Cooling glass beads to study quantum entanglement


University of Adelaide researchers are part of an international team making advances in understanding the quantum world. Your chosen examination method? A “micro fridge” capable of chilling tiny objects to the coldest temperature in the universe.

Quantum mechanics has helped shape our understanding of the fact that the universe is very strange over the past hundred years.

For example, the “superposition principle” tells us that the properties of the particle do not take on a single value unless observed directly in an experiment, but a “superposition” of probable values, each with its own probability.

If superpositions are distributed over several particles, one speaks of an “entanglement” of the particles. Quantum entanglement means that the physical states of two or more entangled particles remain connected regardless of the distance separating them.

Quantum mechanics is only observable in the tiny world of particles, atoms and molecules. The direct observation of quantum effects also requires extremely low temperatures. In fact, the lowest temperatures in the universe.

Research published in optics Journal aims to help science answer the question of why quantum effects are not visible on the scale of our daily experience. Quantum entanglement of everyday objects is still science fiction.

“Quantum mechanics describes the behavior of extraordinarily small objects at very low temperatures,” says senior author and project leader Professor Kishan Dholakia from the University of Adelaide and the University of St Andrews, UK. He describes quantum entanglement as “remarkable”.

Einstein called it “spooky action at a distance”. This effect couples the fate of separate objects: when you take a measurement on one object, you immediately know 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 effort to realize quantum computing and quantum-based encryption.”

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To study entanglement, the team had to be in the ‘quantum regime’. This meant that they had to cool the objects to be examined. The bigger the object, the colder they had to get it.

So far, entanglement has only been observed in exceptionally small and cold objects, such as small clouds of atoms or molecules.

researchers from the University of St Andrews; the University of Adelaide, Australia; The project involved the University of Arizona, USA and the Institute of Scientific Instruments of the Czech Academy of Sciences, Czech Republic.

But this international team of researchers developed a method for cooling two or more glass beads – each the size of a red blood cell (just a few microns in diameter) – to be chilled to temperatures below the coldest regions of space.

At this length, the temperature is directly related to the speed of movement of the object. So slowing down the vibrations in the beads has the effect of cooling them.

“We used lasers to slow down one of the beads, which then acted like a refrigerator for other beads,” says first author Dr. Yoshihiko Arita, also from the University of St Andrews.

So the microfridge is not a box into which the glass beads are placed – it is itself one of the refrigerated beads.

“By reducing the temperature of the ‘fridge’, the other globules were chilled to less than a degree above absolute zero, −273.15 °C, the coldest attainable temperature in the universe.

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

Dholakia explains that with such experiments on supercooled objects in the micron range, they are “ready to offer a paradigm shift for terrestrial sensing of fundamental forces and quantum physics. They could even lead to desktop gravitational wave sensors,” explains Dholakia. “This work will inspire researchers to explore the utility of multiple particles for a range of studies in this emerging field.”


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