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Experimental Passive Decoy-state Quantum Key Distribution

Qi-Chao Sun, Weilong Wang, Yang Liu, Fei Zhou, Jason S. Pelc, M. M. Fejer, Cheng-Zhi Peng, Xianfeng Chen, Xiongfeng Ma, Qiang Zhang, Jian-Wei Pan

Published Jan 1, 2014• 1 citation
Publisher

Abstract

By employing low dark count up-conversion single photon detectors, we have experimentally demonstrated the passive decoy-state method over a 50-km-long optical fiber and have obtained a key rate of about 100 bit/s.
DOI
10.1364/cleo_at.2014.jth2a.121

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The decoy-state method is widely used in practical quantum key distribution systems to replace ideal single photon sources with realistic light sources by varying intensities. Instead of active modulation, the passive decoy-state method employs built-in decoy states in a parametric down-conversion photon source, which can decrease the side channel information leakage in decoy state preparation and hence increase the security. By employing low dark count up-conversion single photon detectors, we have experimentally demonstrated the passive decoy-state method over a 50-km-long optical fiber and have obtained a key rate of about 100 bit/s. Our result suggests that the passive decoy-state source is a practical candidate for future quantum communication implementation.
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.
To increase dramatically the distance and the secure key generation rate of quantum key distribution (QKD), the idea of quantum decoys--signals of different intensities--has recently been proposed. Here, we present the first experimental implementation of decoy state QKD. By making simple modifications to a commercial quantum key distribution system, we show that a secure key generation rate of 165 bit/s, which is 1/4 of the theoretical limit, can be obtained over 15 km of a telecommunication fiber. We also show that with the same experimental parameters, not even a single bit of secure key can be extracted with a non-decoy-state protocol. Compared to building single photon sources, decoy state QKD is a much simpler method for increasing the distance and key generation rate of unconditionally secure QKD.
Long distance quantum key distribution links are essential for the development of a widely spread fiber-based quantum network. For instance, it would allow for two remote users to communicate securely without the need for a trusted intermediate node. In this work, we focus on enhancing the receiver side of a quantum key distribution system, deploying state-of-the-art superconducting nanowire single photon detectors with ultra low dark count (< 1Hz) and very high efficiency (> 90%), achieving a secure key distribution over a 55 dB losses channel with 0.6 bps rate. This result opens new possibilities for long distance secure communications.
Today’s society heavily relies on secure communications, which can be guaranteed by Quantum Key Distribution (QKD), the most mature quantum technology. However, achieving long-distance links without relying on trusted nodes is still challenging. An important limitation is the non-ideality of detection systems, where intrinsic dark counts can hinder key extraction. This work proposes using state-of-the-art superconducting nanowire singlephoton detectors (SNSPD) with ultra-low dark count rates (<1 Hz) to reduce the quantum bit error rate (QBER) and achieve a higher secret key rate. Together with a high-rate QKD transmitter and a self-stabilizing receiver, we enabled a key exchange over 55 dB, corresponding to 340 km over an ultra-low-loss optical fiber.