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Now for a really cool micro-drum solo: Boffins chill gizmo below quantum limit

Neat microwave trick could be key to reaching absolute zero

Physicists working at the US National Institute of Standards and Technology (NIST) have developed a way to theoretically cool an object to absolute zero.

This groundbreaking technique, detailed in Nature today, has been used to chill a vibrating aluminium membrane to 360 microKelvin, a temperature below the “quantum limit.” That 360µK reading is just a smidge away from zero Kelvin or absolute zero, the point where molecular activity stops.

This quantum limit was believed to be the lowest temperature you could practically go according to the laws of physics. Now NIST's boffins say they have broken that barrier using superconductors and microwaves.

Several techniques have been used in the past to super-cool objects, but this new technique from NIST reaches lower temperatures by reducing pesky quantum fluctuations – random variations in energy – that result in heat. The key is to “squeeze light” out of a tiny aluminium drum.

“There is a quantum limit on how cold one could ever cool an object using light. While laser cooling – or sideband cooling – is a well-known technique for cooling, the quantum noise of the light means that you can never get perfectly cold, even in principle,” John Teufel, who led the experiment and is coauthor of a paper on the research, told The Register.

NIST's 360µK aluminium drum ... Click to enlarge (Source: Teufel/NIST)

Measuring 20 micrometres in diameter and 100 nanometres thick, the small drum is embedded in a superconducting circuit to create an electromagnetic cavity. When microwave light is directed onto the drum, its motion affects the way microwaves bounce inside the cavity.

The microwave light inside the cavity alters its frequency to match the vibrational frequency of the cavity. This is described as the cavity’s natural “tone,” similar to how applying the right frequency to a tuning fork produces a pure sound.

The researchers apply a microwave frequency below the cavity’s frequency to drive an electric charge in the circuit to make the drum beat. As it beats, the drum generates photons at a higher frequency than the microwave frequency to match the cavity’s frequency, by taking a small amount of extra energy from the vibrations of the cavity.

As the cavity fills up with photons, some photons leak out. For every photon that escapes, one phonon – a vibrational unit of energy – leaves too, reducing the energy of the vibration and thus making the drum cooler.

Squeezing moves noise, or quantum fluctuations “from a useful property of light to another that doesn’t affect the experiment.”

“Noise gives random kicks or heating to the thing you’re trying to cool,” Teufel explained. “We are squeezing the light at a ‘magic’ level – in a very specific direction and amount – to make perfectly correlated photons with more stable intensity."

The theory shows that squeezed light removes the cooling limit, allowing researchers to reach temperatures lower than predicted by quantum physics. If the light could be perfectly “squeezed” then it would be possible to cool the vibrational motions “arbitrarily close” to absolute zero, Teufel told The Register.

There are far-reaching benefits to cooling objects, Teufel added. “The colder you can get the drum, the better it is for any application.=," he said. "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.” ®

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