An ultradilute quantum liquid made from ultra-cold atoms

December 14, 2017, ICFO
Artistic view of a quantum liquid droplet formed by mixing two gases of ultracold potassium atoms. Credit: ICFO/ Povarchik Studios Barcelona

ICFO researchers created a novel type of liquid 100 million times more dilute than water and 1 million times thinner than air. The experiments, published in Science, exploit a fascinating quantum effect to produce droplets of this exotic phase of matter.

Liquids and gases are two different phases of matter. While gases are dilute, compressible and take the size of their container, liquids are dense, have a fixed volume and, in small quantities, form droplets. These are ensembles of particles that remain bound by themselves, and have a free surface that separates them from the environment. By increasing the temperature, it is possible to induce a phase transition between liquid and gas. This is what happens when boiling water in a pan.

But are gases always dilute and liquids always dense? Although in normal conditions the answer to this question is yes, things can become very different at ultra-low temperatures. In a recent study published in Science, ICFO researchers created a liquid 100 million times more dilute than water and 1 million times thinner than air.

The team cooled down a gas of to -273.15 degrees Celsius, very close to absolute zero. Although at these temperatures, the atoms behave as waves and follow the rules of mechanics, they still conserve an intrinsic property of a gas: They expand in the absence of containment. In contrast, when two such gases are mixed together and attract each other, the atoms instead form liquid droplets. Cesar Cabrera, first author of the study, says, "In many respects, our potassium droplets are very similar to those of water: They have their own size and shape, regardless of where we put them, but they are much colder and their properties are quantum."

An ultradilute quantum liquid made from ultra-cold atoms
A mixture of two ultracold gases that attract each other can form an ultra-dilute liquid, which remains self-bound in a droplet even in the absence of any confinement. Credit: ICFO

Indeed, the existence of these liquid droplets is entirely due to quantum fluctuations, a fascinating intrinsic quantum effect. Furthermore, due to quantum mechanics, the atoms forming a droplet cannot stay completely at rest inside it. This is forbidden by Heisenberg's uncertainty principle. They thus remain in perpetual motion, leading to a quantum pressure that makes very small droplets unstable and evaporates them into an expanding gas. Prof. Leticia Tarruell says, "These are fascinating macroscopic objects: even if they are made up of thousands of particles, their behavior is still fully determined by and correlations. By observing the phase transition between liquid and gas, we measure very precisely these surprising quantum effects."

The unique combination of diluteness and "quantumness" makes quantum an ideal testbed to better understand quantum systems made of many interacting particles, and comprehend features they share with liquid Helium, neutron stars or other complex materials.

Explore further: Quantum particles form droplets

More information: Quantum liquid droplets in a mixture of Bose-Einstein condensates, C. R. Cabrera, L. Tanzi, J. Sanz, B. Naylor, P. Thomas, P. Cheiney and L. Tarruell, Science, 14 December 2017.

Related Stories

Quantum particles form droplets

November 28, 2016

In experiments with magnetic atoms conducted at extremely low temperatures, scientists have demonstrated a unique phase of matter: the atoms form a new type of quantum liquid or quantum droplet state. These so called quantum ...

Stirring up a quantum spin-liquid with disorder

December 13, 2017

Disorder is generally thought to be detrimental to creating materials with unusual magnetism or other quantum phenomena. However, a team found that weak disorder surprisingly stabilizes a rare quantum state called a quantum ...

Physicists' prediction of gas 'droplets' confirmed

November 10, 2016

Ground-breaking theoretical work by University of Otago physics researchers showing that under certain conditions gases can form into stable droplets – as liquids do – has now been confirmed experimentally by scientists ...

Recommended for you

Unusual sound waves discovered in quantum liquids

July 20, 2018

Ordinary sound waves—small oscillations of density—can propagate through all fluids, causing the molecules in the fluid to compress at regular intervals. Now physicists have theoretically shown that in one-dimensional ...

A phonon laser operating at an exceptional point

July 20, 2018

The basic quanta of light (photon) and sound (phonon) are bosonic particles that largely obey similar rules and are in general very good analogs of one another. Physicists have explored this analogy in recent experimental ...

A physics treasure hidden in a wallpaper pattern

July 20, 2018

An international team of scientists has discovered a new, exotic form of insulating material with a metallic surface that could enable more efficient electronics or even quantum computing. The researchers developed a new ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

Steelwolf
not rated yet Dec 15, 2017
I would like to see the Heisenberg principle beaten and see an Ice form of this liquid. It obviously happens with other materials, but to get it to form an 'ice' at such dilution level and density would be interesting indeed.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.