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

TL;DR

This paper reports the discovery of a novel parametric down-conversion process producing ring-shaped spectra, revealing unique spatiotemporal coherence and entanglement where photon pairs never occupy the same space-time point.

Contribution

It introduces a new type of parametric down-conversion with ring-shaped spectra and demonstrates entangled photon pairs that are spatially and temporally separated.

Findings

Ring-shaped wavelength-angular spectra observed in anomalous dispersion regime

Photon pairs exhibit unique spatiotemporal entanglement with no overlap in space-time

Phase control enables creation of entangled states with photons never at the same point

Abstract

We report on the observation of an unusual type of parametric down-conversion. In the regime where collinear degenerate emission is in the anomalous range of group-velocity dispersion, its spectrum is restricted in both angle and wavelength. Detuning from exact collinear-degenerate phasematching leads to a ring shape of the wavelength-angular spectrum, suggesting a new type of spatiotemporal coherence and entanglement of photon pairs. By imposing a phase varying in a specific way in both angle and wavelength, 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.