‘Micro-fridge’ helps investigate behaviour of atoms and molecules

Purple lights.

University of Adelaide researchers are part of an international team of experts who have taken a major step in testing quantum mechanics by cooling glass beads the size of a blood cell to the coldest temperature in the Universe.

This research, published in the journal Optica, could help address the long-standing open question in physics - why mysterious quantum effects that govern the behaviour of atoms and molecules are not seen on an everyday scale.

“Quantum mechanics describes the behaviour of exceptionally small objects at very low temperatures. Among the remarkable effects of quantum mechanics is quantum entanglement,” said Professor Kishan Dholakia from the University of Adelaide and the University of St Andrews, who led the team.

“Referred to by Einstein as “spooky action at a distance”, this effect couples the destiny of separated objects: performing a measurement of one object instantaneously tells you the result of the same measurement on the other object, even if it is exceptionally far away.

“Quantum entanglement is a key phenomenon behind the current drive to realise quantum computers and quantum-based encryption.”

To see entanglement between two objects, they first need to be in the quantum regime. This means they need to be incredibly cold – and the bigger the object, the colder it must be. For this reason, entanglement has only ever been demonstrated with exceptionally small and cold objects, such as small clouds of atoms or molecules. Entanglement of everyday objects remains in the realm of science fiction.

The researchers from the University of St Andrews, UK; the University of Adelaide, Australia; The University of Arizona, USA and the Institute of Scientific Instruments of the Czech Academy of Sciences, Czech Republic, have now developed a way to allow two or more glass beads, each the size of a red blood cell, to be cooled to temperatures colder than the depths of outer space.

“This work will inspire researchers to explore the merit of multiple particles for a range of studies in this burgeoning area.”Professor Kishan Dholakia from the University of Adelaide.

For objects this size, the speed of their motion is related to their temperature, so slowing an object down is effectively cooling it.

Dr Yoshihiko Arita, Research Fellow in the School of Physics and Astronomy at the University of St Andrews was the first author of the study.

“We used lasers to slow one of the beads, which then acted like a refrigerator for additional beads. To achieve this we used light scattering between the beads to couple their motion,” said Dr Arita.

“Reducing the temperature of the laser-cooled ‘refrigerator’ cooled the other beads to less than one degree above absolute zero, −273.15 °C, the coldest temperature achievable in the Universe.

“This experiment shows a new path by which we may cool two or more objects. It is exciting that the approach is compatible with many current experiments in the field and it offers a potential route to seeing entanglement in objects that are at the edge of what we can see with the naked eye.”

Professor Dholakia said: “Levitated particles are poised to offer a paradigm shift for terrestrial sensing of fundamental forces and quantum physics. They could even lead to table-top sensors of gravitational waves.”

“This work will inspire researchers to explore the merit of multiple particles for a range of studies in this burgeoning area.”

The team’s work is funded by an ARC Discovery Project grant for research that will be undertaken at the University of Adelaide over the next few years which will advance understanding in this area.

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