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, to our knowledge 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 0.1% slowed 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.
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