Quantum Activity “Heard” With Record Low Entropy

Just as the darkest patches of the night sky are actually teeming with hundreds of galaxies’ ancient light, the quietest sounds are actually filled with activity. Physicists from UC Berkeley detected quantum activity by cooling a gas to reduce background noise to a level low enough for them to detect even the minuscule quantum effects, usually drowned out by cold fluids’ racketing around at similar frequencies.


When you hear a noise, it’s because of some sort of impact creating subtle vibrations in the surrounding gas, and that wave moving through the gas to your ears. But if you look closely, even the little waves in gas are composed of particles which collide with one another, thus allowing the wave to transfer from one group of gas particles to a neighboring one, and so on, to your ears.

The impact of these particles creates a small amount of energy through friction (i.e., heat), and so when the particles are slowed, you have much less energy created, thus less heat, and less collisions. This means that, when you cool a gas to a low enough temperature, you reduce the frequency of impacts between particles, and thus reduce the noise it generates, clearing the field to listen for the subtlest noises. In other words, this is why it helps for your friends to quiet down while you search for your lost phone by listening for its ringtone; its faint noise can then be recognized with ease.


On this note, despite the gas’ being outdone in coldness by another gas that was once brought to half this one’s temperature, the one used in this experiment did set a record for lowest entropy ever measured. Entropy is a noun which denotes a system’s degree of disorder. This means that low temperature alone isn’t enough to guarantee an absence of noise.

“This ‘lowest entropy’ or ‘lowest noise’ condition means that the quantum gas can be used to bring forth subtle quantum mechanical effects which are a main target for modern research on materials and on many-body physics,” explained co-author Dan Stamper-Kurn, a UC Berkeley professor of physics. “When all is quiet and all is still, one might discern the subtle music of many-body quantum mechanics.”


The gas used to detect the quantum activity is a Bose-Einstein condensate, and is composed of a million rubidium atoms constrained together by a beam of light, separated from the outside atmosphere in a vacuum, and then is cooled to its lowest energy state. The temperature was brought so low that scientists had to create a new kind of thermometer just to measure it.

“One of the holy grails of modern physics is to understand these exotic materials well enough to design one that is superconducting without requiring any cooling at all,” explained UC Berkeley graduate Ryan Olf. “By studying the properties of low-entropy gases in various configurations, our community of researchers hope to learn what makes these fascinating materials work the way they do.”

Olf added that the temperature isn’t really the measuring stick when it comes to things as delicate as quantum effects. Instead, the entropy per particle is used to compare systems, and the scientists toiled to bring the entropy far enough down to really test their models for such materials.

“In a very real sense, this constitutes the coldest gas ever produced, at 50 times lower than the temperature at which quantum statistical effects become manifest, the Bose-Einstein condensation temperature,” Olf remarked.

This experiment, which was recently published online, will be released in a future print edition of the periodical Nature Physics.

Source: UC Berkeley

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