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Frontiers in Optics

Modified coherent state for improvement of quantum cryptography and generation of 3-photon entangled state

Z. Y. Ou

Published Jan 1, 2004• 0 citations
Publisher

Abstract

We study photon number distribution for Yuen’s two-photon coherent state. We find a multi-photon interference effect will cancel either the two-photon or the three-photon terms. Such a modified coherent state can be used for improving the security of quantum cryptography and for creating a three-photon entangled state.
DOI
10.1364/fio.2004.fmo4

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A modified coherent state is created by mixing a coherent state with optical field from parametric down-conversion in a beam splitter. A two-photon interference effect will cancel the two-photon state. This modified coherent state has ad-vantage over the conventional weak coherent state currently used widely in quantum cryptography. We make numerical analysis on the improvement in secure data rate and transmission distance with the modified coherent state. With a three-photon interference effect to cancel the three-photon state, further improvement is shown to be possible in a newly proposed protocol for quantum cryptography.
Weak coherent states as a photon source for quantum cryptography have a limit in secure data rate and transmission distance because of the presence of multiphoton events and loss in transmission line. Two-photon events in a coherent state can be taken out by a two-photon interference scheme. We investigate the security issue of utilizing this modified coherent state in quantum cryptography. A 4-dB improvement in the secure data rate or a nearly twofold increase in transmission distance over the coherent state are found. With a recently proposed and improved encoding strategy, further improvement is possible.
To increase the security distance of quantum communication,a new scheme of decoy state quantum key distribution with a modified coherent state is proposed.The probability of multi-photon pulse in weak coherent source can be decreased by canceling two photon pulses to improve the security distance.Based on the math model of the decoy state QKD,the formula for the yield and error rate of the single photon pulse with the Modified Coherent State is deduced,and substituted in GLLP to calculate the generation rate of quantum keys.Results show that the decoy state with the Modified Coherent State can increase the secure communication distance about 10%,which is very close to that of the perfect decoy state QKD.
By combining a coherent state with fields from a number of parametric down-conversion processes, we are able to generate arbitrary NNN-photon entangled states, including the maximally entangled photon-number state or the so-called NOON state $(\ensuremath{\mid}N,0⟩+\ensuremath{\mid}0,N⟩)$. This is achieved through destructive quantum interference for the cancellation of the unwanted states.