- An increasing amount of questions about the quantum world gives a new push to research close to absolute zero. Quantum physicists are moving back to the domain of ultra-low temperatures, where atoms nearly come to a standstill and are therefore easier to study. In line with Leiden tradition, scientists from the Leiden Institute of Physics (LION) have built a fridge that approaches absolute zero up to one milliKelvin. Tjerk Oosterkamp now uses his device—recently on tv show Het Klokhuis—to perform his first measurements on diamond.
Back in 1908, it was Heike Kamerlingh Onnes who became the first person ever to liquefy helium, in his lab in Leiden. For this he needed a temperature of minus 269 degrees Celsius—only four degrees above absolute zero. This earned him the Physics Nobel Prize in 1913. Following his footsteps, LION spin-off Leiden Cryogenics keeps the tradition alive by making fridges that reach temperatures of eight milliKelvin. This provides physicists nowadays with the opportunity to simply buy a device and do what was such a great endeavor for Kamerlingh Onnes. In that sense, life became a lot easier for them. But researchers wouldn’t be researchers if they didn’t want to go further into the extreme. And so Oosterkamp started working on his fridge—together with Leiden Cryogenics—to get it as cold as one milliKelvin. In that intense cold it is possible to conduct groundbreaking science. ‘Ultra-cold research is back and booming,’ he shouts with his device buzzing loudly behind him. ‘For a long time it has been considered less interesting to go towards absolute zero. But now we have many questions that can only be answered in extreme cold.’
Close to absolute zero, particles practically stop moving, so scientists can perform accurate measurements. For example on new combinations of materials they want to use in future quantum computers. Or they dive into questions about the laws of nature on the interface of quantum mechanics and the theory of general relativity—the theory that says space and time curve in the presence of mass. By extremely cooling a small mechanical oscillator and then manipulate it, physicists try to place an object at two positions simultaneously. In that scenario it is unclear which mass distribution determines the curvature of space-time. This gives rise to a new kind of uncertainty and might hold a clue to a large mystery in physics: the collapse of a wave function. Experimental research on this theory could lead to a groundbreaking result.
It is safe to say that there are enough reasons for Oosterkamp to create an ultra-cold environment. And indeed there should be, as it is a giant tour de force to go from room temperature to one milliKelvin. First, they set the fridge to four Kelvin. For this the research team uses a compressor that alternately makes helium (4He) compress and expand in a small chamber. The resulting low temperature is the starting point to let a container of 4He condense inside a second cooling system. They add the helium isotope 3He to this container, producing a mixture of 3He-4He. This requires mixing enthalpy, so it extracts energy from its surroundings. Next, a pump separates the 3He atoms from their heavier counterparts, after which the researchers re-use them in the same process.
In the meantime, it is of vital importance for the setup to stay as well isolated as possible. Oosterkamp’s group uses vacuum layers, heat shields and many ultra-thin layers of plastic and aluminum foil. Now in order to push through to truly extreme temperatures, they take a piece of metal and cool it down inside a magnetic field of two Tesla. A magnetic field creates order and therefore low entropy, because it makes all the metal’s nuclear magnets point in the same direction. When they slowly turn off the field, the nuclear magnets start to point in random directions again. This chaos increases the entropy, which cools the system even further. ‘In the end we reach 1 mK,’ says Oosterkamp. ‘It’s no world record, but that’s not why we do it. It is about the research we’re able to conduct at that temperature, and that is booming once again. Twenty years ago you saw the people at conferences on this area growing older and older, but now we have a new generation of young scientists studying old cooling techniques. With all the new research questions in quantum mechanics, science moves back to ultra-low temperatures. And for this Leiden is both historically and currently the place to be.’
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Publ. 25-01-2016 10:05