Scientists with the U.S. Department of Energy's Lawrence Berkeley
National Laboratory (Berkeley Lab) and the University of California (UC)
Berkeley, using a unique optical trapping system that provides
ensembles of ultracold atoms, have recorded the first direct
observations of distinctly quantum optical effects -- amplification and
squeezing -- in an optomechanical system. Their findings point the way
toward low-power quantum optical devices and enhanced detection of
gravitational waves among other possibilities.
Berkeley Lab researchers directly observed quantum optical effects
-- amplification and ponderomotive squeezing -- in an optomechanical
system. Here the yellow/red regions show amplification, the blue regions
show squeezing. On the left is the data, on the right is the
theoretical prediction in the absence of noise.
"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," says Daniel Brooks, a scientist with Berkeley
Lab's Materials Sciences Division and UC Berkeley's Physics Department.
Brooks, a member of Dan Stamper-Kurn's research group, is the corresponding author of a paper in the journal Nature
describing this research. The paper is titled "Nonclassical light
generated by quantum-noise-driven cavity optomechanics." Co-authors were
Thierry Botter, Sydney Schreppler, Thomas Purdy, Nathan Brahms and
Stamper-Kurn.
Light will build-up inside of an optical cavity at specific resonant
frequencies, similar to how a held-down guitar string only vibrates to
produce specific tones. Positioning a mechanical resonator inside the
cavity changes the resonance frequency for light passing through, much
as sliding one's fingers up and down a guitar string changes its
vibrational tones. Meanwhile, as light passes through the optical
cavity, it acts like a tiny tractor beam, pushing and pulling on the
mechanical resonator.
If an optical cavity is of ultrahigh quality and the mechanical
resonator element within is atomic-sized and chilled to nearly absolute
zero, the resulting cavity optomechanical system can be used to detect
even the slightest mechanical motion. Likewise, even the tiniest
fluctuations in the light/vacuum can cause the atoms to wiggle. Changes
to the light can provide control over that atomic motion. This not only
opens the door to fundamental studies of quantum mechanics that could
tell us more about the "classical" world we humans inhabit, but also to
quantum information processing, ultrasensitive force sensors, and other
technologies that might seem like science fiction today.
"There have been proposals to use optomechanical devices as
transducers, for example coupling motion to both microwaves and optical
frequency light, where one could convert photons from one frequency
range to the other," Brooks says. "There have also been proposals for
slowing or storing light in the mechanical degrees of freedom, the
equivalent of electromagnetically induced transparency or EIT, where a
photon is stored within the internal degrees of freedom."
Already cavity optomechanics has led to applications such as the
cooling of objects to their motional ground state, and detections of
force and motion on the attometer scale. However, in studying
interactions between light and mechanical motion, it has been a major
challenge to distinguish those effects that are distinctly quantum from
those that are classical -- a distinction critical to the future
exploitation of optomechanics.
Brooks, Stamper-Kurn and their colleagues were able to meet the
challenge with their microfabricated atom-chip system which provides a
magnetic trap for capturing a gas made up of thousands of ultracold
atoms. This ensemble of ultracold atoms is then transferred into an
optical cavity (Fabry-Pferot) where it is trapped in a one-dimensional
optical lattice formed by near-infrared (850 nanometer wavelength) light
that resonates with the cavity. A second beam of light is used for the
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 says. "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."
The Berkeley research team first applied classical light modulation
to a low-powered pump/probe beam (36 picoWatts) entering their optical
cavity to demonstrate that their system behaves as a high-gain
parametric optomechanical amplifier. They then extinguished the
classical drive and mapped the response to the fluctuations of the
vacuum. This enabled them to observe light being squeezed by its
interaction with the vibrating ensemble and the atomic motion driven by
the light's quantum fluctuations. Amplification and this squeezing
interaction, which is called "ponderomotive force," have been
long-sought goals 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 says. "The
ponderomotive squeezing we saw, while narrow in frequency, was a natural
consequence of having radiation-pressure shot noise dominate in our
system."
Since squeezing light improves the sensitivity of gravitational wave
detectors, the ponderomotive squeezing effects observed by Brooks,
Stamper-Kern and their colleagues could play a role in future detectors.
The idea behind gravitational wave detection is that a ripple in the
local curvature of spacetime caused by a passing gravitational wave will
modify the resonant frequency of an optical cavity which, in turn, will
alter the cavity's optical signal.
"Currently, squeezing light over a wide range of frequencies is
desirable as scientists search for the first detection of a
gravitational wave," Brooks explains. "Ponderomotive squeezing, should
be valuable later when specific signals want to be studied in detail by
improving the signal-to-noise ratio in the specific frequency range of
interest."
The results of this study differ significantly from standard linear
model predictions. This suggests that a nonlinear optomechanical theory
is required to account for the Berkeley team's observations that
optomechanical interactions generate non-classical light. Stamper-Kern'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 says. "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."
This research was funded by the Air Force Office of Scientific Research and the National Science Foundation.
From sciencedaily