# How should I interpret an article about light being slowed and compressed?

I've found an article describing a method that allowed the light to be "slowed and compressed"... That sounds really strange, which made me google a lot and find this PDF document. However for me, this one is too complicated to understand given my English skills. There is also an article Slow light on wikipedia, but I'm not sure if this is related to the previous texts.

I first thought that the light is being absorbed and released later, but none of the texts suggests anything like this.

Could anyone tell me how should I interpret this information?

Edit: There also seems to be a related question, however this one seems to use the re-emition method that I've talked about.

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The linked article refers to cesium gas heated to 100° Celsius so it isn't a BEC. I too would be interested to know how the caesium gas is slowing the light. – John Rennie Jan 21 '14 at 18:25
For those who have no idea what's BEC (like me), here's a Wikipedia article. Sounds like sci-fi though. – Tomáš Zato Jan 21 '14 at 18:30

The popular media article is describing this PRL paper

http://arxiv.org/abs/physics/0701297
http://prl.aps.org/abstract/PRL/v98/i4/e043902
All-Optical Delay of Images using Slow Light

by Ryan M. Camacho, Curtis J. Broadbent, Irfan Ali-Khan, John C. Howell. Check the arXiv version that there is the "UR" image on the last page of the paper.

The achievement is that the optical pulses lasting 2 ns may be delayed by 10 ns using some clever interference between the delayed images and local oscillators.

The popular article's focus on the "information stored in a single photon" is utterly misleading. A single photon only carries 3 continuous degrees of freedom we may choose, e.g. the momentum vector, and 1 qubit of information, e.g. the linear $x/y$ or the circular L/R polarization. This is clearly insufficient for a natural encoding of an image.

The actual point of the paper related to the "single photon" was that the interference used to reconstruct the delayed images only relied on the wave character of "whatever represents the light". One may derive that it works for a classical electromagnetic field. But the same gadget inevitably works even if the electromagnetic field is so weak that at most one photon is present at each moment. They had something like 0.5 photons per pulse in average.

It works because a single photon is described by a probabilistic wave function whose behavior completely mimics the behavior of the classical electromagnetic field in these interference experiments. So everything will work even if the "rate of photons per second" is diluted to a tiny number. This disclaimer is analogous to the observation that the interference pattern in a double-slit experiment exists even if we send the photons "one by one" – so the interference pattern can't possibly have anything to do with the interactions of many photons. However, to reconstruct the whole image "UR" in their setup, one still needs many photons that will paint the image as their probabilistic distribution. One photon wouldn't be enough to refresh the whole image.

"Slow light" is a necessary prerequisite and context in which they did their work – a method to undo the "harmful" time evolution of the photon in the dispersive slow-light medium. But the slow light mediums themselves were invented by my ex-colleague Lene Hau in 1999, over 7 years before the paper we are discussing here.

Linguists may summarize the mechanism as: Tomáš Zato Žeseptáš Wolle. Apologies to readers who don't understand it, they may easily ignore this part of the answer.

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I didn't take the fact about the image being recontructed from 1 photon for serious. But I wasn't sure about the light being slowed - and if it's slowed, than how and why. – Tomáš Zato Jan 21 '14 at 18:44
Dear Tomáši, the new result of the paper isn't slow light itself. Slow light is an old and well-known situation in optics where the group velocity is very low – which may be achieved because the group velocity is $\partial \omega/ \partial k$. They use the cesium vapor cell - invented by someone else - to achieve that. I was answering your question what did the popular article describe. The popular article was describing a paper by Camacho et al. that shows how to reconstruct an image even though it's been "diluting" in the dispersive slow-light environment for a long time, 10 ns. – Luboš Motl Jan 21 '14 at 18:50
So, a very simple interpretation could be that they were just testing how much the light changes while being slowed? (btw.: I can't read the articles you refer to. I'm asked to login to download them) – Tomáš Zato Jan 21 '14 at 18:58

All of the resources you point to are indeed related, and are examples of what is called slow light. This phenomenon refers to the extraordinarily slow speeds that can be enforced on light pulses when they traverse gas cells that are under the influence of additional light beams.

However, you do not need all that fancy apparatus to make light go slower than $c$. Indeed, the speed of light within any normal material medium will be slower than $c$ by a factor of the medium's index of refraction. This means that you can use, say, glass, to 'slow and compress' a light pulse, though of course by much, much less than what you've read.

There are multiple ways to understand why this happens, and it is important to stress that they all apply both to the exotic gas-phase experiments and to ordinary glass. I would tend to phrase it in terms of an electromagnetic wave trying to propagate through a region that contains electric charges, which changes the propagation equation.

You can also see it, though, as the absorption and re-emission of photons by the material medium. As the light beam reaches each atom in the medium, there is a small chance that it will be absorbed and re-emitted. The corresponding time delay induces a phase shift between the original and the re-emitted beam, and when the two interfere the result will be a phase shift in the light, if it is monochromatic. To get slow light, you need a medium whose refractive index rises (steeply) with frequency, which means that the higher-frequency components of your pulse accumulate a bigger phase shift than the slower ones. When you add all the phase-shifted components together, the net result is a slower pulse. Thus, there's several 'layers' involved, but it is in the end all down to photons being absorbed and re-emitted after a certain delay.

For the more inquisitive, here's the original reference:

All-Optical Delay of Images using Slow Light. Ryan M. Camacho, Curtis J. Broadbent, Irfan Ali-Khan, and John C. Howell. Phys. Rev. Lett. 98 no. 4, 043902 (2007). arXiv:physics/0701297. U. Rochester e-print.

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Dear Emilio, I just want to emphasize that your answer isn't really discussing the paper by Camacho et al. at all. They didn't invent slow light, did they? They did a particular thing - "undoing" the (dispersive, image destroying) time evolution in the slow-light environment by some clever harmonic oscillators. Slow light has been known and done since the 1999 work by my ex-colleague Lene Hau. – Luboš Motl Jan 21 '14 at 18:52
You are correct that they didn't invent it, but they are using it. I preferred addressing the OP's concerns instead. – Emilio Pisanty Jan 21 '14 at 19:09
I can't see into his brain but I can see the question and the question was about the meaning of a popular article describing Camacho et al. So I can't really understand how an answer completely ignoring the paper may be an answer to OP's question - even if he thought it is. ;-) – Luboš Motl Jan 22 '14 at 8:07