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

Effects of loss and dispersion on fiber-based quantum key distribution system

Shu Yang, Wei Liu, Yu‐Hong Han, Bojun Yang

Published Oct 17, 2008• 3 citations
Publisher

Abstract

Studied are the effects of fiber dispersion and loss and single-photon detector dark counts on key transmission rate and bit error rate of quantum key distribution system through theoretical analysis and computation. Research demonstrates that fiber loss is the main factor that influences quantum key transmission rate, while quantum bit error rate is related to fiber loss, dispersion, pulse width, dark counts and Rayleigh backscattering. Theoretical calculation indicates that in the current single-mode fiber-based plug & play quantum key distribution system with a dark counts of 10 -5/ns, the utilization of short pulse with 1550 nm wavelength and a 50 ps width can achieve the maximum transmission distance of 131 km. If we reduce dark counts or use G.653 dispersion-shifted fiber with short pulse, we can further improve key transmission rate and reduce the quantum bit error rate, thus effectively enhance the transmission distance of quantum key distribution system.

DOI
10.1117/12.804060

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Studied are the effects of fiber dispersion and loss and single-photon detector dark counts on key transmission rate and bit error rate of quantum key distribution system through theoretical analysis and computation. Research demonstrates that fiber loss is the main factor that influences quantum key transmission rate, while quantum bit error rate is related to fiber loss, dispersion, pulse width, dark counts and Rayleigh backscattering. Theoretical calculation indicates that in the current single-mode fiber-based plug & play quantum key distribution system with a dark counts of 10 -5/ns, the utilization of short pulse with 1550 nm wavelength and a 50 ps width can achieve the maximum transmission distance of 131 km. If we reduce dark counts or use G.653 dispersion-shifted fiber with short pulse, we can further improve key transmission rate and reduce the quantum bit error rate, thus effectively enhance the transmission distance of quantum key distribution system.
The effects of fiber dispersion and loss and single-photon detector dark counts on key transmission rate and BER of quantum key distribution system through theoretical analysis and computation are studied . Research demonstrates that fiber loss is the main factor that influences quantum key transmission rate, while quantum BER is related to fiber loss, dispersion, pulse width, dark counts and Rayleigh backscattering. In the experiment, we should try to reduce fiber loss, dispersion and single-photon detector dark counts, and meanwhile choose appropriate initial pulse width, thereby enhancing quantum key transmission rate and reducing BER, thus effectively enhance the transmission distance of quantum key distribution system.
High-speed single-photon detectors are crucial for efficiently performing quantum key distribution. We demonstrate fiber-optic quantum key distribution at 1550 nm using single-photon detectors operating at 10 MHz. Discharge pulse counting is adopted for single-photon detection in order to suppress after-pulse generation. Furthermore, by setting a dead time in single-photon detection, we discard after-pulses to reduce the number of bit errors in the quantum key distribution. Using discharge pulse counting and after-pulse discarding in the quantum key distribution, we can achieve a key rate of 45 kHz with a bit-error rate of 2%. The fiber length is 10.5 km.
One of our major target is quantum key distribution (QKD,) which is closest to the practical use. Though QKD can be performed only with current technology, there still remain many thing to be improved. One of the most important devices that determine the system performance is photon detectors. It limits the transmission distance in optical fiber communication wavelength. A photon detector combining the two avalanche photon diodes (APD) has been demonstrated for qubit discrimination in 1550 nm. Spikes accompanied with the signals in gated-mode were canceled by balanced output from the two APDs. The balanced APD photon detector shows very low dark count rate (2x10-7) counts/pulse) with 10 % detection efficiency. We have also developed a highly stable interferometer on a planer-lightwave-circuit (PLC.) We have achieved single-photon interference over 150 km using time-division interferometers for quantum cryptography, which were composed of the two integrated-optic asymmetric Mach-Zehnder interferometers, and the balanced gated-mode photon detectors. The observed visibility was more than 80 % after 150-km transmission, which refers to a quantum bit error rate of less than 10 %. We will also describe temperature insensible QKD system and high speed (100 bps) key transmission over 40 km fiber.
The distance for quantum key distribution using optical fibers is limited to around 100 km by the transmission loss, background level in the optical fibers, and birefringence in the fibers. Free space is an ideal medium for transmitting photon-based quantum information; this is considered to be the key advantage of quantum cryptography. In fiber-based systems, there is an optical loss in the fiber of about 0.2 dB/km for a telecom wavelength of 1.5 µm. That is, after a propagation of 150 km, the transmission loss in the fiber becomes 30 dB. On the other hand, free-space transmission obeys the diffraction theory of laser light; therefore, the free-space transmission loss is inversely proportional to the square of the distance, and this transmission has an advantage at longer distances over fiber-based transmission. The final key rates for free-space and fiber-based transmission systems are compared with respect to the parameters for the dark count rate, the imperfection of the optical system, and so on. In this paper, the trade-off of free-space and fiber-based systems is presented.
Phase encoding in quantum key distribution (QKD) enables long-distance information-theoretic secure communication in optical fibers. We present a novel theoretical model characterizing errors from various sources in practical phase encoding-based QKD systems, namely the laser linewidth, detector dark counts, and channel dispersion. This model provides optimized optical pulse parameters and less distortion in pulses, which eliminates system imperfections and leads to a reduced quantum bit error rate (QBER) for practical QKD scenario. This analysis is applicable to various fiber-based phase and time encoding protocols. In particular, we implement this to a differential phase shift (DPS) QKD scheme operating at a 2.5 GHz clock, which produces a secure key rate of 193 bits/s at a fiber length of 265 km and an unprecedented QBER < 1[Formula: see text] up to 225 km length with standard telecom components. We show that by adjusting the quantum efficiency and dark count rates of detectors, proposed system can establish secure keys up to 380 km distance using standard telecom grade fiber with a QBER of 1.48%. Moreover, the system is compatible with existing optical fiber networks and capable of establishing a secure key exchange between two cities 432 km apart using ultra-low-loss (ULL) specialty fiber.