Trapping Light and Saving It for Later Scientists manage to stop light, hold it trapped in a cloud of chilled atoms known as a Bose Einstein condensate, and then release it in a second cloud a short distance away. We'll talk about the work and its potential applications in information processing.

Trapping Light and Saving It for Later

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IRA FLATOW, host:

You're listening to TALK OF THE NATION: SCIENCE FRIDAY. I'm Ira Flatow.

For the rest of the hour, finishing up this week, quantum physics and working with light. Yeah, you know, one of the things teachers like to tell you is that you can't go faster than the speed of light, right? That's one of Einstein's great foundations of physics.

But that statement comes with a lot of qualifiers, and while you can't go faster than light, it is possible to slow light down. And under the right conditions, you can slow it down a whole lot and even stop it, and then send it on its way again.

Well, sound strange? Here's something that might sound even stranger. This week in the journal "Nature," researchers described an experiment in which they stopped light and trapped it in a cloud of chilled atoms. And then some of those atoms were moved a short distance away to a second cloud and the researchers were able to then recall the original beam of light from the second cloud. In other words, essentially they transferred light to matter and then back to light again.

I know you're scratching your head. We'll talk about it a little bit more. Joining me now is Lene Hau at the - she is the Mallinckrodt professor of physics and applied physics at Harvard, and she's one of the authors of "The Professor." Lene Hau joins us from a studio at Harvard campus.

Dr. Hau, welcome to SCIENCE FRIDAY.

Dr. LENE HAU (Physics, Harvard University): Thank you very much.

FLATOW: Is it possible for a lay person to understand what you did?

Dr. HAU: Well, I think so. The short version of the story is that we can basically stop a light part in one part of space, extinguish it there, and then later revive it in a completely different location. And you described the process actually very well. We do that by creating two extremely cold clouds of atoms, actually called Bose-Einstein condensate, superfluids - it's a superfluid state of matter.

So each of the two clouds separated in space turned into a superfluid where the atoms behave in lock step. Then we can send the light parts into the first condensate, the first atom cloud, stop it there, and then create an imprint of the light parts that will then fly out in space and travel across as a perfect matter copy of the light pulse to the second condensate, to the second cold cloud; and then we can revive it in that second cloud and send the light pulse back on its way as if nothing had happened.

FLATOW: Could you theoretically pick it up and put it in a box and carry it some place else and then let the light out?

Dr. HAU: Sure.

(Soundbite of laughter)

FLATOW: Wow! It sounds almost like science fiction.

Dr. HAU: Yes. Yes. No, we've had a lot of fun in the lab for sure.

FLATOW: Well, tell us about the fun. Do you actually think about doing that? How far have you actually moved the light stored as matter?

Dr. HAU: It's - we have moved it. We create these two cold atom clouds. They are separated by 0.2 millimeters. And so the atom clouds are completely separated, absolutely no overlap, and then this matter copy moves 0.2 millimeters in space, which is a long distance for atoms and a long distance for light.

And we could -the important point is then that once that imprints, that matter copies out in free space like you're indicating, you can then grab on to it with a laser beam and in some sense put it on the shelf, store it there for a while, and then grab it again and send it back on its way into the second cloud.

And the curious thing is that we, you know, this will be a good way of storing information for long periods of time. But we can actually also manipulate - and that's very important - we can actually start massaging, pinching these matter copy with laser beams, for example, and then change the shape of that matter copy and send it on its way; and then we can revive the light pulse with a changed shape, so we can actually manipulate light in this way very powerfully.

FLATOW: When you say - let me just give out the number 1-800-989-8255 talking about moving light changing into matter. When you say manipulate it in shape, what exactly do you mean?

Dr. HAU: It could take a completely - the light pulse - actually, I should sort of back off a little bit because when we start out with the light pulse and free space, it actually starts out being about a mile long. And then we send it into the first cold atom cloud and the light parts slows down to 15 miles an hour and compresses to only 1,000th of an inch.

So it's less than half the thickness of a hair. And then when we form them, then we extinguished that light pulse and the matter copy of the light pulse starts to fly out. And that matter copy has exactly the same shape as the original light pulse. And it's precisely that shape, that imprint, that we can start to change, and we can also change what is called the face information in the matter copy and thereby in the light pulse.

So that means that we start to have some very, very powerful ways of manipulating light, which, for example, could be very, very interesting in connection with optical communication.

FLATOW: Does this have anything to do with the equivalence of matter and energy?

Dr. HAU: Yeah, and certainly you can say in these experiments we kind of take it from - we take information from one form and make a perfect copy in another form, in this case from light to matter and back to light. And we preserve all the energy in doing that, and that's really the magical thing, that we don't lose information in this process.

FLATOW: And where would you eventually - I mean, people always want to know hey, what are the practical applications that are going to come out of this thing. Have you got any?

Dr. HAU: Well, I mean I think we have here - partly why I'm so excited about it is that we have some new territory here and it's really opening up a lot of doors. There's so many things where we could go - or so many things we could do from here. And in terms of applications, now we - of course this is guesswork on my part, predicting about the future because of course, what we have here is a prototype system where we have some new territory.

But I think there is a possibility, really, for making, you know, using this in some interesting ways, some interesting applications in the future. And coming back to optical communication, we love to send light down optical fibers, code information in light, and to get very high data transfer rates. For example if you want - you're sitting at your computer and you want to download movies, the best way of sending information over long distances is through optical fibers.

But then the problem is light is great for transport, but how do you change it? How will you process? And that's where this comes in, because we can then take it into matter form in, say, in a node, at a node position in an optical network, we could convert the light information perfectly into matter, manipulate it, process it, and then send it back on its way down another optical fiber.

And you can really compare it to that if you have a computer, you like two main things in a computer: memory and a CPU processor. And what we have done with this research, I would say, is we have moved toward not just being able to store light in a memory, but we're actually moving towards being able to make a processor.

FLATOW: Wow. So it would be an optical processor.

Dr. HAU: Yes.

FLATOW: And a very…

Dr. HAU: For processing of optical information, exactly.

FLATOW: But a very cold one.

Dr. HAU: A very cold one.

FLATOW: I mean, this light is how cold? The matter is almost at zero, right?

Dr. HAU: Yes. The matter itself - when we cool the atoms down they are a few billionths of a degree above absolute zero. But the curious thing is that it's in our little atom cloud that lives in a vacuum chamber, but our vacuum chamber itself is actually sitting at room temperature. So it's just that tiny little atom cloud that has to be cold.

FLATOW: And how big is the cloud, I mean physically?

Dr. HAU: It's about 0.1 millimeters.

FLATOW: Tiny little thing.

Dr. HAU: Tiny little thing.

FLATOW: Do you actually see the light escape once you let it go? I mean, can you see it?

Dr. HAU: Yes, yes, absolutely. We actually, we record it with a light detector. And what we can also do is when the matter copy is out in free space, when it's traveling across, we can actually photograph it, and we have images of that directly.

FLATOW: Wow. 1-800-989-8255. Let's go to the phones. Let's go to Carson(ph) in Raleigh, North Carolina. Hi, Carson.

CARSON (Caller): Hey, thanks for letting me on. I was just wondering if turning light into matter, if this would bring us any closer to turning matter into light and transporting objects like we see in science fiction, like Star Trek, and transporting and all that.

FLATOW: A transporter beam. I'll bet you get asked this at least three times a day, (unintelligible). Dr. Hau, have you got the answer down yet for this?

(Soundbite of laughter)

Dr. HAU: Yes, well it's an interesting question, and certainly it's - we can really now treat matter and light at, you know, at an equal footing. Taking light to matter, back to light, back to matter. And the curious thing is also that we can kind of - we got this as manipulating light with matter, but just as well we manipulate matter with light.

And we can actually - and this is kind of coming back to the question - we can actually pick up little pieces of one of the cold atom clouds, one of the condensates, and move it over to the second condensate and dump it there in a controlled location. So for sure we can transport matter back and forth, and we can actually shuttle little pieces back and forth in space.

CARSON: Awesome.

FLATOW: What do you think of that, Carson?

CARSON: That's pretty mind-boggling thinking about this, man. Thank you very much.

FLATOW: Thanks for calling.

CARSON: No problem.

FLATOW: 1-800-989-8255 is our number. So where do you go from here, Dr. Hau?

(Soundbite of laughter)

Dr. HAU: Well, it's - as I said, there's so many paths we want to go down, and it's question of, you know, trying to figure out which one should we pick. We're very, very excited about it. We have had a tremendous amount of fun in the lab, but also in terms in thinking about how to understand this process.

So exactly what we are doing next, we have to decide that in the lab, you know, poke at the system and then see what happens because we just have complete new territory to work within now.

FLATOW: So you don't know what could happen here. You're just going to - very basic research, poke at it and see what comes out.

Dr. HAU: That's - when you have new territory, then you are bound to find new effects. And I believe we really have new territory here, and we could, you know, find new effects in, you know, basic research and also potentially in application.

FLATOW: Okay, let's go to the phones. Interesting questions coming in. Dan from Iowa. Hi, Dan, welcome to SCIENCE FRIDAY.

DAN (Caller): Yes, thank you very much, Ira.

FLATOW: Go ahead.

DAN: I was (unintelligible) quantum entanglement and separation - have you had quantum photons in two different clouds? And how far you can separate them where they still have the quantum entanglements take effect? Can you separate it by half a block of half a continent, how far can you go?

FLATOW: Good question. Are these clouds entangled?

Dr. HAU: Well, that's a very interesting question, actually. And we could certainly - what happens is that we actually create - when we create that imprint of the light pulse, the atoms actually go into what is called quantum mechanical super-position states.

So when part of the - that means that part of the atom is actually moving out in free space, and when the atoms are in that traveling matter copy, the atoms - this is really quantum mechanics - the atoms are partly out there in free space traveling along, and at the same time the atoms are stuck back in the original atom cloud, the original condensate.

And of course if we then go and make a measurement on the system, the atoms will have to make up their mind: Am I here or am I there? But if we want to transfer the information, we just don't do a measurement on the system.

And in terms of getting back to the entanglement, the curious thing is that we could actually send a classical light pulse in without entanglement into the first condensate, let it travel out in free space, and then get back and massage it, manipulate it, so that it actually turns into an entangled atomic state. And then we could read that into the second cloud and get a nice entangled light state out of the system. So there are a tremendous amount of possibilities here.

FLATOW: Why do you need two clouds? Why can't you just leave it in the first one and carry it around in the first cloud?

Dr. HAU: Well, it's really to get matter copy separated, get the matter copy of the light pulse out, separate it in space. Because it's at that point you can start to hold on to it, manipulate it in very powerful ways, and then send it on. It's so that you can get your hands on it and get it separated from its mother, so to speak.

FLATOW: Everybody seems to be interested in this topic. Let's go get another call here. Let's go to Al(ph) in Rochester Hills. Hi, Al.

JOSH (Caller): Yes, hello.

FLATOW: Yes, go ahead. Go ahead.

JOSH: With who am I speaking?

FLATOW: You're on SCIENCE FRIDAY. Go ahead.

JOSH: Oh, well, my name's Josh from California. I was just curious what the after effects on the atom are after the light has been transferred from one cloud to another. If the light has been transferred from one cloud to another through an atom, is the atom identical after that transfer has taken place?

FLATOW: Interesting question. Have you…

Dr. HAU: That's a very interesting question, yeah, right. Because it's actually - when we start out with a condensate, all the atoms in the first condensate are in a particular internal state because we have to deal here both with the atom's internal structure and also the motion of the atoms as a whole. And when we create the matter imprint, we are actually promoting part of the atom into an excited internal state. And when the matter copy travels across, it's actually in that excited internal state.

And then we (unintelligible) the light pulse in the second cloud, the atoms go back to their original ground state, internal ground state. So in that sense, they become identical to the atoms that are in the second condensate and also identical to the atoms that were in the first condensate.

And that's extremely important because now we have a set of identical particles, and that's the reason that this whole process can happen. Because quantum mechanically, if we have a set of identical particles, they are completely indistinguishable. They lose their individuality, independence. And now the atoms in the matter copy, once they get turned back into their original internal state, they want to join the other atoms in that condensate because all those guys are bosons, and bosons love to be in the same quantum state. The more the merrier for bosons.

(Soundbite of laughter)

Dr. HAU: So this is…

FLATOW: We lost her. Well, we were running out of time. I'm sorry, she just dropped out. Dr. Lene Hau is Mallinkrodt Professor of Physics and Applied Physics at Harvard University in Cambridge, Massachusetts. I guess she wanted to be back with her colleagues there at Harvard.

I'm Ira Flatow in New York.

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