Focus on research: physicist Jook Walraven

The research of experimental physicist Jook Walraven on quantum gases is considered such an important achievement that his colleagues nominated him for a fellowship at the American Physical Society (APS). A great honour, since only a half percent of all members of this American association of physicists may call themselves fellows. What scientific discoveries has Walraven made?

‘It was a great surprise to hear that I was elected as a fellow’, says Walraven enthusiastically. ‘Especially since I didn’t actively seek after it.’ The American Physical Society is comparable to the Dutch Physical Society, the Nederlandse Natuurkundige Vereniging. Walraven is a member of both. If someone’s research attracts attention, he may be elected special member of the APS in recognition of the research carried out. The initiative for Walraven’s nomination came from his colleagues in the United States. ‘That was the nicest thing about the nomination, that it came as such a surprise. Some colleagues put my name on the list and others decided to vote for me. There was no need to lobby!’
Scientific research is almost always the work of more than one person. Walraven had a group of good researchers around him when he was doing his work on quantum gases. Among the people he worked with were Tom Hijmans, Merrit Reynolds and Ike Silvera. Walraven carried out the research, which was funded mainly by the FOM and the UvA, between 1980 and 1996.

Not pellets, but waves

Photo: Bob Bronshoff

What exactly are quantum gases? The term is almost self-explanatary: they are gases which act in a quantum-mechanical manner. If you examine a small atom cloud, you can no longer describe the atoms as small pellets; you need to view them as waves. And two colliding waves exhibit very different behaviour than two colliding pellets. Gas, and in fact all matter, acts in a quantum-mechanical manner when there is very little thermal energy in the system, for example if you make it extremely cold.’

The cold gases in Walraven’s experiments are of great importance for the precision of quantum or atomic clocks. Telecommunication and satellite navigation make use of these clocks. The more precisely you are able to measure time, the better you are able to determine your location. Atomic clocks are extremely precise. If an atomic clock had ticked throughout the existence of the universe, it would now be only a hundred seconds off time. ‘We expect that atomic clocks will become a hundred times more exact within a decade. It is now possible to determine your location with the navigation system on your car to within a few metres, then it will be possible to do so within a couple of centimetres.’

The clock and the gas

‘I’ve never built a clock myself,’ says Walraven, laughing ‘But I do know that if you want to make a useful instrument, every clock - whether it is classical or hyper-modern - needs a pendulum and a pointer. It is difficult to measure time exactly with a clock that ticks only once a second. It you let the pendulum move e.g. 1000 times faster, you would be able to measure a second with an accuracy of one in a thousand. The faster the pendulum motion, the more accurate the clock. The best way to get the pendulum move the quickest is to strike an atom rather than striking the pendulum. We do this with light pulses: you strike the atom with light. Then you count the number of times the atom vibrates. And since the atom vibrates incredibly quickly, you are able to count an incredible number of ticks within that one second, and so measure very accurately.’

A requirement for a stable clock is that the atom keeps moving accurately. The atoms you strike must all vibrate with a constant frequency. If they collide, they start to go ‘out of time’, just as the pendulum of your clock at home would if you tapped it. The extent to which a clock goes out of time depends on the number of times you strike it and how hard you do so. The same goes for atoms, where the constancy of the vibrating atoms depends on the temperature which, after all, determines how fast they move.

Making atoms cold

A good atomic clock needs very cold, slow-moving atoms. Gas atoms at room temperature move at the rate of a few thousand metres per second, quantum gases move only a few millimetres per second. ‘We need to keep hold of the cold cloud of gas in a magnetic field, otherwise it would drop like a stone. We then study the way the atoms in the gas collide. The better we understand these collisions, the better atomic clocks will become. You can e.g. choose an atom that is insensitive to collisions. We were the first ever in the world to make a quantum gas.
There are two ways to make a gas extremely cold. The first works in approximately the same way as your cup of coffee cools. The coffee molecules with the most energy escape as steam from your cup, leaving your coffee a little colder. Walraven and his group studied and improved these damping techniques so much that a few years later it was possible to make the first Bose-Einstein condensate using this method.
The second technique is optical cooling, which makes clever use of radiation pressure and the Doppler Effect. ‘For this technique, you need a small atom cloud which you light from all sides with a certain frequency. When an atom moves out of the group towards one of the lasers, it absorbs a photon which makes it move in the opposite direction. So if you ‘shoot’ an atom cloud from all sides, the cloud will remain in the middle and will, eventually, come to a standstill. The size of the cloud tells you how cold the atoms are.’

The beauty of the subject/profession

Walraven measures the properties of atom clouds containing about a million atoms. ‘All those atoms interact with each other by means of quantum mechanics, which are fascinating and fundamental processes The fact that it is possible to use a magnet to levitate such a gas cloud, which has no walls, and nobody touching it - that is truly beautiful. It is an extremely useful subject, where we compare theory and experiment in great detail, thus eliminating errors in the theory. And when we have figured out how the theory works, it can be applied to other systems.’

According to Walraven, a good researcher seeks out unknown fields, a poor researcher follows the beaten track. Walraven himself has set a good example. After a period as director of the FOM Institute AMOLF, he has returned to the UvA and is now again active on his newest research project. He would like to create a new quantum gas made up of a mixture of various atoms. ‘These act in a totally different manner. The great thing about the kind of research I do is that you are so free. When you work on large-scale experiments such as CERN or in space research, where there is lots of money involved and so many people collaborating, you need to be much more careful. You can’t just go and try out some wild idea, and this makes the research more conservative. Here we can be much more playful with nature.’

Published by  Faculty of Science

21 August 2012