Squeezed light produced using silicon micromechanical system Optomechanical device.

Squeezed light produced using
silicon micromechanical system
Optomechanical device.
a, Scanning
electron microscope image of a
waveguide-coupled zipper
optomechanical cavity. b, Left: close-
up of the coupling region between
one of the cavities and the
waveguide. Right: finite-element
method (FEM) simulation of the
cavity field leaking into the waveguide
(log scale). c, Top: FEM simulation
showing the in-plane electrical field
of the fundamental optical cavity
mode. Bottom: FEM simulation of the
displacement of the fundamental in-
plane differential mode of the
structure. The mechanical motion,
modifying the gap between the
beams, shifts the optical cavity
frequency, leading to optomechanical
coupling. Credit: (c) Nature, doi:
10.1038/nature12307
One of the many counterintuitive and
bizarre insights of quantum
mechanics is that even in a vacuum
—what many of us think of as an
empty void—all is not completely
still. Low levels of noise, known as
quantum fluctuations, are always
present. Always, that is, unless you
can pull off a quantum trick. And
that's just what a team led by
researchers at the California Institute
of Technology (Caltech) has done.
The group has engineered a
miniature silicon system that
produces a type of light that is
quieter at certain frequencies—
meaning it has fewer quantum
fluctuations—than what is usually
present in a vacuum.
This special type of light with fewer
fluctuations is known as squeezed
light and is useful for making precise
measurements at lower power levels
than are required when using normal
light. Although other research groups
previously have produced squeezed
light, the Caltech team's new system,
which is miniaturized on a silicon
microchip, generates the ultraquiet
light in a way that can be more easily
adapted to a variety of sensor
applications.
"This system should enable a new set
of precision microsensors capable of
beating standard limits set by
quantum mechanics," says Oskar
Painter, a professor of applied
physics at Caltech and the senior
author on a paper that describes the
system; the paper appears in the
August 8 issue of the journal Nature .
"Our experiment brings together, in a
tiny microchip package, many aspects
of work that has been done in
quantum optics and precision
measurement over the last 40 years."
The history of squeezed light is
closely associated with Caltech. More
than 30 years ago, Kip Thorne,
Caltech's Richard P. Feynman
Professor of Theoretical Physics,
Emeritus, and physicist Carlton Caves
(PhD '79) theorized that squeezed
light would enable scientists to build
more sensitive detectors that could
make more precise measurements. A
decade later, Caltech's Jeff Kimble,
the William L. Valentine Professor
and professor of physics, and his
colleagues conducted some of the
first experiments using squeezed
light. Since then, the LIGO (Laser
Interferometer Gravitational-Wave
Observatory) Scientific Collaboration
has invested heavily in research on
squeezed light because of its
potential to enhance the sensitivity of
gravitational-wave detectors.
In the past, squeezed light has been
made using so-called nonlinear
materials, which have unusual optical
properties. This latest Caltech work
marks the first time that squeezed
light has been produced using silicon,
a standard material. "We work with a
material that's very plain in terms of
its optical properties," says Amir
Safavi-Naeini (PhD '13), a graduate
student in Painter's group and one of
three lead authors on the new paper.
"We make it special by engineering
or punching holes into it, making
these mechanical structures that
respond to light in a very novel way.
Of course, silicon is also a material
that is technologically very amenable
to fabrication and integration,
enabling a great many applications in
electronics."
In this new system, a waveguide
feeds laser light into a cavity created
by two tiny silicon beams. Once
there, the light bounces back and
forth a bit thanks to the engineered
holes, which effectively turn the
beams into mirrors. When photons—
particles of light—strike the beams,
they cause the beams to vibrate. And
the particulate nature of the light
introduces quantum fluctuations that
affect those vibrations.
Typically, such fluctuations mean that
in order to get a good reading of a
signal, you would have to increase
the power of the light to overcome
the noise. But by increasing the
power you also introduce other
problems, such as introducing excess
heat into the system.
Ideally, then, any measurements
should be made with as low a power
as possible. "One way to do that,"
says Safavi-Naeini, "is to use light
that has less noise."
And that's exactly what the new
system does; it has been engineered
so that the light and beams interact
strongly with each other—so strongly,
in fact, that the beams impart the
quantum fluctuations they experience
back on the light. And, as is the case
with the noise-canceling technology
used, for example, in some
headphones, the fluctuations that
shake the beams interfere with the
fluctuations of the light. They
effectively cancel each other out,
eliminating the noise in the light.
"This is a demonstration of what
quantum mechanics really says: Light
is neither a particle nor a wave; you
need both explanations to
understand this experiment," says
Safavi-Naeini. "You need the particle
nature of light to explain these
quantum fluctuations , and you need
the wave nature of light to
understand this interference."
In the experiment, a detector
measuring the noise in the light as a
function of frequency showed that in
a frequency range centered around
28 MHz, the system produces light
with less noise than what is present
in a vacuum—the standard quantum
limit. "But one of the interesting
things," Safavi-Naeini adds, "is that
by carefully designing our structures,
we can actually choose the frequency
at which we go below the vacuum."
Many signals are specific to a
particular frequency range—a certain
audio band in the case of acoustic
signals, or, in the case of LIGO, a
frequency intimately related to the
dynamics of astrophysical objects
such as circling black holes. Because
the optical squeezing occurs near the
mechanical resonance frequency
where an individual device is most
sensitive to external forces, this
feature would enable the system
studied by the Caltech team to be
optimized for targeting specific
signals.
"This new way of 'squeezing light ' in
a silicon micro-device may provide
new, significant applications in
sensor technology," said Siu Au Lee,
program officer at the National
Science Foundation, which provided
support for the work through the
Institute for Quantum Information
and Matter, a Physics Frontier
Center. "For decades, NSF's Physics
Division has been supporting basic
research in quantum optics , precision
measurements and nanotechnology
that laid the foundation for today's
accomplishments."
More information: The paper is
titled "Squeezed light from a silicon
micromechanical resonator." http://
dx.doi.org/10.1038/nature12307
Provided by California Institute of
Technology