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Object Cooled to Quantum Ground State with Laser Light
Oct 2011
PASADENA, Calif., Oct. 12, 2011 — California Institute of Technology (Caltech) researchers, in collaboration with a team from the University of Vienna, have cooled a miniature mechanical object to its lowest possible energy state — its quantum ground state — using laser light. The researchers say this achievement paves the way for the development of highly sensitive detectors as well as for quantum experiments.

“We've taken a solid mechanical system — one made up of billions of atoms — and used optical light to put it into a state in which it behaves according to the laws of quantum mechanics,” said Oskar Painter, a professor of applied physics at Caltech. “In the past, this has only been achieved with trapped single atoms or ions.”

The team engineered a nanoscale object — a tiny mechanical silicon beam — such that laser light of a carefully selected frequency can enter the system and, once reflected, can carry thermal energy away, cooling the system.

A scanning electron microscope image (a) of the nanoscale silicon mechanical resonator used in the laser cooling experiment. The outer “cross” patterning forms the shield, while the central beam region, the SEM image of which is shown in (b), forms an optical cavity where laser light is used to cool the mechanical motion of the beam. Numerical simulations of the localized optical field and mechanical breathing motion of the nanobeam are shown in panels (c) and (d), respectively. (Image: Painter/Caltech)

By carefully designing each element of the beam as well as a patterned silicon shield that isolates it from the environment, the team used the laser cooling technique to bring the system down to the quantum ground state, where mechanical vibrations are at an absolute minimum. Such a cold mechanical object could help detect very small forces or masses, whose presence would normally be masked by the noisy thermal vibrations of the sensor.

To reach the ground state, the researchers had to cool its mechanical beam to a temperature below 100 mK (-273.15 °C). That's because the beam is designed to vibrate at gigahertz frequencies (corresponding to a billion cycles per second) — a range where a large number of phonons are present at room temperature. Phonons are the most basic units of vibration, just as the most basic units or packets of light are called photons. All of the phonons in a system must be removed to cool it to the ground state.

“What we've done is used the photons — the light field — to extract phonons from the system,” said Jasper Chan, lead author of the study and a graduate student in Painter's group. To do so, the investigators drilled tiny holes at precise locations in their mechanical beam so that, when they directed laser light of a particular frequency down the length of the beam, the holes acted as mirrors, trapping the light in a cavity and causing it to interact strongly with the mechanical vibrations of the beam.

Because a shift in the frequency of the light is directly related to the thermal motion of the mechanical object, the light — when it eventually escapes from the cavity — also carries with it information about the mechanical system, such as the motion and temperature of the beam. Thus, the researchers have created an efficient optical interface to a mechanical element— or an optomechanical transducer — that can convert information from the mechanical system into photons of light.

Importantly, because optical light, unlike microwaves or electrons, can be transmitted over large, kilometer-length distances without attenuation, such an optomechanical transducer could be useful for linking different quantum systems; e.g.,a microwave system with an optical system. Although Painter's system involves an optical interface to a mechanical element, other teams have been developing systems that link a microwave interface to a mechanical element.

What if those two mechanical elements were the same?

“Then I could imagine connecting the microwave world to the optical world via this mechanical conduit one photon at a time,” Painter said.

The Caltech team isn't the first to cool a nanomechanical object to the quantum ground state, but the new work is, however, the first in which a nanomechanical object has been put into the ground state using optical light.

The work was published in Nature.

For more information, visit:

quantum mechanics
The science of all complex elements of atomic and molecular spectra, and the interaction of radiation and matter.
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