Observation of subluminal twisted light in vacuum

TL;DR

This paper experimentally demonstrates that twisted light pulses can travel slightly slower than the speed of light in vacuum, supporting theoretical predictions and with implications for quantum communication technologies.

Contribution

First experimental observation of subluminal twisted light in vacuum, confirming theoretical models of vacuum group velocities for twisted optical beams.

Findings

Twisted light pulses are slowed by 0.1% relative to c in vacuum.

Supports theoretical predictions of vacuum group velocities for twisted beams.

Relevance to quantum information and communication applications.

Abstract

Einstein's theory of relativity establishes the speed of light in vacuum, c, as a fundamental constant. However, the speed of light pulses can be altered significantly in dispersive materials. While significant control can be exerted over the speed of light in such media, no experimental demonstration of altered light speeds has hitherto been achieved in vacuum for ``twisted'' optical beams. We show that ``twisted'' light pulses exhibit subluminal velocities in vacuum, being slowed by 0.1\% relative to c. This work does not challenge relativity theory, but experimentally supports a body of theoretical work on the counterintuitive vacuum group velocities of twisted pulses. These results are particularly important given recent interest in applications of twisted light to quantum information, communication and quantum key distribution.