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Conference on Lasers and Electro-Optics

Quantum cryptography with bright entangled beams

Christine Silberhorn, Natalia Korolkova, Gerd Leuchs

Published Aug 29, 2005• 4 citations
Publisher

Abstract

Summary form only given. Recently, a scheme for the generation of bright entangled beams has been suggested. The EPR-like quantum correlations are produced by letting two amplitude-squeezed bright optical fields interfere on a 50:50 beamsplitter. The phase (PM) and amplitude (AM) uncertainty measurements exhibit strong correlations and anticorrelations which can be detected by recording the difference (sum) of the PM (AM) photocurrents. In this paper we present a simple version of the quantum key distribution protocol. We believe that the suggested scheme provides an efficient and novel approach to continuous variable quantum communication.

DOI
10.1109/iqec.2000.907757

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Summary form only given. Recently, a scheme for the generation of bright entangled beams has been suggested. The EPR-like quantum correlations are produced by letting two amplitude-squeezed bright optical fields interfere on a 50:50 beamsplitter. The phase (PM) and amplitude (AM) uncertainty measurements exhibit strong correlations and anticorrelations which can be detected by recording the difference (sum) of the PM (AM) photocurrents. In this paper we present a simple version of the quantum key distribution protocol. We believe that the suggested scheme provides an efficient and novel approach to continuous variable quantum communication.
Quantum cryptography with the predetermined key was experimentally realized using Einstein-Podolsky-Rosen(EPR) correlations of continuously bright optical beams. Only one of two EPR correlated beams is transmitted with the signals modulated on quadrature phases and amplitudes, and the other one is retained by the authorized receiver. The modulated small signals are totally submerged in the large quantum noise of the signal beam, therefore nobody except the authorized receiver can decode the signals. Usability of imperfect quantum correlation, high transmission and detection efficiencies, and security provided by quantum mechanics are the favorable features of the presented scheme.
In this paper we present a scheme in which the amplitude-quadrature difference and phase-quadrature difference of two bright nonclassical beams serve as a pair of observed complementary variables that can be measured directly and can form a new squeezed state. Mixing such two quadrature-difference squeezed-state beams on a beam splitter produces characteristic of the Einstein-Podolsky-Rosen quantum entanglement. The experimental schemes producing such nonclassical beams and its possible applications in quantum information are discussed.
We proposed a scheme of continuous-variable quantum key distribution, in which the bright Einstein-Podolsky-Rosen entangled optical beams are utilized. The source of the entangled beams is placed inside the receiving station, where half of the entangled beams are transmitted with round trip and the other half are retained by the receiver. The amplitude and phase signals modulated on the signal beam by the sender are simultaneously extracted by the authorized receiver with the scheme of the dense-coding correlation measurement for continuous quantum variables, thus the channel capacity is significantly improved. Two kinds of possible eavesdropping are discussed. The mutual information and the secret key rates are calculated and compared with those of unidirectional transmission schemes.
Correlations of the type discussed by EPR in their original 1935 paradox for continuous variables exist for the quadrature phase amplitudes of two spatially separated fields. These correlations were first experimentally reported in 1992. We propose to use such EPR beams in quantum cryptography, to transmit with high efficiency messages in such a way that the receiver and sender may later determine whether eavesdropping has occurred. The merit of the new proposal is in the possibility of transmitting a reasonably secure yet predetermined key. This would allow relay of a cryptographic key over long distances in the presence of lossy channels.