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We report on the observation of an unusual type of parametricdown-conversion. In the regime where collinear degenerate emission is in theanomalous range of group-velocity dispersion, its spectrum is restricted inboth angle and wavelength. Detuning from exact collinear-degeneratephasematching leads to a ring shape of the wavelength-angular spectrum,suggesting a new type of spatiotemporal coherence and entanglement of photonpairs. By imposing a phase varying in a specific way in both angle andwavelength, one can obtain an interesting state of an entangled photon pair,with the two photons being never at the same point at the same time.

Ring-shaped spectra of parametric down-conversion and entangled photons that never meet