21 August 2012
The first direct observations of distinctly quantum optical effects — amplification and squeezing — have been recorded in an optomechanical system. The step forward points the way to low-power quantum optical devices and enhanced detection of gravitational waves, among other applications.
The first-of-its-kind experiment uses an innovative optical trapping system that provides a cluster of ultracold atoms to record amplification and squeezing in an optomechanical system. It was conducted by scientists from the Lawrence Berkeley National Laboratory and the University of California, Berkeley.
“We’ve shown for the first time that the quantum fluctuations in a light field are responsible for driving the motions of objects much larger than an electron and could in principle drive the motion of really large objects,” said physicist Daniel Brooks, a member of Dan Stamper-Kurn’s research group.
Using light to move large objects has long been a staple of science fiction: think the tractor beam used in both “Star Trek” and “Star Wars.” While these tractor beams remain science fiction, beams of light today are being used to mechanically manipulate atoms or tiny glass beads, with rapid progress being made to control increasingly larger objects.
The team’s microfabricated atom-chip system provides a magnetic trap that captures a gas comprising thousands of ultracold atoms. This ensemble of atoms is shifted into a Fabry-Perot optical cavity, where the atoms are confined in a one-dimensional optical lattice created by near-infrared light that resonates with the optical cavity. A second light beam is used as a pump/probe.
“Integrating trapped ensembles of ultracold atoms and high-finesse cavities with an atom chip allowed us to study and control the classical and quantum interactions between photons and the internal/external degrees of freedom of the atom ensemble,” Brooks said. “In contrast to typical solid-state mechanical systems, our optically levitated ensemble of ultracold atoms is isolated from its environment, causing its motion to be driven predominantly by quantum radiation-pressure fluctuations.”
A low-power (36 picowatts) pump/probe beam entering the optical cavity was first applied to classical light modulation to illustrate that the system acts as a high-gain parametric optomechanical amplifier. The researchers then turned off the classical drive and mapped the response to the vacuum’s fluctuations. This allowed them to observe the squeezing of light by its interplay with the vibrating ensemble and the atomic movement triggered by the light’s quantum fluctuations. Amplification and the squeezing interaction, which is also known as ponderomotive force, have been sought-after objectives of optomechanics research.
“Parametric amplification typically requires a lot of power in the optical pump, but the small mass of our ensemble required very few photons to turn the interactions on/off,” Brooks said. “The ponderomotive squeezing we saw, while narrow in frequency, was a natural consequence of having radiation-pressure shot noise dominate in our system.”
The study’s results differ substantially from standard linear model predictions, indicating that a nonlinear optomechanical hypothesis is required to explain the team’s findings that optomechanical interactions create nonclassical light.
Stamper-Kurn’s research group is now considering further experiments involving two ensembles of ultracold atoms inside the optical cavity.
“The squeezing signal we observe is quite small when we detect the suppression of quantum fluctuations outside the cavity, yet the suppression of these fluctuations should be very large inside the cavity,” Brooks said. “With a two-ensemble configuration, one ensemble would be responsible for the optomechanical interaction to squeeze the radiation-pressure fluctuations, and the second ensemble would be studied to measure the squeezing inside the cavity.”
The study appeared in Nature.