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International Conference on Transparent Optical Networks

Photon statistics determination for single photon based quantum key distribution

Ágoston Schranz, Eszter Udvary, Z. Kis

Published Jul 1, 2016• 3 citations
Publisher

Abstract

In practice the most advanced field of quantum information science is quantum cryptography. The best known example of quantum cryptography is quantum key distribution (QKD) which offers an information-theoretically secure solution to the key exchange problem. There are QKD protocols which rely on a quantum channel that is able to transmit single photons. This can be implemented by using very low-power coherent light field, where values of bits are encoded into the polarization state of photons. Therefore, it is important to know the photon number distribution of the light source. In this paper we present the reconstruction of photon statistics for different pulsed light sources radiating at 850 nm (a common choice for free space quantum communication), using only a single on-off photon counting detector.

DOI
10.1109/icton.2016.7550483

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In practice the most advanced field of quantum information science is quantum cryptography. The best known example of quantum cryptography is quantum key distribution (QKD) which offers an information-theoretically secure solution to the key exchange problem. There are QKD protocols which rely on a quantum channel that is able to transmit single photons. This can be implemented by using very low-power coherent light field, where values of bits are encoded into the polarization state of photons. Therefore, it is important to know the photon number distribution of the light source. In this paper we present the reconstruction of photon statistics for different pulsed light sources radiating at 850 nm (a common choice for free space quantum communication), using only a single on-off photon counting detector.
Quantum key distribution (QKD) theoretically offers unconditional security. Unfortunately, the gap between theory and practice threatens side-channel attacks on practical QKD systems. Many well-known QKD protocols use weak coherent laser pulses to encode the quantum information. These sources differ from ideal single photon sources and follow Poisson statistics. Many protocols, such as decoy state and coincidence detection protocols, rely on monitoring the photon statistics to detect any information leakage. The accurate measurement and characterization of photon statistics enable the detection of adversarial attacks and the estimation of secure key rates, strengthening the overall security of the QKD system. We have rigorously characterized our source to estimate the mean photon number employing multiple detectors for comparison against measurements made with a single detector. Furthermore, we have also studied intensity fluctuations to help identify and mitigate any potential information leakage due to state preparation flaws. We aim to bridge the gap between theory and practice to achieve information-theoretic security.
Abstract In quantum cryptography, fundamental laws of quantum physics are exploited to enhance the security of cryptographic tasks. Quantum key distribution (QKD) is by far the most studied protocol to date, enabling the establishment of a secret key between trusted parties. Many practical use-cases in communication networks, however, involve parties who do not trust each other. The most fundamental quantum cryptographic building block in such a distrustful setting is quantum coin flipping, which has been studied only in few experimental studies to date, all of which used probabilistic quantum light sources facing fundamental limitations. Here, we experimentally implement a quantum strong coin flipping protocol using single-photon states and demonstrate an advantage compared to both classical realizations and implementations using faint laser pulses. We achieve this by employing a state-of-the-art deterministic single-photon source based on the Purcell-enhanced emission of a semiconductor quantum dot in combination with fast polarization-state encoding with sufficiently low quantum bit error ratio. The reduced multi-photon emission of the single-photon source yields a smaller bias of the coin flipping protocol compared to an attenuated laser implementation, both in simulations and in the experiment. By demonstrating a single-photon quantum advantage in a cryptographic primitive beyond QKD, our work represents a major advance towards the implementation of complex cryptographic tasks in a future quantum internet.
The goal of cryptography is to encode data in such a way that no one can decode it unless they are the intended recipient. With the emergence of quantum computing, classical cryptography is not reliable enough to protect sensitive information. Nevertheless, there are alternatives. Quantum Key Distribution (QKD) is a method to secure communication based on the laws of quantum physics. Due to being impossible to measure the quantum state of a system without disturbing it, QKD provides a way to communicate between two parties while being, in theory, immune to eavesdropping. In this work, a free-space high-speed QKD setup is proposed, with the objective of implementing the polarization three-state one-decoy variant of the BB84 protocol. An assessment of the best type of laser diode to use for free-space QKD is made between several options. The system’s performance was assessed, incorporating a quantitative analysis of system losses and an estimation of the number of photons traversing the quantum channel. The system achieved a Quantum Bit Error Rate (QBER) of 1.58% for the Z basis and 1.46% for the X basis, for an average reception of two photons per pulse. Preliminary QBER measurements were also undertaken in the context of Quantum Keyless Private Communication protocols, focusing on the average number of photons per pulse.
Quantum key distribution introduces a new way of key exchange through a public quantum channel between two communicating nodes. However, the first introduced BB84 protocol was theoretically verifying security based on single photon sources. At this time, it was hard to achieve such device. Therefore, it was a challenging aspect to prove the security of QKD system using coherent light sources or “multi emitting photon source”, which introduces Photon number splitting attacks. We are interested to simulate and investigate the QKD system using coherent laser sources and study the PNS attack. The idea involves that; Eve can crash a beam of light, absorbing some photons using QND measurement and transmit the others. Such attack can make Eve dominant to the situation and gain much information of the key without the fear of getting exposed to the communicating parties. Herein, this paper discusses some useful solution to detect such attack and proves the security of QKD even with weak laser sources.
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%.
Quantum cryptography allows distributing secure keys between two users so that any performed eavesdropping attempt would be immediately discovered. However, in practice an eavesdropper can obtain key information from multi-photon states when attenuated laser radiation is used as a source of quantum states. In order to prevent actions of an eavesdropper, it is generally suggested to implement special cryptographic protocols, like decoy states or SARG04. In this paper, we describe an alternative method based on monitoring photon number statistics after detection. We provide a useful rule of thumb to estimate approximate order of difference of expected distribution and distribution in case of attack. Formula for calculating a minimum value of total pulses or time-gaps to resolve attack is shown. Also formulas for actual fraction of raw key known to Eve were derived. This method can therefore be used with any system and even combining with mentioned special protocols.