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Quantum Information and Measurement (QIM)

Optical Power Limiters for Securing Practical Quantum Communication Systems

Gong Zhang, Ignatius William Primaatmaja, Jing Yan Haw, Xiao Gong, Chao Wang, Charles Ci Wen Lim

Published Jan 1, 2021• 2 citations
Publisher

Abstract

We propose and experimentally demonstrate a passive optical power limiter device used to protect practical quantum communication systems. The device is robust against different signal variations and has minimum impact on the quantum signals.
DOI
10.1364/qim.2021.m2c.6

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Controlling the energy of unauthorized light signals in a quantum cryptosystem is an essential task for implementation security. Here, we propose a passive optical power limiter device based on thermo-optical defocusing effects, which provides a reliable power limiting threshold that can be readily adjusted to suit various quantum applications. In addition, the device is robust against a wide variety of signal variations (e.g. wavelength, pulse width), which is important for implementation security. To show its practical utility for quantum cryptography, we demonstrate and discuss three potential applications: (1) measurement-device-independent quantum key distribution with enhanced security against a general class of Trojan-horse attacks, (2) using the power limiter as a countermeasure against bright illumination attacks, and (3) the application of power limiters to potentially enhance the implementation security of plug-and-play quantum key distribution.
Controlling the energy of unauthorized light signals in a quantum cryptosystem is an essential criterion for implementation security.Here, we propose a passive optical power limiter device based on thermooptical defocusing effects, providing a reliable power limiting threshold that can be readily adjusted to suit various quantum applications.In addition, the device is robust against a wide variety of signal variations (e.g., wavelength, pulse width), which is important for implementation security.We experimentally show that the proposed device does not compromise quantum communication signals.It only has a minimal impact (if not, negligible impact) on the intensity, phase, or polarization degrees of freedom of the photons, thus making it suitable for general communication purposes.To show its practical utility for quantum cryptography, we demonstrate and discuss three potential applications: (1) measurement-device-independent quantum key distribution with enhanced security against a general class of Trojan-horse attacks, (2) using the power limiter as a countermeasure against bright illumination attacks, and (3) the application of power limiters to potentially enhance the implementation security of plug-and-play quantum key distribution.
Quantum communication protocols are considered secure provided that all devices included in the system are fully characterized, and side channels are closed. However, as a result of laser radiation exposure, it is possible to change quantum communication systems components’ characteristics. This leads to vulnerabilities appearing in the quantum key distribution system. In this paper, we consider the effect of pulsed laser radiation on fiber-optic isolators used in quantum communication systems. Isolators protect the source of the system from attacking optical radiation coming from the “eavesdropping” side via the quantum channel. Lowering the isolation factor can bring the entire system out of a secure state. This gives an eavesdropper access to information about the secret key. The scenario of the most probable attack to the source of the quantum key distribution system via a pulsed laser was simulated. The experimental setup provided exposure of fiber isolators with pulsed laser radiation at a wavelength of 1064 nm (within the transparency window of the isolators) with a mean power up to 840 mW in four different pulse generation modes. The isolation factor and throughput of tested samples were monitored using a laser diode with a wavelength of 1550 nm and average power of 10.5 mW. Spectrally selective splitters were used to separate the lasers. It is shown that the isolation factor (isolator attenuation in the direction from the quantum channel to the system) at a wavelength of 1550 nm decreases from the initial value of 59.1 dB to 24.5 dB. The throughput (in the direction from the system to the quantum channel) at the same wavelength decreases from 0.6 dB to 1.2–12.3 dB or remains the same, depending on the acting pulsed laser radiation parameters. Temperature monitoring showed that the temperature of the isolator body changes insignificantly when exposed to pulsed radiation. The obtained effects of changing the isolation coefficient and throughput can be explained by the presence of nonlinear effects in the magneto-optical crystal of the isolator. The results of the work can be applicable in the practical evaluation of quantum communication systems security, in particular, the security evaluation of quantum key distribution systems. The results can be used to prepare the standards for certification procedures for assessing the security of quantum communication systems. The work gives recommendations for enhancing signal source unit security in quantum communication systems. Namely, as countermeasures to protect against the effects of pulsed laser radiation, it is proposed to set optical fuses with a limited threshold power, detectors for monitoring input optical radiation power, and narrow-band optical filters at the entrance of the quantum communication system.