Changing the speed of optical coherence in free space

TL;DR

This paper introduces the concept of coherence group velocity in free space, demonstrating experimentally that it can be tuned using spectral structure, with measured velocities ranging from 12c to -6c.

Contribution

It defines and experimentally verifies the tunability of coherence group velocity in free space using spectral structuring of broadband light.

Findings

Coherence group velocity can be significantly faster or slower than c.

Spectral structuring enables control over coherence propagation speed.

Measured velocities range from 12c to -6c.

Abstract

It is typically assumed that the fluctuations associated with a stationary broadband incoherent field propagate in free space at the speed of light in vacuum c. Here we introduce the concept of 'coherence group velocity', which -- in analogy to the group velocity of coherent pulses -- is the speed of the peak of the coherence function. We confirm experimentally that incorporating a judicious spatio-temporal spectral structure into a field allows tuning its coherence group velocity in free space. Utilizing light from a super-luminescent diode, we interferometrically measure the group delay encountered by the cross-correlation of a structured field synthesized from this source with the unstructured diode field. By tracking the propagation of this cross-correlation function, we measure coherence group velocities in the range from 12c to -6c.