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Solid light -

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Solid light

Solid light describes a method of obtaining a phase transition in light. At a critical point light
crystallises. This form of light was not predicted! It is a new exotic state for light. Despite
some precursor experiments, the idea is to create a system where this effect can be observed. This
would consist of a trap where a single photon reacts with a single atom.

Transcript

This transcript was typed from a recording of the program. The ABC cannot guarantee its complete
accuracy because of the possibility of mishearing and occasional difficulty in identifying
speakers.

Robyn Williams: Let's start with solid light. No, I don't understand it either, but just when you
thought physics was over it re-emerges with double the mystery. Andrew Greentree is senior lecturer
in physics at the University of Melbourne.

Andrew Greentree: Solid light, it's how we describe a method for getting a phase transition, that
we can make the light bounce off other particles of light, photons can bounce off each other, and
then at some critical point the photons will suddenly crystallise out. When it crystallises you'll
find that there's only a certain number of photons, and this is a very exotic state for light, it's
a very new state.

Robyn Williams: It's almost like...you talk about phase transition...when water becomes ice, but
water is supposed to become ice. Light is not supposed to form crystals.

Andrew Greentree: Yes, exactly, it was quite a surprise. It wasn't something that had been
predicted until we started looking at it. This is why it's so interesting. Physics is really
interesting when you can look at taking one system that you know well (light, for example)
comparing it to another system that you know fairly well (for example, a solid) and find the
connections. It's a very fertile ground. In particular we think that we can use this to understand
more about quantum mechanics and the quantum mechanics of many interacting particles, because this
is something that is really difficult to understand, and yet particle-particle interactions are all
about us. The natural world is made by it, that's what defines chemistry. We understand the rules
of the game but we can't see them in action, we can't look at these very complicated systems. So
the main application of solid light, we believe, is to get us a system that we can observe.

Robyn Williams: You did say in the beginning, of course, it's theoretical. Has anyone here or in
Cambridge or elsewhere actually observed this happening in the real world?

Andrew Greentree: No. The precursor experiments have been seen, so the very first photon-photon
repulsion at the one photon level, it's an effect called photon blockade, that was observed at
Caltech, I think just last year. However there are a few groups now trying to build the structures
we're talking about, and here in Melbourne we're trying to do that as well by looking at diamond
and making optical structures in diamond. There are other people looking at this in all kinds of
different structures. So it hasn't been done yet but there's definitely experimental work underway
to try and do it.

Robyn Williams: What do you need to do this experiment? An apparatus the size of Switzerland?

Andrew Greentree: No, this is about things that are very small. The structures that we want to
build in diamond would actually be very small indeed. A thin layer of diamond, some 200 nanometres
thick, maybe ten micrometres or ten-millionths of a metre on either side. I know of one group who
are looking at similar structures that maybe could be used for this who are using what are called
micro-traps; two mirrors very close to each other and a single atom is in there.

Robyn Williams: You have these photons which presumably are travelling at the speed of light
because they are light, and I'd imagine the crystals are not travelling very fast and they're
sitting there, so what happens to the energy?

Andrew Greentree: The trick actually is to stop the photons. We want to trap the photons so they
can interact with the atoms for a long time. So we don't think that the best way is with mirrors,
but if you just put two mirrors next to each other, photons can bounce between them. If you make
something called an optical cavity you can trap a photon for a certain length of time. The longer
you can trap it, the better it is. So what you want to do is make a very small trap, put a single
photon in there, have a single atom in there, the photon interacts with the atom and it can
interact for a long time. Gradually it leaks out. In fact we describe it by 'hopping', we say it
hops out from one cavity and goes into the next. You want to make sure that when it hops out it
hops to another cavity and then it can interact with another atom. Then when you start to pile in
the photons, then you can start to see the change in the properties of the photons.

Robyn Williams: See? I was so stunned by all that that I failed to ask Andrew Greentree what he
plans to do with the lumps of light when he obtains them. He's Queen Elizabeth Fellow at the
University of Melbourne.