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Journal of Optoelectronics·laser

Preparation of high fidelity polarization EPR entangled state

Xiaoqin Wang

Published Jan 1, 2011• 0 citations
OpenAlex

Abstract

By optimizing the scheme of EinsteinPodolsky-Rsoen(EPR) by spontaneous parametric down-conversion and using the technique of coincidence counting and post selection,the high fidelity polarization EPR entangled state has been prepared.The results show that the fidelity of EPR entangled state approach as almost 0.99,and the coincidence rate can be more than 3 500 Hz.It overcomes the limit that the fidelity and intensity of entanged state can′t increase synchronously.A stationary entangle source is supplied for preparation of the multi-photon entangled state and two-photon quantum communication.

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By optimizing the scheme of EinsteinPodolsky-Rsoen(EPR) by spontaneous parametric down-conversion and using the technique of coincidence counting and post selection,the high fidelity polarization EPR entangled state has been prepared.The results show that the fidelity of EPR entangled state approach as almost 0.99,and the coincidence rate can be more than 3 500 Hz.It overcomes the limit that the fidelity and intensity of entanged state can′t increase synchronously.A stationary entangle source is supplied for preparation of the multi-photon entangled state and two-photon quantum communication.
A simple experimental setup consisting of a spontaneous parametric down-conversion source and passive linear optics is proposed for conditional preparation of a maximally entangled polarization state of two photons. Successful preparation is unambiguously heralded by coincident detection of four auxiliary photons. The proposed scheme utilizes the down-conversion term corresponding to the generation of three pairs of photons. We analyze imperfect detection of the auxiliary photons and demonstrate that its deleterious effect on the fidelity of the prepared state can be suppressed at the cost of decreasing the efficiency of the scheme.
We report on a high-intensity source of polarization-entangled photon pairs with high momentum definition. Type-II noncollinear phase matching in parametric down conversion produces true entanglement: No part of the wave function must be discarded, in contrast to previous schemes. With two-photon fringe visibilities in excess of 97%, we demonstrated a violation of Bell's inequality by over 100 standard deviations in less than 5 min. The new source allowed ready preparation of all four of the EPR-Bell states.
We have demonstrated entanglement swapping with 1550 nm polarization-entangled photon pairs created by spontaneous parametric down conversion in two spatially separated type-II periodically poled lithium niobate bulk crystals. In a partial Bell-state measurement, entanglement was observed between two photons from separate sources. We employed sinusoidally gated InGaAs/InP avalanche photodiodes operated at a repetition frequency of 1.28 GHz, which increased the four-fold coincidence count rate without degrading the fidelity of entanglement swapping. The obtained fidelity exceeds 90%, which indicates that the entangled states obtained in this experiment are high enough for immediate further use in quantum communication protocols such as quantum teleportation or quantum cryptography.
We propose a scheme to generate a four-photon polarization-entangled cluster state by using only linear optical elements and four-photon coincidence detection. The scheme requires two single-photon states and one two-photon polarization entangled state as input resources. Then, we consider the realistic implementation of the scheme by using the spontaneous parametric down-conversion as photon resources. It is shown that under certain conditions, the four-photon polarization cluster state can be generated with a high fidelity, and the scheme is feasible by current experimental technology.
We present a theoretical and experimental study of preparing maximally entangled two-photon polarization states, or Bell states, using femtosecond-pulse-pumped spontaneous parametric down-conversion (SPDC). First, we show how the inherent distinguishability in femtosecond-pulse-pumped type-II SPDC can be removed by using an interferometric technique without spectral and amplitude postselection. We then analyze the recently introduced Bell-state preparation scheme using type-I SPDC. Theoretically, both methods offer the same results, however, type-I SPDC provides experimentally superior methods of preparing Bell states in femtosecond-pulse-pumped SPDC. Such a pulsed source of highly entangled photon pairs is useful in quantum communications, quantum cryptography, quantum teleportation, etc.
We experimentally realize a high-intensity three-photon Greenberger-Horne-Zeilinger (GHZ) entanglement source directly following the proposal by Rarity and Tapster [J. G. Rarity and P. R. Tapster, Phys. Rev. A 59, R35 (1999)]. The threefold coincidence rate can be more than 200Hz200\phantom{\rule{0.3em}{0ex}}\mathrm{Hz}200Hz with a fidelity of 0.811, and the intensity can be further improved with moderate fidelity degradation. The GHZ entanglement is characterized by testing the Bell-Mermin inequality and using an entanglement witness operator. To optimize the polarization-entangled source, we theoretically analyze the relationship between the mean photon number of the single-photon source and the probability of parametric down-conversion.