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Journal of Experimental and Theoretical Physics

On the passive probing of fiber optic quantum communication channels

A. V. Korol’kov, Konstantin G. Katamadze, S. P. Kulik, S. N. Molotkov

Published Apr 1, 2010• 4 citations
Publisher

Abstract

Avalanche photodetectors based on InGaAs:P are the most sensitive and only detectors operating in the telecommunication wavelength range 1.30–1.55 μm in the fiber optic quantum cryptography systems that can operate in the single photon count mode. In contrast to the widely used silicon photodetectors for wavelengths up to 1 μm operating in a waiting mode, these detectors always operate in a gated mode. The production of an electron-hole pair in the process of the absorption of a photon and the subsequent appearance of an avalanche of carriers can be accompanied by the inverse processes of the recombination and emission of photons. Such a backward emission can present a potential serious problem for the stability of fiber optic quantum cryptography systems against passive probing. The results of analyzing the detection of backscattered radiation are reported. The probability of such an emission has been estimated.

DOI
10.1134/s1063776110040023

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Avalanche photodetectors based on InGaAs:P are the most sensitive and only detectors operating in the telecommunication wavelength range 1.30–1.55 μm in the fiber optic quantum cryptography systems that can operate in the single photon count mode. In contrast to the widely used silicon photodetectors for wavelengths up to 1 μm operating in a waiting mode, these detectors always operate in a gated mode. The production of an electron-hole pair in the process of the absorption of a photon and the subsequent appearance of an avalanche of carriers can be accompanied by the inverse processes of the recombination and emission of photons. Such a backward emission can present a potential serious problem for the stability of fiber optic quantum cryptography systems against passive probing. The results of analyzing the detection of backscattered radiation are reported. The probability of such an emission has been estimated.
This paper gives experimental results of quantum key distribution on a fiber-optic setup at a telecom wavelength of 1555 nm. A self-compensated two-channel optical circuit is used. Quantum key distribution was performed by coding the phase states of single photons radiated by a pulsed semiconductor laser in two alternative nonorthogonal bases. Specially developed single photon counters based on InGaAs: InP avalanche photodiodes were employed as high-sensitivity photodetectors. The results of investigation of the quantum efficiency, probability of afterpulses, and noise level for various operating modes of the detectors at temperatures from −40 to −60°C are given. A key distribution rate of 450 bit/s was obtained for a single-mode fiber-optic quantum communication channel between the receiver and sender 25 km long at a laser pulse clock frequency of 5 MHz and an average number of photons per pulse of about 0.2. For the achieved photodetector characteristics, the average number of errors in the quantum key did not exceed 3.7%.
Abstract In order to realize long‐distance quantum cryptography based on standard optical fiber network, we have developed a single‐photon detector at 1550 nm using InGaAs/InP avalanche photodiodes (APDs). To reduce the dark count probability, APDs were cooled and operated with a Gated Passive Quenching Circuit. A quantum efficiency of 13.7% was obtained at −55 °C while keeping the dark count probability per gate (2 ns) as low as 2.4 × 10−5. In this paper we will discuss the performance of the developed single‐photon detector in view of application to quantum cryptography. © 2003 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 86(5): 10–15, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/ecjb.10115
Quantum key distribution (QKD) at telecom wavelengths (1260 - 1625 nm) has the potential for fast deployment due to existing optical fibre infrastructure and mature telecom technologies. At these wavelengths, Indium Gallium Arsenide (InGaAs) avalanche photodiode (APD) based detectors are the preferred choice for photon detection. Similar to their Silicon counterparts used at shorter wavelengths, they exhibit fluorescence from recombination of electron-hole pairs generated in the avalanche breakdown process. This fluorescence may open side channels for attacks on QKD systems. Here, we characterize the breakdown fluorescence from two commercial InGaAs single photon counting modules, and find a spectral distribution between 1000 nm and 1600 nm. We also show that by spectral filtering, this side channel can be efficiently suppressed.
In the description of quantum key distribution systems, much attention is paid to the operation of quantum cryptography protocols. The main problem is the insufficient study of the synchronization process of quantum key distribution systems. This paper contains a general description of quantum cryptography principles. A two-line fiber-optic quantum key distribution system with phase coding of photon states in transceiver and coding station synchronization mode was examined. A quantum key distribution system was built on the basis of the scheme with automatic compensation of polarization mode distortions. Single-photon avalanche diodes were used as optical radiation detecting devices. It was estimated how the parameters used in quantum key distribution systems of optical detectors affect the detection of the time frame with attenuated optical pulse in synchronization mode with respect to its probabilistic and time-domain characteristics. A design method was given for the process that detects the time frame that includes an optical pulse during synchronization. This paper describes the main quantum communication channel attack methods by removing a portion of optical emission. This paper describes the developed synchronization algorithm that takes into account the time required to restore the photodetector’s operation state after the photon has been registered during synchronization. The computer simulation results of the developed synchronization algorithm were analyzed. The efficiency of the developed algorithm with respect to synchronization process protection from unauthorized gathering of optical emission is demonstrated herein.
A study was made of the characteristics of photodetectors for fiber-optic communication lines using quantum cryptographic systems – germanium avalanche photodiodes operating in the photon counting mode. In particular, it was established at what highest temperature the implementation of the photon counting mode is possible, and the influence of temperature and overvoltage on the sensitivity of photodiodes is also considered. An experimental setup has been developed for the research. It has been determined that the highest ambient temperature at which LFD-2 germanium avalanche photodiodes operate in the photon counting mode is 243 K. It has also been found that the highest sensitivity of germanium avalanche photodiodes corresponds to the optical radiation wavelength range of 1310÷1490 nm. Lowering the temperature leads to an increase in the sensitivity of germanium avalanche photodiodes. It was found that the dependence of the signal-to-noise ratio on overvoltage has a maximum corresponding to overvoltage ΔU = 0.1 V. Lowering the temperature led to an increase in sensitivity and signal-to-noise ratio. Since there was no shift in the maximum dependence of the signal-to-noise ratio on the overvoltage, it was therefore concluded that when the avalanche photodiode operates in the photon counting mode, in order to ensure maximum sensitivity, it is necessary to select the overvoltage corresponding to the maximum signal-to-noise ratio. The results obtained can be used in quantum cryptographic systems, technical means of protecting information transmitted over fiber-optic communication lines, and for the metrology of single-photon radiation sources.