Using a squeezed state of coherent light at microwave frequencies, NIST researchers were able to cool a tiny aluminum drum—20 microns in diameter and 100 nanometers thick—to below the quantum backaction limit. [Image: Teufel/NIST]
Researchers at the U.S. National Institute of Standards and Technology (NIST) have used the quantum trick of “squeezing” light to cool a mechanical resonator to less than a fifth of a single quantum of mechanical energy—that is, a fifth of a phonon (Nature, doi: 10.1038/nature20604). That’s below the so-called quantum backaction limit imposed on conventional laser-cooling schemes by quantum vacuum fluctuations.
The technique, according to the scientists, could in principle allow cooling of even low-frequency oscillators to levels “arbitrarily close to the motional ground state.” And that capability, they suggest, could potentially enable hybrid approaches to quantum computing and open new areas of inquiry in the quantum physics of macroscopic systems.
Sideband cooling
The resonator in question is a small aluminum drum, 20 μm in diameter and 100 nm thick. The NIST team had already reported some success in cooling that object to below-single-quantum levels six years ago, through the technique known as sideband cooling.
Sideband cooling works by removing mechanical energy through anti-Stokes scattering. In essence, sub-resonance photons in a coherent microwave field are added the cavity to drive it toward its resonance frequency. Some of those incident drive photons are inelastically scattered and up-converted to the higher resonance frequency, consuming a phonon in the process to give the required energy kick to up-convert the photon. As up-converted photons leak out of the resonant cavity, they carry the phonon (mechanical) energy with them, cooling the cavity as a result.
Sideband cooling can take the resonator’s temperature down only so far, however. That’s because random quantum vacuum fluctuations tend to stimulate some level of countervailing Stokes scattering, which down-converts the microwave photons and creates new mechanical phonons that raise the cavity’s temperature. This leads to the so-called quantum backaction limit on the lowest temperatures reachable in such a macroscopic mechanical object through conventional laser-cooling techniques.
Squeezed light to the rescue
The NIST team wanted to see if it could get past that limit. To do so, they employed a so-called squeezed state of light. “Squeezing” light—perhaps better understood as squeezing quantum noise—involves moving the uncertainty or noise associated with quantum fluctuations from one property of the system to another. More specifically, amplitude or intensity fluctuations are reduced, at the cost of an increase in phase fluctuations.
The researchers were able to show numerically that, by driving the system with a microwave field in a pure squeezed state, they should be able to push the Stokes scattering from quantum vacuum fluctuations to zero, eliminating the effect of phonon heating and overcoming the backaction limit on cooling. They then set up a proof of concept in the lab, using a superconducting Josephson parametric amplifier to create the squeezed microwave field. Through heterodyne spectroscopy, they were able to confirm that the thermal occupancy of the mechanical resonator could be ratchted down to as little as 0.19 phonons—a level that blows through the lower limit of conventional laser-cooling techniques without squeezed light.
Toward “hybrid” quantum computers
While the aluminum drum in the experiments was microscopic in size, the researchers point out that the experiments could in principle “immediately improve the cooling of any cavity optomechanical system,” even larger, lower-frequency resonators. According to lead researcher John Teufel at NIST, the ability to do so could benefit a range of technologies.
“The colder you can get the drum, the better it is for any application,” Teufel said in a NIST press release. “Sensors would become more sensitive.… If you were using it in a quantum computer, then you would compute without distortion, and actually get the answer you want.” Indeed, Teufel notes that the NIST drum could ultimately become a component in hybrid quantum computers based partly on mechanical elements.