This story is over 5 years old.

The Coldest Matter in the World Can Act as a Near-Absolute Zero Fridge

The ultracold world can now be seen by the naked eye.
November 25, 2014, 12:00pm
​Image: Gramicidin/​Flickr

A team of physicists has devised a new technique for using ultra-cold atomic gas to chill large-scale matter to less than a single degree above absolute zero. It's a vast, new doorway into a bizarre physical realm where time itself seems to stop along with the motion of particles.

Reaching temperatures as cold as this is not in itself a new thing, but what the researchers, led by University of Basel physicist Philipp Treutlein, seem to have achieved is a way of scaling up the cooling technique, with the result being the ability to freeze systems as large as a single millimeter. The implications are fully bizarre: a way of making quantum worlds macro-scale.


Temperature at a certain point doesn't have very much to do with "temperature" and instead is a measurement describing energy and disorder. The atoms making up different sorts of matter are never really resting; instead, they shake and tremble. Even this stupid desk is an atomic riot. As room temperature matter, it's bursting with enthalpy (internal/thermal energy) and good old entropy, which is a measurement of disorder or a system's possible states/arrangements.

the supercooled world is a strange, strange realm. It can get stranger

As things get colder and colder, the atomic world slows down. That's even what "colder and colder" properly means: slowing down, a minimum state of enthalpy and entropy. Atoms oscillate, gaining a sort of internal resonance thanks to electrons bouncing up and down the atom's different energy levels. Ultra-cold atoms are slower atoms, which means they can be more precisely probed, which is helpful if those atoms are the atoms of an atomic clock.

This sort of freezing is actually done with lasers. Fire a beam of photons (particles of light) at some atom and sometimes the atom will absorb one of those particles. All of the junk inside the atom winds up relatively excited, thermally, and so the atom kicks out a couple of photons. The photons it winds up getting rid of as light happen to have a higher average energy than the photon it absorbed, which is just a conversion of the original photon's momentum to heat.


Atoms eventually become completely drained of thermal energy and as a result they stop acting like the large-scale objects they are. (An atom compared to a photon is like an entire universe for the light particle.) Atoms start doing quantum things, like occupying superpositions, e.g. being in many places at once. These atoms are no longer "things," but wavepackets, indeterminate arrangements of chance. This is pretty weird and against the rules for something as big as an atom, but the supercooled world is a strange, strange realm. It can get stranger.

Image: Treutlein et al

There's no such thing as a refrigerator for near-absolute zero, and the laser scheme is limited to, so far, systems of a mere several billion atoms—which is still smaller than a grain of sand. The atomic gas developed by Treutlein offers something a bit closer to the desired quantum fridge. The gas is created via the laser technique above and is still very, very small, but what the physicists discovered is that it can be used to supercool much bigger objects.

In this case, the object was a thin silicon membrane. This sort of membrane has the interesting property of behaving like a tiny drumhead, oscillating in time to the thermal vibrations of whatever energy state/temperature it happens to be in. It's a sort of measurement instrument in itself.


The membrane in the experiment is linked to the atomic cloud via a laser beam, which is key. "To date, cooling with atoms and atomic ions has been used to cool other microscopic particles such as different atoms or molecular ions," Treutlein and team write. "In these experiments, the coolant and the target species thermalize through short-range collisional or electrostatic interactions in a trap. A large difference in their mass reduces the cooling performance, which has prevented extension to more massive objects."

Imagine the cloud having a certain resonance that filters the light in such a way that the membrane acquires that same resonance from the laser. At nearly a millimeter in size, this membrane is relatively enormous, many billions of times the size of the cloud. It's visible to the eye, yet able to behave as a quantum mechanical system, like the laser-cooled atoms in atomic clocks. The laser acts as a bridge of sorts, linking systems with vast differences in mass.

"The trick here is to concentrate the entire cooling power of the atoms on the desired vibrational mode of the membrane," explained Andreas Jöckel, one of the physicists behind the current study, in a statement. "The laser light exerts forces on the membrane and atoms. Vibration of the membrane changes the light force on the atoms and vice versa."

Absolute zero, or the fully strange realm just above it, is no longer the distant concept it once was. And it would seem neither is the quantum realm.