Bose-Einstein condensates, explained

What is the fifth state of matter? Two physicists tackle the subject...
09 June 2020

Interview with 

Rob Nyman, Imperial College London; Lindsay LeBlanc, University of Alberta


We're celebrating the 25th anniversary of a Nobel Prize-winning achievement: the creation of the 'fifth state of matter', a bizarre soup of particles that’s known as a Bose-Einstein condensate. It’s one of the more complicated concepts in physics, but that very complexity is what makes it mysterious and exciting - with implications for quantum computers, dark matter, and the very fabric of the universe. Where does this idea come from? Chris Smith heard from two Bose-Einstein condensate physicists: Lindsay LeBlanc from the University of Alberta, and Rob Nyman at Imperial College London...

Rob - Satyendra Nath Bose was an Indian mathematician who made some realisations about the statistics when you count the ways of arranging identical particles. In quantum mechanics, there's a particular concept of identical particles. So identical that you couldn't stick a label on them, even in principle. And that's what Satyendra Nath Bose realised. And he tried to get his work published. He wrote to lots of journals and they all said no, because they didn't understand his work. And eventually in despair, he wrote to Einstein and said, "can you please see if you understand it and maybe get this published?" And Einstein immediately understood it, and did some calculations himself and realised that one of the consequences is Bose-Einstein condensation. He translated the paper into German and helped get it published in a German article.

Chris - And how long ago was that?

Rob - Satyendra Nath Bose wrote to Einstein in 1924, and Einstein published his article in 1925.

Chris - So this is another example of work which was theorised almost a century ago, but has taken until almost the modern era and technology to catch up so we can actually start working on the ideas that these people had?

Rob - Absolutely. It's a simple idea, where Einstein had a thought experiment about simply adding a few more particles to a system. And they all collapse into one state, which is the essence of a Bose-Einstein condensate. He had that idea, and it took 75 years before we could actually make them into a real experiment rather than just a thought experiment.

Chris - As I mentioned, Lindsay LeBlanc is also with us. Lindsay is at the University of Alberta, she works on this too. Lindsay - what actually is a Bose-Einstein condensate, though? If we were to actually see one in a dish, what would it look like?

Lindsay - Well for first thing, it's quite small. And so you need a microscope of sorts to see it. It's a collection of atoms where all of these atoms are no longer individual particles anymore. They become almost like a super atom - they all behave together. They work together and they create this sort of new quantum soup that has different characteristics than the other kinds of matter that we know about.

Chris - Now, when you say that they all behave as though they're one - when we're just got individual atoms, they would just be buzzing around all over the place, going in random directions. How do you actually turn them then into a Bose-Einstein condensate so they behave as though they're just one atom?

Lindsay - The first thing that we need to do to make a Bose-Einstein condensate is to make them very cold. The ideas of quantum mechanics start to emerge when things are cold, because we are able to get rid of the randomness associated with temperature. So we think of hot things as, exactly, buzzing around in a random way. To get to the Bose-Einstein condensate, we need to get rid of that randomness. And so we need to cool the atoms down to very low energies. And this comes back to the original ideas of Bose and Einstein, where: why does this happen? It happens because the world is quantum, and we could only see this at these very low energies when things are very cold.

Quantum means that there are discrete levels. And so you can have, say, zero, or one energy unit, or two energy units; but you can't have like 1%, you can't have just a little bit, you have to have either zero or one. And so if you get cold enough, suddenly these particles don't have enough energy to be in that first energy unit. They have to be in the bottom one. And so they're all acting together in this condensate. Experimenters like me and Rob in the lab, we have to figure out ways to get those atoms that cold. And that was really the technical achievement that was made possible by lasers and all the developments through the 80s and 90s that brought us the first Bose-Einstein condensates in the lab.

Chris - And Rob, when we actually think about the way these things are behaving, are there any good sort of analogies that you can give me, which would enable me to imagine atoms all behaving as though they're one giant atom instead of a collection of atoms?

Rob - Yeah, I think probably the clearest analogy for me is that a Bose-Einstein condensate is to an ordinary balloon full of gas, as a laser is to a light bulb. Very, very intense and everything is moving together. So lasers come in beams; light bulbs spread out. And lasers can be much brighter than light bulbs. Just like Bose-Einstein condensates can have high densities, but also the particles move together.

Chris - And Lindsay, when we get down to this low temperature and we make atoms get into this configuration, what sorts of properties do they have that get physicists like you excited?

Lindsay - The fact that they are quantum particles, but there are many of them means that I can work with them. So they're big enough that as an experimentalist, I can do something useful with them. There's many different things that people have figured out what to do with them over the last 25 years. Some examples include quantum memory, which is something we work on in my lab. We take the atoms and we use their quantumness to store quantum information, which is very much related to this revolution in quantum computing that is going on around us. We're able to use the quantum properties of this macroscopic object, to grab onto information, hold onto it and then retrieve it sometime later. So it's almost like RAM or a hard disc for a quantum computer. Other applications include generally understanding the fabric of our universe. So this is a quantum object that we can manipulate, that we can study, that we can take pictures of. And what we can do in the lab is sort of poke and prod it in different ways to see what happens to quantum objects under these conditions. And so we use these condensates as a simulator of other quantum systems to learn more about how many body quantum systems act in a variety of contexts.


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