For the first time, researchers at the University of Basel used an ultracool atomic gas to cool a very thin membrane to less than one degree Kelvin. The new technique might enable novel investigations of quantum mechanics phenomena and precision measuring devices.

Coldest matter in the world lends its freeze

A cloud of ultracold atoms (red) is used to cool the mechanical vibrations of a millimeter-sized membrane (brown, in black frame). The mechanical interaction between atoms and membrane is generated by a laser beam and an optical resonator (blue mirror). Credit: Tobias Kampschulte, University of Basel

In the ultracold world, produced by methods of laser cooling and trapping, atoms move at a snail’s pace and behave like matter waves. Typically, lasers are used to trap atoms inside a vacuum chamber, almost grounding all atomic vibrations to a halt and thus lower temperature close to less than 1 millionth of a degree above absolute zero. In this state, atoms behave differently – governed by laws of spooky quantum mechanics – and move in small wave packets. This means superposition or being in several places at once.

Ultracooled atoms are usually used in so called atomic clocks that only lose a second every couple hundred millions of years. These are very useful for syncing GPS satellites, for instance, but can ultracool atoms be used to refrigerate some other matter? It’s a very interesting idea, but only if one can surpass the challenges. Even the largest ultracool atom clouds, which can number billions of particles, aren’t larger than a grain of sand. Because the surface area is so small, it’s very difficult to transfer heat and cool objects.

There are workarounds, however. Swiss researchers successfully cooled the vibrations of a millimeter-sized membrane using ultracool atoms. The membrane, a silicon nitride film of 50 nm thickness, oscillates up and down like a small square drumhead. Such mechanical oscillators are never fully at rest but show thermal vibrations that depend on their temperature. Although the membrane contains about a billion times more particles than the atomic cloud, a strong cooling effect was observed, which cooled the membrane vibrations to less than 1 degree above absolute zero, as reported in Nature Nanotechnology.

“The trick here is to concentrate the entire cooling power of the atoms on the desired vibrational mode of the membrane,” explains Dr. Andreas Jöckel, a member of the project team.

A laser light was shone which changed the vibration of the membrane and transmitted the cooling effect over a distance of several meters. The effect was amplified by an optical resonator made of two mirrors, with the membrane sandwiched in between. Previously, systems that use light to couple ultracold atoms and mechanical oscillator had been proposed theoretically, but this is the first time it’s been demonstrated experimentally.

The take away is that such a system might be employed to experience quantum mechanical system in macrosized objects – the kind that you can see with the naked eye.

It may also be possible to generate what are known as entangled states between atoms and membrane. A membrane’s vibrations could be measured with unprecedented detail, and along with the improvement would follow a new class of highly sensitive sensors for small forces and masses.

“The well-controlled quantum nature of the atoms combined with the light-induced interaction is opening up new possibilities for quantum control of the membrane,” says Treutlein.