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Optimal Receiver for Quantum Cryptography with Two Coherent States

Konrad Banaszek, Bs U

Published Jan 1, 1998• 28 citations
OpenAlex

Abstract

A setup for discriminating between arbitrary two coherent states of a single light mode with the highest success rate allowed by quantum mechanics is presented. Its application to time-multiplexed quantum key distribution is discussed. PACS numbers: 03.67.Dd, 42.50.\\Gammap, 03.65.Bz From the conceptual point of view, the simplest quantum protocol for establishing a cryptographic key is a transmission of a random sequence of two non-orthogonal states [1]. What makes this protocol secure, is the impossibility of discriminating between these two states with certainty. Thus, any attempt of eavesdropping inevitably generates noise in the transmission channel, which can be detected by the communicating parties [2]. A straightforward optical realisation of this protocol is the use of two weak coherent states in an interferometric setup [1,3]. In the two-state protocol, the cryptographic key is constructed from these events, for which the receiving party managed to identify unambiguously the o...

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A setup for discriminating between arbitrary two coherent states of a single light mode with the highest success rate allowed by quantum mechanics is presented. Its application to time-multiplexed quantum key distribution is discussed. PACS numbers: 03.67.Dd, 42.50.\Gammap, 03.65.Bz From the conceptual point of view, the simplest quantum protocol for establishing a cryptographic key is a transmission of a random sequence of two non-orthogonal states [1]. What makes this protocol secure, is the impossibility of discriminating between these two states with certainty. Thus, any attempt of eavesdropping inevitably generates noise in the transmission channel, which can be detected by the communicating parties [2]. A straightforward optical realisation of this protocol is the use of two weak coherent states in an interferometric setup [1,3]. In the two-state protocol, the cryptographic key is constructed from these events, for which the receiving party managed to identify unambiguously the o...
We suggest a protocol for quantum key distribution—a technology allowing two distant parties to create an unconditionally secure cryptographic key. For the creation of the key we suggest to use laser pulses weakened to the single-photon level of duration T , the pulse carrying the value “1” being shifted in time by T /2 compared to the pulse carrying the value “0”. The overlap of the pulses provides their non-orthogonality and, therefore, impossibility to discriminate between them with certainty. Besides the signal pulses the protocol uses coherent decoy pulses, having longer duration than the signal ones and providing a more effective protection from a wide class of attacks. Security of the protocol is based on interferometric control of the pulse coherence at the receiving station. We analyze the security of the protocol against a number of intercept-resend attacks and on the basis of this analysis substantiate the necessity of decoy state implementation.
A new family of two-parameter quantum key distribution protocols is discussed where eavesdropping is detected by using two parameters: bit error rate Q and photon count rate q in control time slots. When a single-photon source is used and mutually orthogonal states are prepared in each basis, the protocol’s maximum tolerable error rate for secure key distribution is the highest, reaching a theoretical upper limit of 50%. When the signal states emitted by the source of attenuated laser light include multiphoton coherent states, the protocol also guarantees secure key distribution over the longest distance as compared to other quantum cryptography systems, up to the channel length for which the channel losses are sufficiently high that all five-photon pulses can be blocked by an eavesdropper.
The cryptographic communication has a lot of important applications, particularly in the magnificent prospects of private communication. As one knows, the security of cryptographic channel depends crucially on the secrecy of the key. The Vernam cipher is the only cipher system which has guaranteed security. In that system the key must be as long as the message and most be used only once. Quantum cryptography is a method whereby key secrecy can be guaranteed by a physical law. So it is impossible, even in principle, to eavesdrop on such channels. Quantum cryptography has been developed in recent years. Up to now, many schemes of quantum cryptography have been proposed. Now one of the main problems in this field is how to increase transmission distance. In order to use quantum nature of light, up to now proposed schemes all use very dim light pulses. The average photon number is about 0.1. Because of the loss of the optical fiber, it is difficult for the quantum cryptography based on one photon level or on dim light to realize quantum key-distribution over long distance. A quantum key distribution based on coherent state is introduced in this paper. Here we discuss the feasibility and security of this scheme.
The safety of a quantum key distribution system relies on the fact that any eavesdropping attempt on the quantum channel creates errors in the transmission. For a given error rate, the amount of information that may have leaked to the eavesdropper depends on both the particular system and the eavesdropping strategy. In this work, we discuss quantum cryptographic protocols based on the transmission of weak coherent states and present a new system, based on a symbiosis of two existing ones, and for which the information available to the eavesdropper is significantly reduced. This system is therefore safer than the two previous ones. We also suggest a possible 1 experimental implementation.
The safety of a quantum key distribution system relies on the fact that any eavesdropping attempt on the quantum channel creates errors in the transmission. For a given error rate, the amount of information that may have leaked to the eavesdropper depends on both the particular system and the eavesdropping strategy. In this work, we discuss quantum cryptographic protocols based on the transmission of weak coherent states and present a system, based on a symbiosis of two existing systems, for which the information available to the eavesdropper is significantly reduced. This system is therefore safer than the two previous ones. We also suggest a possible experimental implementation.
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.