7 LASER cooling: researching quantum effects

So here we are in Open University's quantum physics laboratory. And I'm with my colleague, Calum. Calum, what on Earth goes on in this lab here?

Lots of things go on. What we mostly do is we make atoms very, very cold using lasers. And then when the atoms are really cold, we can do quantum physics with them.

So when the atoms are really, really cold, what part of the quantum world are we looking at with them?

So for us, when the atoms are really, really cold, what's important is that hang around for a very long time. So in a normal gas we associate the temperature with roughly how fast particles are moving. So in the air around us, the nitrogen particles are moving around about 300, 400 metres per second. Maybe even faster.

That's pretty fast, right. Faster than I drive my car.

Probably faster than you drive your car. I have seen you.

[LAUGHTER]

So how fast are the atoms going in this case then?

So we can cool them so they're going around about a centimetre per second.

Oh, wow, a centimetre per second. That's really interesting because later on we're going to look at parabolic flights, where we're working at similar kinds of velocities with particles. So we've got these tiny atoms moving really slowly. Why do we want them to do that?

Well, we need to investigate what they're doing. And if they're going at 300 metres per second, it doesn't take them very long to cross the experiment that we're doing. So if you look at the vacuum chamber, you can see that the chamber is around about 10 centimetres across, maybe 15 centimetres across. The actual region where we do the experiments is actually a few microns across.

And how big is a micron? Just remind me.

A millionth of a metre

A millionth of a metre.

So a thousandth of a millimetre. So your hair is 50 microns thick.

OK. So it's just smaller than a human hair...

Yeah.

...where you're doing the experiment.

It's about the same sort of volume as that.

And what happens to these atoms then when they're in this quantum world, in this tiny volume? What are they doing? They're interacting with your lasers somehow, all these lasers that are around.

Yeah. So the lasers actually are tuned to what we call the D line. So you saw earlier that you were looking at sodium. And you saw the orange line, those two lines.

That's right, those two lines. So do you have sodium here?

No. We have rubidium.

Oh, rubidium, another element. And you tune your lasers to those D lines.

Yes. So in rubidium, the D lines are actually infrared. They're at 790 nanometers, which is just beyond what, well, most of us can see 780, but it's not in focus. It's like a kind of dull red colour.

OK. And so why are you interested in looking at these D lines? Ultimately, what are you trying to get to happen when you've got this transition happening in the atom and you're looking at this energy release and probing it with the lasers?

Well, there's two things we look at. One thing is that to make the atoms cold, we have to make them interact with the lasers. OK. So what's happening, roughly speaking, is the lasers are lots and lots of photons. And they're showering the atoms, millions of photons per second. And the atoms bounce off of the little photons.

And if you arrange the lasers right and you do some clever tricks with some magnetic fields, the net effect is that the atoms get slowed down by the constant collisions with the photons. So that's one thing we do. The other thing we can do is that the lasers drive transitions, so from a low energy state to a high energy state. And we can play clever tricks, which allows us to manipulate which state the atom is actually in. So if we start in the lower state, we can make it into the next one up.

And this, I think, is something that's quite important in quantum communication and quantum cryptography. Eventually, one day, the way we might be communicating in the future.

So what I'm trying to tell you is that these two lowest energy states, you can associate with them a label, if you like. So we label the lower one, 0, and the upper one, 1, which is just like bits in a computer. But our bits obey the laws of quantum mechanics, which means we can do much more physics with them than just with normal bits in a computer, which only obey classical physics. We've got the quantum world at our disposal.

So we've seen that this quantum world works really well, both in the labs, where we can do simple experiments, that students can one day do if they're studying an Open University degree, but also in a research world, like here. So why do people want to take your type of experiments and your type of ideas with cold atoms into microgravity environments, onto the space station?

Well, there's lots of reasons actually. But just think about one. What's beautiful about atoms is that every rubidium atom is the same. And that means those two energy levels, the 0 and 1, are the same, no matter what's going on, OK well, roughly speaking.

So you can make real stable clocks. And the really stable clocks can be then changed just a little bit to make really stable measurements of, say, gravity, or electric fields, or magnetic fields. So if you go into microgravity, the fields are very, very weak, which means you need really precise measurements of the gravity to even know what it is. And cold atoms are the gold standard for gravity measurements.

So that's really interesting. We can measure gravity. But also, we can measure timing. And timing becomes really important in our world, doesn't it? In bank transfers, in cryptography, but also in satellite communication.

So is there a chance in the future we could see this kind of, like, optical clocks and these cold atoms in all of our satellites? Is that a real possibility?

That's actually the goal. GPS, for example, runs on having very precise timing. And the best timing you can have is an atomic clock. And the typical atomic clock is just the gas of rubidium, with a microwave field in it. But if you can improve that by having very cold rubidium atoms and substituting the microwaves for lasers, you get an even better clock.

Wow. So all of this stuff that we see here in this lab is really contributing towards our timing of the future. But I have to say, I have one last question, after everyone's been thinking while we've been talking. There's an awful lot of stuff on this optical bench. Do you really need all these mirrors?

Yeah, we do. Actually, the hardest thing about these experiments is probably setting them up and making them work. They're extremely complicated.

Let me just give you an example. To make the laser cooling work, we have to have our atoms tuned to one part in about 10 to the 8, so one part in 100 million. If you don't have that kind of precision, the laser cooling doesn't work.

And laser cooling isn't the hardest bit of our experiment. It's the first bit we do. It's the thing which we set up on day one.

So I'm guessing that achieving this, when we go into the space station, they'll still need to put it into a tiny satellite. It's going to require an awful lot of technological advance to take all these, I don't know, I guess it must be at least a hundred mirrors here and condense them into a tiny space, where we can actually do that timing.

Yeah. I've not even counted them. But if you compare our lab to the demands of a satellite, we've got like 30 power sockets. So there's a whole bunch of instruments, concealing lots and lots of power. That all has to be condensed, so it's at low power.

If something goes wrong, if a computer goes wrong, well, you know, that's a pain. We go and get another computer. You can't have that on a satellite.

Absolutely, the challenge of space travel. So we can see actually from all of this, how we start off in the laboratory with research here at the Open University. We take the quantum world. And we're potentially applying it at the end of the day, when we get in a microgravity environment, to our understanding of satellite, GPS, better timing, quantum cryptography, more safety for us here on Earth as well.

Calum, thank you very much.

Thanks, Helen.