modulator is designed for our experiment.
DOI
10.1109/qels.2007.4431642

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Phase randomization is an important assumption made in many security proofs of practical quantum key distribution (QKD) systems. Here, the authors present the first experimental demonstration of QKD with reliable active phase randomization. One key contribution is a polarization-insensitive phase modulator, which they added to a commercial phase-coding QKD system to randomize the global phase of each bit. They also proposed a useful method to verify the phase randomization experimentally. Their result shows very low quantum bit error rate (<1%). They expect this active phase randomization process to be a standard part in future QKD setups due to its significance and feasibility.
Coherent state photon sources are widely used in quantum information processing. In many applications, such as quantum key distribution (QKD), a coherent state functions as a mixture of Fock states by assuming that its phase is continuously randomized. In practice, such a crucial assumption is often not satisfied, and therefore the security of existing QKD experiments is not guaranteed. To bridge this gap, we provide a rigorous security proof of QKD with discrete-phase-randomized coherent state sources. Our results show that the performance of the discrete-phase randomization case is close to its continuous counterpart with only a small number (say, 10) of discrete phases. Compared to the conventional continuous phase randomization case, where an infinite amount of random bits are required, our result shows that only a small amount (say, 4 bits) of randomness is needed.
Quantum key distribution (QKD) offers information-theoretic security based on the fundamental laws of physics. However, device imperfections, such as those in active modulators, may introduce side-channel leakage, thus compromising practical security. Attempts to remove active modulation, including passive decoy intensity preparation and polarization encoding, have faced theoretical constraints and inadequate security verification, thus hindering the achievement of a fully passive QKD scheme. Recent research [W. Wang et al., Phys. Rev. Lett. 130, 220801 (2023).PRLTAO0031-900710.1103/PhysRevLett.130.220801; 2V. Zapatero et al., Quantum Sci. Technol. 8, 025014 (2023).2058-956510.1088/2058-9565/acbc46] has systematically analyzed the security of a fully passive modulation protocol. Based on this, we utilize the gain-switching technique in combination with the postselection scheme and perform a proof-of-principle demonstration of a fully passive quantum key distribution with polarization encoding at channel losses of 7.2 dB, 11.6 dB, and 16.7 dB. Our work demonstrates the feasibility of active-modulation-free QKD in polarization-encoded systems.
The decoy-state method could effectively enhance the performance of quantum key distribution (QKD) with a practical phase randomized weak coherent source. Although active modulation of the source intensity is effective and has been implemented in many experiments, passive preparation of decoy states is also an important addition to the family of decoy-state QKD protocols. In this paper, following the theory of Curty et al. [Phys. Rev. A 81, 022310 (2010)], we experimentally demonstrate the phase-encoding passive-decoy-state QKD with only linear optical setups and threshold single-photon detectors. In our experiment, two homemade independent pulsed lasers, with visibility of Hong-Ou-Mandel interference $0.53(\ifmmode\pm\else\textpm\fi{}0.003)$, have been implemented and used to passively generate the different decoy states. Finally, a secret key rate of $1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$/pulse is obtained with 10-km commercial fiber between Alice and Bob.
Abstract The performance of quantum key distribution (QKD) is severely limited by multiphoton emissions, due to the photon-number-splitting attack. The most efficient solution, the decoy-state method, requires that the phases of all transmitted pulses are independent and uniformly random. In practice, however, these phases are often correlated, especially in high-speed systems, which opens a security loophole. Here, we address this pressing problem by providing a security proof for decoy-state QKD with correlated phases that offers key rates close to the ideal scenario. Our work paves the way towards high-performance secure QKD with practical laser sources, and may have applications beyond QKD.