Universiteit Leiden

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Hielco Kuipers

1.3 million euros to catch quantum collapse

Leiden physicists Tjerk Oosterkamp and Dirk Bouwmeester have received a 1.3 million euro Science-Groot grant from NWO to catch a mystery at the core of quantum mechanics, together with Amsterdam physicist Jasper van Wezel.

Quantum mechanics is strange but true: it's a theory that describes atoms, electrons and other small particles extremely successfully. But it also predicts that such particles can be in superpositions, essentially existing in two places at once. 

That is odd since we never see such superpositions in real life: we see real objects in just one place, not two places at the same time. This seems to imply a border between the quantum world and the macroscopic world that we all know.

One leading interpretation says that this is because the act of looking, or 'measuring', forces it to choose between one of the states. Oosterkamp: 'It's unclear how this "collapse" is supposed to work. And what does "measuring" mean?'

Superpositions

These are questions that Bouwmeester, Oosterkamp and Van Wezel want to address. They plan to do this by trying to coax a small but macroscopical object, called a resonator, into a superposition.

For Bouwmeester's lab, the resonator is a micrometre-sized mirror, which enters a superposition when a light particle in superposition hits them. For Oosterkamp's lab, the resonator is a 5-micrometre wide magnetic ball vibrating on the tip of a thin needle.

Refrigerators

To detect quantum effects, the required circumstances are extreme: one is an extremely low temperature, an old Leiden specialisation. Refrigerators need to be built that are able to cool down to 1 millikelvin, or a thousandth of a degree above absolute zero, which is -273,15 degrees Celsius.

Another requirement is keeping vibrations at bay: the experimental setups in the new 'measuring hall' in the Gorlaeus Building are placed on huge ultra-low vibration platforms. Once the setup works, the researchers should be able to test different theories about quantum collapse. For instance, one theory states that the mass of the resonator limits its ability to enter a superposition state.

Philosophical question

Oosterkamp: 'To detect such a state, we have devised a setup where we repeat superposition steps. If the resonator enters superposition, it will interfere with itself, which means different superposition levels cancel each other out.' If there is no superposition, no such interference will be detectable.

Quantum collapse is a highly theoretical, even philosophical question, but could also have implications for the workings of the quantum computers. This new technology, which sees heavy research investment from industry, uses quantum superposition to speed up calculations.

Daunting challenges

The 1.3 million euro grant, from the NWO Open Competition Domain Science-GROOT programme, covers five years and provides funding for three PhD candidates as well as instruments and materials. Oosterkamp: 'The technical challenges are quite daunting, but I think we're finally at a stage where we can address these questions experimentally.'

Text: Bruno van Wayenburg

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