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

A novel switched interferometric quantum key distribution system

Philip A. Hiskett, Charles G. Peterson, Danna Rosenberg, Sae Woo Nam, Adriana E. Lita, Aaron Miller, Richard Hughes, J. E. Nordholt

Published Sep 11, 2007• 1 citation
Publisher

Abstract

A novel system for ultra-long-distance quantum key distribution in optical fiber, incorporating ultra-low-noise transition-edge sensor (TES) photodetectors, is described. Integration of the TES detectors into the system was facilitated with a unique optically switched interferometer design. The performance of the system over 101 km of dark, single-mode fiber at 1550 nm and a clock rate of 1 MHz is described. Secret-key bits were produced after error correction and privacy amplification when using mean photon numbers of 0.01, 0.0148, 0.02, 0.0304, and 0.2 photons/pulse at the output of the transmitter. At a mean photon number of 0.1 photons per pulse at the transmitter, a transmission line loss of 29.92 dB, roughly equivalent to 150 km of optical fiber, could be tolerated and secret bits extracted from the transmitted key.

DOI
10.1117/12.733265

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Jan 11, 2005•0 citations
A novel system for ultra-long-distance quantum key distribution in optical fiber, incorporating ultra-low-noise transition-edge sensor (TES) photodetectors, is described. Integration of the TES detectors into the system was facilitated with a unique optically switched interferometer design. The performance of the system over 101 km of dark, single-mode fiber at 1550 nm and a clock rate of 1 MHz is described. Secret-key bits were produced after error correction and privacy amplification when using mean photon numbers of 0.01, 0.0148, 0.02, 0.0304, and 0.2 photons/pulse at the output of the transmitter. At a mean photon number of 0.1 photons per pulse at the transmitter, a transmission line loss of 29.92 dB, roughly equivalent to 150 km of optical fiber, could be tolerated and secret bits extracted from the transmitted key.
Use of low-noise detectors can both increase the secret bit rate of long-distance quantum key distribution (QKD) and dramatically extend the length of a fibre optic link over which secure keys can be distributed. Previous work has demonstrated the use of ultra-low-noise transition-edge sensors (TESs) in a QKD system with transmission over 50 km. In this study, we demonstrate the potential of the TESs by successfully generating an error-corrected, privacy-amplified key over 148.7 km of dark optical fibre at a mean photon number μ = 0.1, or 184.6 km of dark optical fibre at a mean photon number of 0.5. We have also exchanged secret keys over 67.5 km that is secure against powerful photon-number-splitting (PNS) attacks.
A novel, user-friendly quantum key distribution (QKD) system operating at a wavelength of 1550nm and at a clock rate of 10MHz was constructed to explore the compatibility of this emerging technology with the optical fiber network environment. Custom circuit boards providing the low-level control and sensing functions for both the transmitter and receiver were developed, allowing software-based system reconfiguration via USB interface to personal computers. The computer control allowed the user to change operating parameters such as detector bias voltages and pulse delays and also allowed for self-tuning of the system. Epitaxx avalanche photodiodes, operated in Geiger mode, were used to detect the single photons. A complete QKD protocol stack incorporating the sifting, reconciliation, privacy amplification, authentication and key confirmation functions was implemented in software. The system was tested over twenty five kilometers of dark underground fiber, producing 18.6 million sifted bits, with a sifted bit error rate of 4.9% at an average number of photons per pulse of 0.2, during a continuous 12-hour period of self-sustaining operation: a small portion of the secret bits distilled from each session's sifted bits were used to authenticate the next session. A total of 6.8 million shared secret bits were produced.
Our system is based upon a Mach-Zender interferometer using phase modulation. Photons are generated by a 1.55 pm DFB pulsed laser diode operating with a repetition rate of 2 MHz. The pulses are strongly attenuated so that on average 0. I photons/clock cycle leaves the senders apparatus. Phase modulators. controlled by custom electronics, in the two interfering arms were used to encode the bit information. The signal is multiplexed with pulses from a 1.3 pm clock laser which serves as a timing reference. Cooled InGaAs avalanche photodiodes operating in Geiger mode are used to detect the single photons. It is imperative for operation over long fibres that the dark count rate in the single photon detector is as low as possible. Our detectors typically have a dark count probability of 3.2~10.~ er gate, along with a detection efficiency of 12% at 1.55 pm. This yields a Noise Equivalent Power of I .OXIO-'~ WHz- , whcch IS one of the lowest reported to date. R ' . The interferometer displayed a single photon interference fringe visibility between 99.1% and 99.7% for fibre lengths up to 65km. For longer fibre lengths the visibility falls, but remains around 96% at IOlkm, see Figure 1. The QBER, shown in Figure 2, remains virtually constant at around 3.6% for fibre length up to 50km, suggesting that the dominant contribution derives from inaccuracy in the phase modulation. Beyond 50km the QBER increases with fibre length, due to dark count noise. At 101 km, the QBER averaged over a 2-minute key transfer is typically 7. I %. This error rate is below the 1 I .5% limit, thus allowing us to perform error correction and privacy amplification to form a secure key. The sifted bit rate (shown in Figure 2) decreases with increasing fibre length at a rate of -0.20 dB/km, close to the specified value. At IOOkm the average bit rate was typically 15 Hz, sufficient to allow a cryptographic key to be transferred in several tens of seconds. 100
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.