They create a quantum liquid millions of times more diluted than water

Researchers from the Institute of Photonic Sciences have generated a new type of liquid a hundred million times more diluted than water and a million times less dense than air. To produce the drops in this very exotic phase of matter they have used ultra-cold atoms and a surprising quantum effect. Liquids and gases are two states of matter that are part of our day to day. While the gases are diluted, compressible and occupy all the available space, the liquids are dense, their volume is well determined and in small quantities they form drops. These drops are constituted by a set of particles linked together and isolated from the environment by a well-defined outer surface. When heated they evaporate, resulting in a phase transition from liquid to gas. This is exactly what happens when we boil water when cooking.

Now, can a liquid be ultradiluted? Is it possible to imagine that a liquid contained in a soup spoon could occupy the volume of an Olympic pool? Although under normal conditions this is undoubtedly impossible, at very low temperatures the matter behaves in an unusual and surprising way.

In a recent study published in the journal Science, researchers from the Institute of Photonic Sciences (ICFO) led by Leticia Tarruell (professor Cellex Nest), have created a liquid one hundred million times more diluted than water and a million times less dense than the air.

To achieve this, they have cooled a gas of potassium atoms to -273.15º C, very close to absolute zero. Although atoms at these temperatures behave like waves and obey the laws of quantum mechanics, they still retain an intrinsic property of gases: they occupy all the available volume.

However, by mixing two gases that attract each other at these low temperatures, ultra-diluted liquid droplets can form. “In many ways, our quantum potassium drops are very similar to water droplets: they have a well-defined shape and size. On the other hand, they are extremely cold and have unique quantum properties “explains Cesar R. Cabrera, first author of the article.

In fact, the existence of these drops is due exclusively to quantum fluctuations, a surprising quantum effect. Furthermore, due to Heisenberg’s uncertainty principle, the atoms that form them can never be at absolute rest. This continuous movement generates a small additional energy that makes the very small drops evaporate again becoming a gas.

“These drops are fascinating because, despite being macroscopic objects formed by thousands of particles, their behavior is totally determined by fluctuations and quantum correlations. By observing the phase transition between liquid and gas, we can better measure these quantum effects, “says Tarruell.

The ultra-diluted nature and intrinsically quantum properties of these drops make them ideal for better understanding the behavior of many interacting quantum particles, as well as some of the characteristics common to liquid helium, neutron stars or even some complex materials.

Cesar R. Cabrera, Luca Tanzi, Julio Sanz, Bruno Naylor, Philip Thomas, Pierrick Cheiney and Leticia Tarruell. “Quantum liquid droplets in a mixture of Bose-Einstein condensatesScience, 14 diciembre 2017


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