Apparent superluminal advancement of a single photon far beyond its coherence length

TL;DR

This paper reports an experiment demonstrating apparent superluminal propagation of a single photon through a 4f optical system, showing a shift beyond its coherence length while preserving causality.

Contribution

It introduces a novel experimental setup to observe superluminal effects with single photons and clarifies the origin of the apparent superluminal velocity.

Findings

Photon peak shifted beyond coherence length

Superluminal group velocity detected interferometrically

Causality remains unviolated

Abstract

We present experimental results relative to superluminal propagation based on a single photon traversing an optical system, called 4f-system, which acts singularly on the photon's spectral component phases. A single photon is created by a CW laser light down{conversion process. The introduction of a linear spectral phase function will lead to the shift of the photon peak far beyond the coherence length of the photon itself (an apparent superluminal propagation of the photon). Superluminal group velocity detection is done by interferometric measurement of the temporal shifted photon with its correlated untouched reference. The observed superluminal photon propagation complies with causality. The operation of the optical system allows to enlighten the origin of the apparent superluminal photon velocity. The experiment foresees a superluminal effect with single photon wavepackets.