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Acta Sinica Quantum Optica

A Simple, Stable Experimental System forQuantum Key Distribution (QKD) in Free Space

Cheng-Zu Li

Published Jan 1, 2004• 0 citations
OpenAlex

Abstract

Basing on to B92 protocol, an experimental free-space quantum key distribution (QKD) system has been developed. Based on polarization encoding, we used weak coherent laser of average photon 0.1 per pulse as key carrier, and got a raw key rate of about 12.7 kbit/s and a qubit-error rate (QBER) of about 4%. It shows us a simple, stable, practical and user-friendly system.

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Basing on to B92 protocol, an experimental free-space quantum key distribution (QKD) system has been developed. Based on polarization encoding, we used weak coherent laser of average photon 0.1 per pulse as key carrier, and got a raw key rate of about 12.7 kbit/s and a qubit-error rate (QBER) of about 4%. It shows us a simple, stable, practical and user-friendly system.
Quantum key distribution (QKD) has emerged as a revolutionary technique that can ensure unconditionally secure communication between two distant parties, referred to as Alice and Bob, by exploiting the fundamental principles of quantum physics. Here, we report experimental demonstration of a fully automated QKD system based on the BB84 protocol. A weak coherent pulse source is developed to generate stream of polarization encoded single photons. Our QKD system demonstrates a low QBER of ∼1.2% with sifted key generation rate of ∼70 Kbps at an average photon number per pulse ofμ=0.11\mu=0.11μ=0.11. The final secure key rate of ∼33 Kbps is achieved after performing post processing procedures viz. error correction and privacy amplification. The quantum communication link is established continuously for several hours and the performance is evaluated iteratively. The developed QKD system is shown to have the capability of distributing cryptographic keys securely and seamlessly, under practical operating conditions. Furthermore, we have successfully demonstrated unconditionally secure transmission and reception of an image between Alice and Bob connected over an unsecured public channel by utilizing quantum cryptography.
We report on the progress of our real-time entanglement based free-space quantum key distribution (QKD) system which uses polarization entangled photon pairs sent over a variety of free-space optical telescope links to distribute the key. An experiment with one photon from each pair sent over a 1,325 m long free-space link and the other photon detected locally next to the source is described. The system performs the complete QKD protocol including all error correction and privacy amplification algorithms. Over the course of 6.5 hours of communication at night an average raw key rate of 1,398 bits/s with an average quantum bit error rate (QBER) of 4.58% was observed producing an average final key rate of 244 bits/s. We also performed a Bell inequality experiment over two free-space links of 435 m and 1,325 m respectively, producing a total separation of 1,575 m between the two detectors in the experiment. During the Bell inequality experiment we observed an average Bell parameter of 2.51 ± 0.11.
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.
Employing a polarization compensator, an optical fibre quantum key distribution (QKD) system based on polarization coding has been developed. To obtain the compensator setting parameters, the measurement of the laser pulse polarization is performed with one single photon detector. We obtain a sifted key bit rate of about 2 kbits/s and a qubit error rate lower than 10% within 3.5 h. It is shown that polarization coding can be used for QKD over optical fibres as well. At the same time, the system is simple, easy to operate, practical and user-friendly. It gains more advantages than other systems over optical fibres when used in local area quantum communications and where the functional agility is important.
Quantum Key Distribution (QKD) has become an essential technology in the realm of secure communication, with applications ranging from secure data transmission to quantum networks. This paper presents a simple, compact, and cost-effective setup for undergraduate tutorial demonstrations of QKD. It relies on using weak coherent pulses, which can be readily produced using an attenuated laser. The system employs the simplified three-state BB84 protocol in free space, the states are encoded using linear polarization. Polarization encoding can be done passively or actively, depending on the budget available. Time multiplexing is implemented at the receiver to reduce the number of required detectors. Only two detectors are used to implement measurements on two bases, with a total of four outcomes. The result demonstrates the practicality of the system for free-space quantum communication, and its compact and portable nature makes it particularly suitable for pedagogical demonstrations. This work paves the way for engaging undergraduate students in the field of quantum communication through hands-on laboratory projects.