Just how cold can a mechanical object get? Apparently, way colder than scientists ever imagined possible.
A team of researchers at the National Institute of Standards and Technology in Boulder, Colorado, used "squeezed" light — light that is highly organized so it can dampen, or cancel out, thermal vibrations — to cool a tiny mechanical drum to below the so-called "quantum limit," which was supposed to be the coldest temperature possible for a mechanical object.
In fact, the drum's temperature was lowered to about 360 microkelvin, or 10,000 times colder than the vacuum of space. That likely means it was made much colder than any naturally occurring temperature anywhere in the universe.
As cold as that is, that doesn't make it the coldest object ever, strictly speaking. That title belongs to a Bose-Einstein condensate, a dilute gas of bosons, that can be cooled to temperatures as cold as 500 picoKelvin. But condensates are not mechanical objects, so they operate between a different set of extremes. To get a mechanical object to as low a temperature as the NIST team did is a new record, and it's important because it can be applied to future technology.
"The colder you can get the drum, the better it is for any application," said team leader John Teufel. "Sensors would become more sensitive. You can store information longer. If you were using it in a quantum computer, then you would compute without distortion, and you would actually get the answer you want."
The drum itself was 20 micrometers in diameter and 100 nanometers thick. Why a drum? The scientists weren't attempting the universe's smallest and coldest bongo session. Rather, the drum allowed them to match drumbeats to the resonance of the cavity it was placed within. This made it so that photons generated by the drumbeats slowly leaked out of the cavity as it filled up, and each departing photon takes with it one mechanical unit of energy, or one phonon, from the drum's motion. This process aids in cooling the drum to such extreme low temperatures. It also involves a mechanism that's similar to what's employed in some atomic clocks.
"The results were a complete surprise to experts in the field," said co-author José Aumentado. "It's a very elegant experiment that will certainly have a lot of impact."
The research appears in the journal Nature.