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European Quantum Electronics Conference (EQEC)

Engineering the frequency correlations of entangled two-photon states by achromatic phase-matching

Martin Hendrych, Juan P. Torres

Published Jan 18, 2006• 2 citations
Publisher

Abstract

The paper describes the method of suppressing the distinguishing information contained in the frequency spectrum of the entangled photon pairs generated in spontaneous parametric downconversion (SPDC). The generation of entangled states that make use of only a portion of the total two-photon quantum state, requires the suppression of any distinguishing "which-path" information that otherwise degrades the degree of entanglement.

DOI
10.1109/eqec.2005.1567488

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The paper describes the method of suppressing the distinguishing information contained in the frequency spectrum of the entangled photon pairs generated in spontaneous parametric downconversion (SPDC). The generation of entangled states that make use of only a portion of the total two-photon quantum state, requires the suppression of any distinguishing "which-path" information that otherwise degrades the degree of entanglement.
We put forward a method to suppress distinguishing information contained in the frequency spectrum of entangled photon pairs generated in spontaneous parametric down-conversion. The distinguishing information is shown to be suppressed by employing achromatic phase-matching techniques to tailor the group velocities of the generated signal and idler photons, and thus tuning their spectral properties. The method can be implemented in materials and frequency bands where conventional solutions do not hold, and it can benefit from the dispersion cancellation inherent to entangled photons featuring frequency correlation or anticorrelation. The generation of polarization Bell states is discussed.
In most configurations aimed at generating entangled photons based on spontaneous parametric down conversion (SPDC), the generated pairs of photons are required to be entangled in only one degree of freedom. Any distinguishing information coming from the other degrees of freedom that characterize the photon should be suppressed to avoid correlations with the degree of freedom of interest. However, this suppression is not always possible. Here, we show how the frequency information available affects the purity of the two-photon state in space, revealing a correlation between the frequency and the space degrees of freedom. This correlation should be taken into account to calculate the total amount of entanglement between the photons.
We put forward a new scheme to tailor the frequency correlations of paired photons which allows their spectral properties to be tuned from correlation to anticorrelation, including uncorrelation. The method is based on the proper tailoring of the group velocities of all interacting waves through the use of beams with angular dispersion. The method can be implemented in materials and frequency bands where conventional solutions do not hold. This technique makes possible the generation of frequency correlated photons, heralded single photons with a high degree of purity from pairs of uncorrelated photons, and the suppression of distinguishing information contained in the frequency spectrum of polarization entangled photon pairs.
We present a novel interferometric technique for suppressing distinguishing information in the space-time component of the state vector of an entangled pair of photons by providing two indistinguishable ways for each photon to occupy any given space-time mode. We demonstrate this method by using spontaneous parametric down-conversion to generate a pair of photons in the state with the least distinguishing information consistent with the set of modes available. The technique also allows the preparation of the two photons in a highly entangled space-time state provided certain criteria are met.