Photons that travel in free space slower than the speed of light

TL;DR

This paper demonstrates that the group velocity of single photons in free space can be reduced below the speed of light by introducing transverse spatial structures such as Bessel and Gaussian beams, challenging the notion that light always travels at c.

Contribution

It provides experimental evidence that structured light beams can have a slower group velocity in free space, highlighting the impact of spatial structure on photon propagation.

Findings

Measured delay of several microns over 1 meter propagation

Reduced group velocity observed in Bessel and Gaussian beams

Shows invariance of light speed applies only to plane waves

Abstract

That the speed of light in free space is constant is a cornerstone of modern physics. However, light beams have finite transverse size, which leads to a modification of their wavevectors resulting in a change to their phase and group velocities. We study the group velocity of single photons by measuring a change in their arrival time that results from changing the beam's transverse spatial structure. Using time-correlated photon pairs we show a reduction of the group velocity of photons in both a Bessel beam and photons in a focused Gaussian beam. In both cases, the delay is several microns over a propagation distance of the order of 1 m. Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves. Introducing spatial structure to an optical beam, even for a single photon, reduces the group velocity of the light by a readily measurable amount.