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Electronics Letters

Single photon interference in 10 km long optical fibre interferometer

Paul D. Townsend, John Rarity, P. R. Tapster

Published Apr 1, 1993• 284 citations
Publisher

Abstract

Single-photon interference fringes with greater than 90% visibility were measured using a 10 km long optical fibre interferometer. The experiment employed a pulsed semiconductor laser source operating at a wavelength of 1.3 μm and a novel single-photon counting scheme using high-speed germanium avalanche photodiodes. Interferometers of this type could form the basis of future quantum cryptography systems.

DOI
10.1049/el:19930424

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Single-photon interference fringes with greater than 90% visibility were measured using a 10 km long optical fibre interferometer. The experiment employed a pulsed semiconductor laser source operating at a wavelength of 1.3 μm and a novel single-photon counting scheme using high-speed germanium avalanche photodiodes. Interferometers of this type could form the basis of future quantum cryptography systems.
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.
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
We experimentally demonstrate stable and user-adjustable single-photon interference in a 1 km long fiber-optic Mach-Zehnder interferometer, using an active phase control system with the feedback provided by a classical laser. We are able to continuously tune the single-photon phase difference between the interferometer arms using a phase modulator, which is synchronized with the gate window of the single-photon detectors. The phase control system employs a piezoelectric fiber stretcher to stabilize the phase drift in the interferometer. A single-photon net visibility of 0.97 is obtained, yielding future possibilities for experimental realizations of quantum repeaters in optical fibers and violation of Bell's inequalities using genuine energy-time entanglement.