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Physicists Make 'Slow Light' by Reshaping Photons

Physicists force a slight tweak in the fundamental speed of light in a vacuum.

​A team of physicists from the University of Glasgow has ​devised a new way of tweaking the fundamental, vacuum speed of light by "restructuring" photons—e.g. light particles—themselves. As a bonus, unlike other "slow light" methods, this one should be able to create "slow sound" as well. This is deeper than it might initially appear.

First, we need to clarify something. We can very easily change the speed of some light or some sound by introducing an intervening medium. Different media have ​different refractive indices, which just means that light is bent or absorbed more or less as it travels through a given medium. Air, for example, has a refractive index of about 1.003, which when divided into the speed of light in a vacuum gives us a new speed of light.

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So: the speed of light as it travels through our air on Earth is about 2.981 x 10^8 ms^-1, rather than the vacuum speed limit of 2.999 x 10^8 ms^-1. But how do we really, fundamentally change the speed of light so that it moves slower in a vacuum? It turns out that light particles can be intrinsically tweaked to allow for slow light in a vacuum.

The trick is in manipulating the spatial structure of photons. This is, of course, immediately fishy because we shouldn't even be talking about photons occupying space or having volume (which they don't). So, how can we talking about spatial structure when our subject doesn't take up space in the first place?

It has to do with the peculiarities of living a wavelike existence: a photon might not occupy a volume of space, but as a wave it has a wavelength, which does exist in space as the infinitesimal point of a photon becomes a probabilistic smear of location possibilities. (Remember: in quantum mechanics, a thing can be indefinite and, as a wave, de facto occupying many places at once until measured.) A photon might not occupy space, but it can be found at a particular location. Weird, eh?

What can be shaped then is the particle's wave envelope. Imagine this as a line tracing from wave crest to wave crest, turning a periodic thing into a nice, manageable line. The best envelope explanation is just to see it, as below.

An envelope can be manipulated, which is like setting some maximum limits on a wave's amplitude. Tweaking the envelopes of some traveling photons is how the Glasgow team did their shaping and, ultimately, speed limiting.

The experiment consisted of a photon source that emitted particles in pairs. One member of the duo went just on a straight line to a detector, while the other went through a series of liquid-crystal masks, which smooshed their profile into the passing photon wave (or its envelope). This shaping affects the wave's frequency. Imagine a prisoner pacing back and forth in a cell as the walls slowly move closer together. To exert the same energy in a single oscillation, the prisoner will need to move faster.

The change, in wave terms, will result in a phase shift, where a waveform scoots forward or backward in time (or along an x-axis). This is where the speed change comes from: the manipulated light winds up having to travel a longer distance, even though the distances between the emitter and detector are really the same.

As Jon Cartwright notes in ​Physics World, there are many definitions or possible ways of looking at the speed of light: "phase velocity, peak velocity, information velocity—definitions abound." This is just one: the speed of a waveform's envelope, known as group velocity. For a distance of a single meter, the researchers were able to come up with a delay of just several micrometers. Small but quite real.