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

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.