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Optical Fiber Communication Conference

Quantum cryptography based on realistic single-photon source

Jan Peřina, Ondřej Haderka, Jan Soubusta

Published Jan 1, 2001• 1 citation
Publisher

Abstract

A photon source based on postselection from entangled photon pairs produced by parametric frequency down-conversion is suggested and applied in quantum cryptography. Its ability to provide good approximations of single-photon states is examined. Use of this source for quantum key distribution is discussed. Advantages of this quantum-key-distribution system are clarified.

DOI
10.1364/icqi.2001.pa5

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A photon source based on postselection from entangled photon pairs produced by parametric frequency down-conversion is suggested and applied in quantum cryptography. Its ability to provide good approximations of single-photon states is examined. Use of this source for quantum key distribution is discussed. Advantages of this quantum-key-distribution system are clarified.
Summary form only given. Quantum cryptography uses single photons to allow secure distribution of secret key material to sender and receiver, without the possibility of an undetected eavesdropper. In the protocol suggested by Ekert, each photon of a quantum-mechanically entangled pair is sent to the sender and receiver, who randomly measure the polarization in various bases. If the bases are the same, then sender and receiver will have completely correlated measurement results, which become the shared secret key. Other combinations of measurements are used to test Bell's inequalities, which limit the possible correlations achievable with any local realistic theory. The presence of an intermediate eavesdropper can be readily detected by an inability to violate Bell's inequality. Using the polarization-entangled photon pairs from a novel spontaneous parametric down-conversion source, we have experimentally implemented. Ekert's proposal.
Pairs of entangled photons can be employed for quantum key distribution. For each bit exactly one pair of photons is needed. Unfortunately, the quantum states produced by real sources, like a parametric down conversion, contain also terms with more than only one photon pair. We discuss several aspects of the use of such states for quantum key distribution. It is shown that the presence of multi-pair signals (together with low detection efficiencies) causes errors in transmission even in the absence of al1 eavesdropper. However, the most important result is that an individual eavesdropping on multi-pair signals increases the error rate. This fact represents the important advantage of the entanglement-based quantum key distribution.
Real sources of entangled photon pairs (like parametric down conversion) are not perfect. They produce quantum states that contain more than only one photon pair with some probability. In this paper it is discussed what happens if such states are used for the purpose of quantum key distribution. It is shown that the presence of "multi-pair" signals (together with low detection efficiencies) causes errors in transmission even if there is no eavesdropper. Moreover, it is shown that even the eavesdropping, that draws information only from these "multi-pair" signals, increases the error rate. Information, that can be obtained by an eavesdropper from these signals, is calculated.
Entanglement-based quantum key distribution (EB-QKD) has several advantages over prepare-and-measure protocols, such as eliminating the need for decoy states and active modulators. At present, entanglement sources based on the spontaneous parametric down-conversion process have been widely used in EB-QKD experiments. Most of the existing models regard the photon-number distribution of pulse-pumped entanglement sources as thermal. However, doing so ignores any effects of spectral modes, which will lead to the overestimation of the secure key rate. In this paper, we analyze the correlation between the photon-number distribution and the joint spectral distribution and rewrite the quantum state of the photon pairs. Then we apply the results to EB-QKD and present a general model which can be used to calculate the secure key rate not only for any pulse-pumped systems but even for cw-pumped systems.