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International Journal of Quantum Information

PRACTICAL QUANTUM KEY DISTRIBUTION USING POLARIZATION ENTANGLED STATES

Fabio Antonio Bovino, Pietro Varisco, Anna Martinoli, Paolo De Nicolo, Sandra Bruzzo, A.M. Colla, Giuseppe Castagnoli, Giovanni Di Giuseppe, Alexander V. Sergienko

Published Dec 17, 2004• 3 citations
Publisher

Abstract

We present the architecture and recent experimental results for a quantum key distribution system realized at Elsag spa Quantum Optics Laboratory with a key distribution rate suitable for practical industrial applications. The current system can reliably distribute secure cryptographic keys at a rate of 1,500 bit per second and higher at a few hundred meters, with a quantum bit error rate lower than 1%.

DOI
10.1142/s0219749905000670

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We present the architecture and recent experimental results for a quantum key distribution system realized at Elsag spa Quantum Optics Laboratory with a key distribution rate suitable for practical industrial applications. The current system can reliably distribute secure cryptographic keys at a rate of 1,500 bit per second and higher at a few hundred meters, with a quantum bit error rate lower than 1%.
We present results of experimental demonstration of secure Quantum Key Distribution (QKD) at Elsag spa based on the implementation of BB84 protocol using polarization entangled states produced in the nonlinear process of type-II spontaneous parametric down conversion (SPDC). This enables us to avoid the use of active polarization modulation components and increases the overall key distribution rate. The high quality of polarization-entangled state generated by parametric down conversion and the high efficiency of coupling entangled-photon pairs into a single-mode optical fiber has enabled us to perform QKD with quantum bit-error rate compatible with acceptable security levels. The complete software system architecture includes a QKD protocol implementing all phases of the key distillation process. The system runs in a server and two users configuration on three different PCs connected over a local area network (LAN). Friendly graphical user interfaces (GUI) are available to start and to monitor the whole key generation and distillation process.
In recent years quantum information research has lead to the discovery of a number of remarkable new paradigms for information processing and communication. These developments include quantum cryptography schemes that offer unconditionally secure information transport guaranteed by quantum-mechanical laws. Such potentially disruptive security technologies could be of high strategic and economic value in the future. Two major issues confronting researchers in this field are the transmission range (typically <100km) and the key exchange rate, which can be as low as a few bits per second at long optical fiber distances. This paper describes further research of an approach to significantly enhance the key exchange rate in an optical fiber system at distances in the range of 1-20km. We will present results on a number of application scenarios, including point-to-point links and multi-user networks. Quantum key distribution systems have been developed, which use standard telecommunications optical fiber, and which are capable of operating at clock rates of up to 2GHz. They implement a polarization-encoded version of the B92 protocol and employ vertical-cavity surface-emitting lasers with emission wavelengths of 850 nm as weak coherent light sources, as well as silicon single-photon avalanche diodes as the single photon detectors. The point-to-point quantum key distribution system exhibited a quantum bit error rate of 1.4%, and an estimated net bit rate greater than 100,000 bits-1 for a 4.2 km transmission range.
Employing the fundamental laws of quantum physics, Quantum Key Distribution (QKD) promises the unconditionally secure distribution of cryptographic keys. However, in practical realisations, a QKD protocol is only secure, when the quantum bit error rate introduced by an eavesdropper unavoidably exceeds the system error rate. This condition guarantees that an eavesdropper cannot disguise his presence by simply replacing the original transmission line with a less faulty one. Unfortunately, this condition also limits the possible distance between the communicating parties, Alice and Bob, to a few hundred kilometers. To overcome this problem, we design a QKD protocol which allows Alice and Bob to distinguish system errors from eavesdropping errors. If they are able to identify the origin of their errors, they can detect eavesdropping even when the system error rate exceeds the eavesdropping error rate. To achieve this, the proposed protocol employs an alternative encoding of information in two-dimensional photon states. Errors manifest themselves as quantum bit and as index transmission errors with a distinct correlation between them in case of intercept-resend eavesdropping. As a result, Alice and Bob can tolerate lower eavesdropping and higher system error rates without compromising their privacy.