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

Photon source for quantum cryptography using postselection from entangled quantum states

Ondřej Haderka, Jan Peřina

Published Mar 7, 2001• 0 citations
Publisher

Abstract

A realistic model for a photon source based on postselection from correlated photon pairs produced by parametric frequency down-conversion is suggested. Its efficiency in producing approximation of single photon states is examined. The application of such source in quantum key distribution schemes is discussed and comparison with currently used sources of photons is given. Future prospects of this type of photon source are outlined.

DOI
10.1117/12.417869

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A realistic model for a photon source based on postselection from correlated photon pairs produced by parametric frequency down-conversion is suggested. Its efficiency in producing approximation of single photon states is examined. The application of such source in quantum key distribution schemes is discussed and comparison with currently used sources of photons is given. Future prospects of this type of photon source are outlined.
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.
We present an experimental setup for generation of entangled-photon pairs via spontaneous parametric down-conversion, based on the femtosecond-pulsed laser. Our entangled-photon source utilizes a 76-MHz-repetition-rate, 100-fs-pulsewidth, mode-locked, ultrafast femtosecond laser, which can produce, on average, more photon pairs than a cw laser of an equal pump power. The output infrared pump photons (λ = 810 nm) are first up-converted to blue light (λ = 405 nm) and, subsequently, down-converted in a 1.5-mm-thick, type-II BBO crystal via spontaneous down-conversion. The resulting entangled pairs are counted by a pair of high-quantum-efficiency, single-photon, silicon avalanche photodiodes. The total down-conversion efficiency of our system, corresponding criterion of the pump power for real entangled coincident events, has been calculated to be 0.86 × 10-9. Our apparatus is intended as an efficient source/receiver system for the quantum communications and quantum cryptography applications.