Spatially structured photons that travel in free space slower than the speed of light

Giovannini, D., Romero, J., Potoček, V., Ferenczi, G., Speirits, F. , Barnett, S. M. , Faccio, D. and Padgett, M. J. (2015) Spatially structured photons that travel in free space slower than the speed of light. Science, 347(6224), pp. 857-860. (doi:10.1126/science.aaa3035) (PMID:25612608)

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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 wave vectors 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 in the group velocity of photons in both a Bessel beam and photons in a focused Gaussian beam. In both cases, the delay is several micrometers over a propagation distance of ~1 meter. Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves.

Item Type:Articles
Additional Information:This is the author’s version of the work. It is posted here by permission of the AAAS for personal use, not for redistribution. The definitive version was published in Science in Volume 347(6224):857-8602, DOI: 10.1126/science.aaa3035.
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Padgett, Professor Miles and Speirits, Dr Fiona and Potocek, Dr Vaclav and Barnett, Professor Stephen and Giovannini, Dr Daniel and Romero, Dr Mary and Faccio, Professor Daniele
Authors: Giovannini, D., Romero, J., Potoček, V., Ferenczi, G., Speirits, F., Barnett, S. M., Faccio, D., and Padgett, M. J.
College/School:College of Science and Engineering > School of Physics and Astronomy
Journal Name:Science
Publisher:American Association for the Advancement of Science
ISSN:0036-8075
ISSN (Online):1095-9203
Copyright Holders:Copyright © 2015 The Authors
First Published:First published in Science 347(6224):857-860
Publisher Policy:Reproduced in accordance with the copyright policy of the publisher.
Data DOI:10.5525/gla.researchdata.157

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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
549871Challenges in Orbital Angular MomentumMiles PadgettEngineering & Physical Sciences Research Council (EPSRC)EP/I012451/1P&A - PHYSICS & ASTRONOMY
549872Challenges in Orbital Angular MomentumMiles PadgettEngineering & Physical Sciences Research Council (EPSRC)EP/I012451/1P&A - PHYSICS & ASTRONOMY
624501TWISTS - Twists & more: the complex shape of lightMiles PadgettEuropean Research Council (ERC)340507S&E P&A - PHYSICS & ASTRONOMY