By Dainius Kilda
Where do we look for the coldest atoms? The official record on Earth is -89°C at the Vostok station in Antarctica. Outer space is even colder: a distinctly chilly -270°C, which is only about 3°C above the lowest possible temperature, known as absolute zero. But hidden from our eyes, an even colder spot exists in physics labs, where some adventurous physicists cool atoms down to only a few billionths of a degree (0.000000001°C) above absolute zero!
Cold matter is a big hit in physics for a reason: it is a breeding ground for quantum phenomena, including new phases of matter that only exist because of quantum mechanics. In the everyday world around us, we usually see only the familiar phases of matter: solids, liquids, gases, and plasma. But at extremely low temperatures, atoms in an ordinary gas condense into a single entity – the exotic fifth phase of matter called a Bose-Einstein condensate, in which the atoms behave like a huge quantum wave. Bose-Einstein condensates can flow without friction, cannot be rotated, and may have important applications in precision sensing, quantum computing, and several other areas.
In this article, we’ll describe what it even means for an object to get this cold, how Bose-Einstein condensates are made, and what we can do with them now we’ve got one.
What is temperature, and what does being cold really mean?
The temperature simply tells us how much kinetic energy atoms or molecules have in the matter around us. When the air is hot, the particles have a lot of kinetic energy and move at high speeds; the colder it gets, the less kinetic energy the particles have and the slower they move. Cooling atoms therefore means extracting energy from them and slowing down their motion. Theoretically, at absolute zero everything would come to a complete standstill.
However, absolute zero, like the horizon, can be approached but never reached. This is because the colder the atoms get, the harder it becomes to extract the remaining bit of energy from them. Nonetheless, modern refrigeration technologies can get them within a few billionths of a degree above absolute zero, which is close enough to see some very interesting things!
So let’s imagine taking a few million such atoms, and placing them in a bowl. In fact, we will make the ‘bowl’ out of lasers – if we actually touch the atoms, they’ll immediately form frost, and the experiment will be over! Now we cool the atoms down to very low temperatures. What happens?
When atoms lose their identities
Quantum mechanics has shown us that all the matter in the universe has a wave-like aspect to it. We can thus liken an atom-wave to the ripples spreading out on the water surface after a pebble is dropped into a pond. If we drop two pebbles, their ripples will meet and overlap, creating interesting patterns. Where the peaks of the two waves pass through each other, the ripples add up and become stronger. Where a peak of one wave passes through a trough of the other, the ripples cancel each other out and the water remains flat, as if no pebbles had been thrown into the pond at all!
Why don’t we notice this in our everyday lives? It’s because, at normal temperatures, the peaks and troughs of the waves are so close together (i.e. the wavelength of the wave is so short) that the waves can make tiny packets that look indistinguishable from particles. But as the atoms are cooled, the wavelength of the atom-waves increases. Eventually, the atom-wave of one packet starts to overlap with that of the others. At this point, the atoms start to lose their individual identities. What happens then?
The answer depends on a very peculiar thing: whether the total number of particles (neutrons, protons, and electrons) making up each atom is odd or even! If that number is odd, the atoms are called fermions (after Enrico Fermi), and nothing special happens: the atom-waves smoothly spread out and overlap, but there is no sudden change in the behaviour of the cloud.
However, if that number is even, the atoms are called bosons (after Satyendra Nath Bose), and they behave quite differently upon cooling. As the temperature drops below a certain level – called the transition temperature – the atom-waves start to synchronise in the fundamental mode: the longest wavelength that the bowl can support. As the temperature is lowered further, more and more atom-waves pile into this mode, until at the very lowest temperatures essentially all atom-waves are vibrating in sync. Thus the atoms lose their individual identities and form a single giant matter-wave called the Bose-Einstein condensate.
But how do we actually go about cooling the atoms to such low temperatures? Perhaps surprisingly, the key is to shine lasers at them!
How to cool atoms with lasers in principle
We usually associate laser light with heat; but, with some trickery, we can also make lasers cool atoms down to less than a millionth of a degree above absolute zero. How? By using the force of the laser beam on the atoms to slow them down! Each photon in the laser beam carries some momentum. When an atom moving towards the laser absorbs a photon, it absorbs that momentum, and gets a ‘kick-back’.
But wait a minute! If the atoms coming towards the laser are slowed down, aren’t the atoms moving away from the laser sped up? This is the clever bit: we can use the Doppler effect to make the incoming atoms more likely to absorb the laser light than the outgoing ones.
How does that work? An atom at rest can only absorb a photon that has the right amount of energy (the right frequency) to provoke one of its electrons into a ‘quantum leap’ from the lowest to the next-lowest orbital. But, in the same way that when an ambulance drives towards you you hear its siren at a higher frequency, an atom moving towards the laser will see the photon frequencies shifted to higher values, i.e. to the blue end of the spectrum. On the other hand, an atom moving away from the laser will see the photons shifted towards the red.
So let’s set our laser on the red side of the frequency at which a stationary atom would absorb the light. Then, because of the Doppler effect, atoms moving towards the laser will see the light shifted towards the blue, and so it will have the right frequency for them to absorb it. These atoms will therefore gradually slow down as they absorb and re-emit the photons. The atoms moving away from the laser will ‘perceive’ the light as red-shifted further away from the absorption frequency, and will thus not be able to absorb any photons. Hence they will not interact with the laser beam, and will avoid being heated.
How to cool atoms with lasers in practice
In reality, of course, the atoms of the gas are moving in all directions of 3D space. To give the atoms a ‘kick-back’ in every direction, we will therefore need three pairs of counter-propagating laser beams, one directed along each spatial axis. This set-up is, rather poetically, called optical molasses.
In optical molasses, atoms are left struggling in a viscous fluid of light where they experience resistive forces no matter how they try to move. The laser light there is so intense that it can exert a force on atoms creating a deceleration 10,000 times stronger than the gravitational deceleration that acts on a ball thrown into the air. As a result, in a matter of milliseconds, the room-temperature atoms whizzing by at 1,800 miles per hour will be forced to slow down to ‘crawling’ at 45 miles per hour.
So can we do this forever? Sadly, no: like everything else, laser cooling has its limits. Even when the atoms are moving as slowly as possible, there is still the recoil after photon absorption or emission, which averages to zero but keeps pushing atoms around in a random trembling motion. The cooling comes to a halt when the atom velocities become as small as a kick from a single photon.
How to get even colder
By now, we our atoms have been chilled to temperatures a few millionths of a degree above absolute zero. But even this is still far too hot for Bose-Einstein condensation; and, as we have just discussed, laser cooling cannot take us any further. So what do we do next? Thankfully, the atoms are now cold enough to be transferred to a magnetic trap, where we can continue the cooling with an old-fashioned method known as evaporation. Once we have our atoms sitting comfortably in the trap, we can cool them just as we would cool a cup of tea!
The liquid in a teacup has many molecules flying around; and, as we know, the temperature of the tea is a measure of the average speed of those molecules. But actually the molecules have a broad range of speeds: some are much faster than the average, and some much slower. The fastest-moving molecules escape the cup as steam, carrying away their kinetic energy from the cup, and leaving behind the slower molecules. As a result, the liquid left in the cup has lost heat and the tea cools down by a tiny bit. Likewise, we can cool down the ‘tea’ of atoms in our magnetic ‘cup’, allowing the most energetic atoms to escape while keeping the cooler ones confined in the trap.
Bose-Einstein condensation becomes a reality
Armed with laser cooling and magnetic evaporation, physicists in Colorado and at the Massachusetts Institute of Technology (MIT) finally succeeded in creating the world’s first laser-cooled Bose-Einstein condensate in 1995, seven decades after its conception by Bose and Einstein. To do so, they had to cool the atoms to 20 billionths of a degree above absolute zero (0.00000002°C!).
Containing around 100,000 atoms, and extending a few millimetres across, the condensate behaved like a single giant atom-wave. When two such Bose-Einstein condensates are brought together, they can actually interfere, just like the ripples in a pond. Where the peak of one Bose-Einstein condensate meets the trough of another, atoms appear to ‘vanish’, and where the two peaks meet, we observe four times the original density of atoms. This is a startling quality of Bose-Einstein condensates: they are comparatively large lumps of matter, but still exhibit clear quantum wave properties – properties we would usually only expect to see at the level of individual atoms, electrons, or other minuscule quantum particles.
The ability to cool atoms with lasers and create Bose-Einstein condensates has opened a new window to quantum physics, as recognised by both the 1997 and 2001 Nobel prizes. Ultra-cold atoms continue to intrigue physicists, and have become an inexhaustible source of developments: new types of laser that emit a beam of atoms instead of light; novel ways of controlling matter on the quantum scale; the ongoing quest for building future quantum computers – the sky’s the limit in the cold quantum world!
Fig.1 What is Bose-Einstein condensation? © Copyright W.Ketterle (reproduced with permission). Source: http://www.rle.mit.edu/cua_pub/ketterle_group/intro/whatbec/whtisbec.html
Fig.2 Optical molasses, Andreas Lilius. Licensed under GNU FDL. Source: http://nobelprize.org/nobel_prizes/physics/laureates/1997/illpres/trapping.html
Fig.3 Bose-Einstein condensate, NIST/JILA/CU-Boulder. Public domain image. Source: https://patapsco.nist.gov/imagegallery/details.cfm?imageid=193