We have recently constructed a fiber-optic “plug and play” quantum cryptography system as shown in Fig.1, wherein a Faraday-Mirror was used to compensate the polarization mode dispersion and phase drifts. It worked at 1550 nm and was based on the B92 protocol[1]. We have achieved stable quantum key distribution in a standard telecom fiber with different working-distances of 10, 25 and 50 km, respectively, by using some specifically developed techniques to guarantee long-term stability of our system, such as polarization-independent phase-modulation for stable phase encoding and decoding, hybrid coincidence for efficient detection of single photons with coincidence-gated quenching of the single-photon detectors followed by coincident counting of the avalanche signals, and synchronized detection of single-photon pulses by coincidence counting gated with proper delay and width. These unique techniques greatly enhanced the fringe visibility of single-photon interference. And we have also demonstrated the application of our system in cryptographic communication by sending coded pictures from Alice to Bob using the sifted quantum keys.
We have recently constructed a fiber-optic “plug and play” quantum cryptography system as shown in Fig.1, wherein a Faraday-Mirror was used to compensate the polarization mode dispersion and phase drifts. It worked at 1550 nm and was based on the B92 protocol[1]. We have achieved stable quantum key distribution in a standard telecom fiber with different working-distances of 10, 25 and 50 km, respectively, by using some specifically developed techniques to guarantee long-term stability of our system, such as polarization-independent phase-modulation for stable phase encoding and decoding, hybrid coincidence for efficient detection of single photons with coincidence-gated quenching of the single-photon detectors followed by coincident counting of the avalanche signals, and synchronized detection of single-photon pulses by coincidence counting gated with proper delay and width. These unique techniques greatly enhanced the fringe visibility of single-photon interference. And we have also demonstrated the application of our system in cryptographic communication by sending coded pictures from Alice to Bob using the sifted quantum keys.
We present a time-multiplexed interferometer based on Faraday mirrors, and apply it to quantum key distribution. The interfering pulses follow exactly the same spatial path, ensuring very high stability and self balancing. Use of Faraday mirrors compensates automatically any birefringence effects and polarization dependent losses in the transmitting fiber. First experimental results show a fringe visibility of 0.9984 for a 23km-long interferometer, based on installed telecom fibers. In so-called private-key cryptographic systems, secure transmission through unprotected channels rely on the exchange of secret keys, known only to the sender, Alice, and the receiver, Bob. Quantum cryptography (QC) relies on the properties of quantum mechanics to obtain a provably secure key distribution [1, 2, 3, 4]. Most exisiting implementations rely on either the polarization [2, 5, 6] or the phase [7, 8, 9, 10] of very weak pulses of light as information carrier. To date, the longest transmission spans were obtained in optical fibers, at a wavelength of 1300 nm [6, 9, 10]. The main difficulty with polarization-based systems is the need to keep stable polarizations over distances of tens of kilometers, in standard telecom cables. Indeed, due to the birefringence of the fibers and the effect of the environment, the output polarization fluctuates randomly. Recent experiments [6]
A stable long-distance quantum key distribution system which has been just completed in our laboratory is demonstrated in this paper. The idea of “plug and pl ay" was adopted to automatically compensate the polarization mode dispersion and phase shift. A symmetrical capacitance-balance technique and magic-T network were used in the single-photon detection module, wherein a detection efficiency of more than 5% and a dark count probability of lower than 29×10-6/p ulse were achieved. As a result, we have realized stable (longer than 12 h) quantum k ey distribution experiment in 506 km single mode fiber with mean photon number of 007 and quantum bit error rate of 4%.
In so-called private-key cryptographic systems, secure transmission of confi dential information through unprotected channels relies on the exchange of secret keys, known only to the sender, Alice, and the receiver, Bob.Quantum cryptography is based on the properties of quantum mechanics to obtain a provably secure key distribution.Most existing implement ations rely on either the polarization or the phase of very weak pulses of light sent through optical fibers.Recent results by research groups in British Telecom, Los Alamos National Laboratory and at the University of Geneva, have shown that quantum key distribution is possible over distances of tens of kilometers, using only standard telecom cables.However, due to birefringence and the effects of the environment, the polarization at the output of a fiber fluctuates randomly.Therefore, all existing systems need active polarization control.Systems based on phase also require interferometric detection, which necessitates active stabilization of the interferometers.To eliminate these problems, our team, led by Prof. N. Gisin in the Group of Applied Physics at the University of Geneva, is currently developing a new interferometric system implementing phase-encoded quantum key distribution (US patent pending).A schematic of the setup is given in Fig. 1.Its main features are:1) The system is based on time multiplexing, the interfering pulses following exactly the same spatial path, albeit with a small time delay.The interferometer is thus automatically aligned, and needs neither adjustments nor stabilization of the path lengths.2) Use of Faraday mirrors enables all birefringence effects in the fibers to be cancelled out.Therefore, the system does not require any polarization control.With this new system, Alice and Bob could exchange their cryptographic keys through standard telecom systems.They would be provided with a sending kit and
Summary form only given. Quantum cryptography is a technique for establishing a fundamentally secure cryptographic key, using the encoding of the key on single photon pulses. In the recent years here has been a remarkable progress in experimental implementations at 1300 nm wavelength. However, for 1550 nm wavelength, there are few results, as the performance of single photon detectors for 1550 nm has been considered insufficient. We report on the implementation of a 1550 nm stable plug-and-play interferometric scheme for quantum cryptography, and for simplicity, we have only implemented a two-state encoding.
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.