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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Quantum cryptography on optical fiber networks

Paul D. Townsend

Published Jul 6, 1998• 2 citations
Publisher

Abstract

Quantum cryptography exploits the fact that an unknown quantum state cannot be accurately copied or measured without disturbance. By using such elementary quantum states to represent binary information it is possible, therefore, to construct communication systems with verifiable levels of security that are 'guaranteed' by fundamental quantum mechanical laws. This paper describes recent progress at BT Laboratories in the development of practical optical fiber- based quantum cryptography system. These developments include interferometric systems operating in the 1.3 micrometers - wavelength fiber transparency window over point-to-point links up to approximately 50km in length and on multi-user passive optical networks. We describe how this technology performs on fiber links installed in BT's public network and discuss issues such as cross-talk with conventional data channels propagating at different wavelengths in the same fiber.

DOI
10.1117/12.312632

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Quantum cryptography exploits the fact that an unknown quantum state cannot be accurately copied or measured without disturbance. By using such elementary quantum states to represent binary information it is possible, therefore, to construct communication systems with verifiable levels of security that are 'guaranteed' by fundamental quantum mechanical laws. This paper describes recent progress at BT Laboratories in the development of practical optical fiber- based quantum cryptography system. These developments include interferometric systems operating in the 1.3 micrometers - wavelength fiber transparency window over point-to-point links up to approximately 50km in length and on multi-user passive optical networks. We describe how this technology performs on fiber links installed in BT's public network and discuss issues such as cross-talk with conventional data channels propagating at different wavelengths in the same fiber.
Dense wavelength division multiplexing optical networks transport a huge aggregate traffic exceeding Tbps per single fiber, and passive optical network technology has been extended to include the residential and enterprise network layer with fiber-to-the premises. Because this network allows for multimedia and extremely high data rates to end-users, optical communications security is important not only on the client and the node management layers, but also on the physical layer. Thus, cyber-security becomes also important in optical networks, including unbreakable cipher keys, cipher key distribution, identification of malicious actors, source authentication, physical link signature, and countermeasure strategies. In this paper we overview the security of the WDM optical network. We discuss advanced cryptographic methods that stem from quantum mechanics, known as quantum cryptography, and we identify specific vulnerabilities. We also look into the security of the next-generation optical network.
The secure distribution of the secret random bit sequences known as {open_quotes}key{close_quotes} material, is an essential precursor to their use for the encryption and decryption of confidential communications. Quantum cryptography is an emerging technology for secure key distribution with single-photon transmissions, nor evade detection (eavesdropping raises the key error rate above a threshold value). We have developed experimental quantum cryptography systems based on the transmission of non-orthogonal single-photon states to generate shared key material over multi-kilometer optical fiber paths and over line-of-sight links. In both cases, key material is built up using the transmission of a single-photon per bit of an initial secret random sequence. A quantum-mechanically random subset of this sequence is identified, becoming the key material after a data reconciliation stage with the sender. In our optical fiber experiment we have performed quantum key distribution over 24-km of underground optical fiber using single-photon interference states, demonstrating that secure, real-time key generation over {open_quotes}open{close_quotes} multi-km node-to-node optical fiber communications links is possible. We have also constructed a quantum key distribution system for free-space, line-of-sight transmission using single-photon polarization states, which is currently undergoing laboratory testing. 7 figs.
The secure distribution of the secret random bit sequences known as `key' material, is an essential precursor to their use for the encryption and decryption of confidential communications. Quantum cryptography is an emerging technology for secure key distribution with single-photon transmissions: Heisenburg's uncertainty principle ensures that an adversary can neither successfully tap the key transmissions, nor evade detection (eavesdropping raises the key error rate above a threshold value). We have developed experimental quantum cryptography systems based on the transmission of non-orthogonal single-photon states to generate shared key material over multi-kilometer optical fiber paths and over line-of-sight links. In both cases, key material is built up using the transmission of a single- photon per bit of an initial secret random sequence. A quantum-mechanically random subset of this sequence is identified, becoming the key material after a data reconciliation stage with the sender. In our optical fiber experiment we have performed quantum key distribution over 24-km of underground optical fiber using single-photon interference states, demonstrating that secure, real-time key generation over `open' multi-km node-to-node optical fiber communications links is possible.
Keeping in mind the ubiquitous standard optical fiber for long‐distance transmission and the widespread availability of efficient active and passive fiber devices, we have been developing telecom‐band resources for practical quantum communication and cryptography in wave‐division‐multiplexed (WDM) optical networks. In this talk I present our recent results on two fronts: i) telecom‐band in‐fiber entanglement generation, storage, and long‐distance distribution and ii) quantum‐noise protected high‐speed data encryption through an optically‐amplified WDM line. Along the first front, with our in‐fiber entanglement source all four Bell states can be readily produced and we have demonstrated violation of Bell’s inequalities by up to 10 standard deviations of measurement uncertainty. With such a source we have demonstrated storage of entanglement for up to 1/8 of a millisecond. Furthermore, when each photon of the entangled pair is propagated in separate 25km‐long standard fibers, high visibility quantum interference is still observed, demonstrating that this system is capable of long‐distance (> 50 km) entanglement distribution. Along the second front, we have implemented a new quantum cryptographic scheme, based on Yuen’s KCQ protocol, in which the inherent quantum noise of coherent states of light is used to perform the cryptographic service of data encryption. In this scheme a legitimate receiver, with use of a short, shared, secret‐key, executes a simple binary decision rule on every transmitted bit. An eavesdropper, on the other hand, who does not possess the secret‐key, is subjected to an irreducible quantum uncertainty in each measurement, even with the use of ideal detectors. We have implemented this scheme to demonstrate quantum‐noise‐protected data encryption at 650 Mbps through a 200 km, in‐line amplified, WDM line. The line simultaneously carried two 10 Gbps standard data channels, 100 GHz on either side of the encrypted channel, which shows that this scheme is compatible with the widely deployed WDM fiber‐optic infrastructure.