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International Conference on Computer and Communication Engineering

Characterization of mean photon number for key distribution system using faint laser

Salim Ali Al-Kathiri, Wajdi Fawzi Mohammed Al-Khateeb, Mohd Hafizulfika, Mohamed Ridza Wahiddin, Suhairi Saharudin

Published May 1, 2008• 3 citations
Publisher

Abstract

Key distribution system using faint laser is very likely to become the first application exploiting the particular properties of single quanta called photons. In experimental key distribution system, it is highly demanded to use single-photon source for secure key distribution. Since there is not yet a laser source capable of producing such faint state, most experiments uses low coherent pulse laser, which is attenuated to produce a mean photon number mu. In this paper, we present an experimental setup to determine the mean photon number using a 300-ps low coherent pulse laser at 1550 nm for a practical crypto system. We will also present the effect of trigger frequency, detection efficiency and dark counts on average photon counts.

DOI
10.1109/iccce.2008.4580803

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Key distribution system using faint laser is very likely to become the first application exploiting the particular properties of single quanta called photons. In experimental key distribution system, it is highly demanded to use single-photon source for secure key distribution. Since there is not yet a laser source capable of producing such faint state, most experiments uses low coherent pulse laser, which is attenuated to produce a mean photon number mu. In this paper, we present an experimental setup to determine the mean photon number using a 300-ps low coherent pulse laser at 1550 nm for a practical crypto system. We will also present the effect of trigger frequency, detection efficiency and dark counts on average photon counts.
The demand for single photon sources in quantum key distribution (QKD) systems has necessitated the use of weak coherent pulses (WCPs) characterized by a Poissonian distribution. Ensuring security against eavesdropping attacks requires keeping the mean photon number (µ) small and known to legitimate partners. However, accurately determining µ poses challenges due to discrepancies between theoretical calculations and practical implementation. This paper introduces two experiments. The first experiment involves theoretical calculations of µ using several filters to generate the WCPs. The second experiment utilizes a variable attenuator to generate the WCPs, and the value of µ was estimated from the photons detected by the BB84 detection setup. The second experiment represents an accurate method for estimating the value of µ because of using single photon detectors with high timing resolution and low dark counts, in addition to using a Time-to-digital convertor with a bin size of 81 ps.
Among various implementations of single-photon sources, faint-pulse single-photon sources provide a simple and practical approach based on attenuated coherent laser pulses. This work presents an experimental realization of a faint-pulse single-photon source using a 785-nm Grating Stabilized Laser Diode system, driven by a laser diode controller and modulated by a Field Programmable Gate Array (FPGA). The system's performance was evaluated by analyzing the photon detection rate as a function of pulse width and driving current. Results indicated a linear relationship between the single-photon detection rate and pulse width and an exponential dependence on driving current. The system achieved a photon emission probability of 10% per pulse, aligning with the requirements of practical quantum key distribution systems. This implementation demonstrates a cost-effective, scalable, and accessible platform for single-photon experiments, providing a foundation for future implementations of quantum communication protocols.
Single photon source is indispensable for realization of photon based quantum communication technologies such as quantum key distribution (QKD). Weak coherent pulse source is a practical, efficient and robust solution to generate stream of single photons, probabilistically. In this paper, a methodology for design and development of weak coherent pulse (WCP) source with high count rate of upto 10 Mcps is proposed and implemented. The generation of optical pulses with narrow pulse width and repetition rate of up to 100 MHz is achieved by developing a miniaturized pulse laser module. The weak coherent pulses are generated by heavily attenuating the laser pulses so as to reach the mean photon no. per pulse, μ, close to 0.1. WCP source performance in terms of mean photon number, second order coherence and photon count rate is also investigated. The measured g(2)[0] of ~0.98 shows that the developed WCP source has a very good quality.
This paper presents a brief review of experimental studies in quantum cryptography and quantum key distribution by single photons in atmospheric and fiber-optic quantum communication channels. Two experimental setups for quantum key distribution developed at the Rzhanov Institute of Semiconductor Physics SB RAS are described. The dependence of the quantum key distribution rate on the average number of photons μ in the laser pulse was studied. For μ > 0.3, there is a discrepancy between theory and experiment. The reasons for this are, first, the nonzero probability of multiphoton pulses occurring in quantum transmission and counted as single photons by single-photon detectors and, second, the rejection of the cases where several single-photon detectors click simultaneously in quantum key sifting because the measurement result is not determined in these cases.
Decoy-state quantum key distribution (QKD) requires information about the photon number distribution of the light sources. The photon number distribution of weak coherent photon sources has been assumed to follow a Poisson distribution. However, it is not yet clear whether picosecond laser pulses, used in high-speed QKD systems, satisfy this assumption due to the significant and rapid changes in carrier density and photon density in the laser cavity. It is necessary to establish a method to experimentally evaluate the photon number distribution of laser pulses with a duration of less than 100 ps. We have implemented three methods to identify a suitable approach for evaluating light sources in quantum key distribution systems. Our findings indicate that all three methods conclude that the photon numbers of the gain-switching laser pulse are consistent with a Poisson distribution. We then compared these methods based on their accuracy of photon number probability and ease of implementation. These considerations will establish a method suitable for the security certification of high-speed quantum key distributed devices.
A measurement-device-independent quantum key distribution (MDI-QKD) protocol is immune to all detection side-channel attacks and guarantees the information-theoretical security even with uncharacterized single photon detectors. A weak coherent source is used in the current MDI-QKD experiments, it inevitably contains a certain percentage of vacuum and multi-photon pulses. The security issues introduced by these source imperfections can be avoided by applying the decoy state method. Here, through modeling experimental devices, and taking into account the weak coherent source and the threshold detectors, we have evaluated the gain, the probability to get successful Bell measurement and incorrect Bell measurement, and the quantum bit error rate (QBER), given a practical setup. In our simulation, we show how QBER varies with different transmission distances in the cases when the average photon numbers per pulse from Alice and Bob are symmetric and asymmetric. Result shows that the multi-photon pulses do not cause error in the Z basis of polarization encoding scheme, but produce a large QBER in phase encoding scheme and in the X basis of polarization encoding scheme. QBER is affected by the dark count rate and the system optical error associated with the multi-photon pulses. For different encoding schemes, QBER caused by each kind of average photon numbers from Alice and Bob increases to different degrees with the transmission distance, and finally is close to 50%. With the increase of the transmission distance, the average photon number per pulse decreases and the fraction of the dark count rate causing QBER gradually increases. Under the same effect of the dark count rate, the smaller the average photon number per pulse, the bigger the QBER. After a certain transmission and at the same transmission distance, the QBER is largest when average photon numbers used by Alice and Bob are both smallest. For the short distance transmission of phase encoding scheme and the X basis, we find that QBER is larger when average photon numbers from the two arms are asymmetric, as compared to the symmetric case. For the Z basis, the QBER caused by the system optical error and the dark count rate is very small.