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Conference on Lasers and Electro-Optics

Experimental implementation of time-coding quantum key distribution at telecommunication wavelength

Olivier Morin, Simon Fossier, Thierry Debuisschert

Published Jan 1, 2009• 0 citations
Publisher

Abstract

A time-coding quantum key distribution protocol at telecommunication wavelength is investigated over a 25 km channel. Measured 98 % visibility and 2 % QBER enable 200 km security range and 1 kbit/sec secret key rates.
DOI
10.1364/cleo.2009.jwa107

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A six-user quantum key distribution network implemented on a bus topology is experimentally demonstrated. The network employs the BB84 protocol to transmit cryptographic keys encoded unto the phase states of highly attenuated laser light to distances of up to 31 km in a standard telecommunication-grade fiber. Each user on the network is assigned a unique wavelength for communication with the network server at a time. The measured quantum bit error rate and sifted key rate compare favorably with theoretical results
Quantum cryptography is a technique for establishing a fundamentally secure key for cryptography using the encoding of the key on single photon states in such a way that an eavesdropper cannot acquire any information about the key without impairing detectable errors in the communication channel [1]. In the recent years there has been a remarkable progress in experimental implementations at 850 nm and 1300 nm wavelengths with secret key generation at kbits/s up to 48 km distance. However, for 1550 nm wavelength, where fiber losses are as smallest, progress has been hampered by the lack of good single photon detectors.
We have implemented an experimental setup in order to demonstrate the feasibility of time-coding protocols for quantum key distribution. Alice produces coherent 20-ns faint pulses of light at 853 nm. They are sent to Bob with delay 0 ns (encoding bit 0) or 10 ns (encoding bit 1). Bob directs at random the received pulses to two different arms. In the first one, a 300 ps resolution Si photon counter allows Bob to precisely measure the detection times of each photon in order to establish the key. Comparing them with the emission times of the pulses sent by Alice allows one to evaluate the quantum bit error rate (QBER). The minimum obtained QBER is 1.62%. The possible loss of coherence in the setup can be exploited by Eve to eavesdrop the line. Therefore, the second arm of Bob setup is a Mach-Zehnder interferometer with a 10 ns propagation delay between the two paths. Contrast measurement of the output beams allows one to measure the autocorrelation function of the received pulses that characterizes their average coherence. In the case of an ideal setup, the value expected with the pulses sent by Alice is 0.576. The experimental value of the pulses autocorrelation function is found to be 0.541. Knowing the resulting loss of coherence and the measured QBER, one can evaluate the mutual information between Alice and Eve and the mutual information between Alice and Bob, in the case of intercept-resend attacks and in the case of attacks with intrication. With our values, Bob has an advantage on Eve of 0.43 bit per pulse. The maximum possible QBER corresponding to equal informations for Bob and Eve is 5.8%. With the usual attenuation of fibres at 850 nm, it shows that secure key distribution is possible up to a distance of 2.75 km, which is sufficient for local links.
Work presents the brief review of the contemporary state of quantum cryptography science. Main principles of the quantum cryptography protocol development are illustrated by giving the example of BB84 protocol. Examples of experimental setups for the quantum key distribution through the atmosphere and fiber quantum channel are described in this work. The work presents results of experiments conducted on the developed setup based on single photon polarization coding. Problem of single photon detection in the visible and near IR area is considered. Work describes developed fiber based quantum cryptography working with the telecom wavelength 1555 nm. With the use of this experimental setup using the 25 km single mode telecom fiber (1555 nm wavelength) between sender and receiver we achieved the 450 bit/s quantum key generation with the use of 5 MHz pulse repetition rate, ~0.2 photon/pulse. Given the achieved photodetector parameters an average quantum bit error rate is less than 3.7%.
A single‐chip dual‐wavelength coherent photon source is proposed to introduce additional parametric uncertainty and improve the security scaling of single‐photon differential binary‐phase‐shift‐keying quantum key distribution (D‐BPSK‐QKD). Simultaneous lasing of two longitudinal modes in the same semiconductor laser cavity generates two coherent photon groups at different wavelengths, and attenuation of their combined output to the single‐photon level inevitably produces wavelength uncertainty at the receiver. Optical injection control enables stabilized D‐BPSK encoding on one selected mode while the other mode remains free‐running, which supports mode‐selective coding and a decoy scenario during single‐photon streaming. This intermodal wavelength switching/selection provides another degree of freedom for QKD security and increases the difficulty of eavesdropping attacks. The dual‐wavelength single‐photon bits deliver D‐BPSK‐QKD codes with a QBER of <4.1% under 2 10 ‐bit encryption, a sifted key rate of 408.6 kbit/s, and a secure key rate (SKR) of 10.5 kbit/s at an average photon number of 0.22 photons#/bit. The transmission distance can be extended to more than 13.25 km with a SKR of 3.6 kbit/s under a nearly equivalent QBER. This versatile wavelength‐uncertain coding or decoying operation enables flexible and secure D‐BPSK‐QKD transmission over wavelength‐division‐multiplexed channelized single‐mode fiber links.