NIST Physicists Squeeze Light, Cool Microscopic Drum below Quantum Limit

Physicists at the National Institute of Standards and Technology (NIST) have cooled a microscopic mechanical drum to less than one-fifth of a single quantum, or packet of energy. The result, according to the research team, is lower than ordinarily predicted by quantum physics and the technique used could theoretically cool objects to absolute zero, the temperature at which matter is devoid of nearly all energy.

“The colder you can get the drum, the better it is for any application,” says John Teufel, the NIST physicist who led the experiment. “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.”

Teufel’s team embedded the drum, a vibrating aluminum membrane 20 micrometers in diameter and 100 nanometers thick, in a superconducting circuit designed so that the drum motion influences the microwaves bouncing inside a hollow enclosure known as an electromagnetic cavity. Microwaves are a form of electromagnetic radiation, so they are in effect a form of invisible light, with a longer wavelength and lower frequency than visible light. The microwave light inside the cavity changes its frequency as needed to match the frequency at which the cavity naturally resonates. This vibration is the cavity’s natural “tone,” analogous to the musical pitch that a water-filled glass will sound when someone rubs its rim is with a finger or strikes its side with a spoon.

“The results were a complete surprise to experts in the field,” says José Aumentado, Teufel’s group leader and co-author of the paper it published in Nature. “It’s a very elegant experiment that will certainly have a lot of impact.”

NIST scientists previously cooled the quantum drum to its lowest-energy “ground state,” or one-third of one quantum. The researchers used a technique called sideband cooling that involves applying a microwave tone to the circuit at a frequency below the cavity’s resonance. The tone drives electrical charge in the circuit to make the drum beat and the drumbeats generate light particles, or photons, that naturally match the higher resonance frequency of the cavity. These photons leak out of the cavity as it fills up. Each departing photon takes with it one mechanical unit of energy—one phonon—from the drum’s motion. It is the same idea researchers use to laser cool individual atoms, first demonstrated at NIST in 1978 and now widely used in applications such atomic clocks.

Teufel’s team added a novel twist to this established technique, using “squeezed light” to drive the drum circuit. Squeezing is a quantum mechanical concept in which noise, or unwanted fluctuations, is moved from a useful property of the light to another aspect that doesn’t affect the experiment. These quantum fluctuations limit the lowest temperatures that can be reached with conventional cooling techniques. The NIST team used a special circuit to generate microwave photons that were purified or stripped of intensity fluctuations, thus reducing inadvertent heating of the drum. “Noise gives random kicks or heating to the thing you’re trying to cool,” Teufel says. “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. These photons are both fragile and powerful.”

The NIST theory and experiments indicate that squeezed light removes the generally accepted cooling limits, according to Teufel, including those for large objects or ones that operate at low frequencies. Teufel says engineers could use the drum in applications such as hybrid quantum computers, combining both quantum and mechanical elements.