The universe, as seen through the lens of quantum mechanics, is a
noisy, crackling space where particles blink constantly in and out of
existence, creating a background of quantum noise whose effects are normally
far too subtle to detect in everyday objects.
Now for the first time, a team led by researchers at MIT LIGO Laboratory has
measured the effects of quantum fluctuations on objects at the human scale.
In a paper published in Nature, the researchers report observing that
quantum fluctuations, tiny as they may be, can nonetheless "kick" an object
as large as the 40-kilogram mirrors of the National Science Foundation's
Laser Interferometer Gravitational-wave Observatory (LIGO), causing them to
move by a tiny degree, which the team was able to measure.
It turns out the quantum noise in LIGO's detectors is enough to move the
large mirrors by 10-20 meters—a displacement that was predicted by quantum
mechanics for an object of this size, but that had never before been
measured.
"A hydrogen atom is 10-10 meters, so this displacement of the mirrors is to
a hydrogen atom what a hydrogen atom is to us—and we measured that," says
Lee McCuller, a research scientist at MIT's Kavli Institute for Astrophysics
and Space Research.
The researchers used a special instrument that they designed, called a
quantum squeezer, to "manipulate the detector's quantum noise and reduce its
kicks to the mirrors, in a way that could ultimately improve LIGO's
sensitivity in detecting gravitational waves," explains Haocun Yu, a physics
graduate student at MIT.
"What's special about this experiment is we've seen quantum effects on
something as large as a human," says Nergis Mavalvala, the Marble Professor
and associate head of the physics department at MIT. "We too, every
nanosecond of our existence, are being kicked around, buffeted by these
quantum fluctuations. It's just that the jitter of our existence, our
thermal energy, is too large for these quantum vacuum fluctuations to affect
our motion measurably. With LIGO's mirrors, we've done all this work to
isolate them from thermally driven motion and other forces, so that they are
now still enough to be kicked around by quantum fluctuations and this spooky
popcorn of the universe."
Yu, Mavalvala, and McCuller are co-authors of the new paper, along with
graduate student Maggie Tse and principal research scientist Lisa Barsotti
at MIT, along with other members of the LIGO Scientific Collaboration.
A quantum kick
LIGO is designed to detect gravitational waves arriving at the Earth from
cataclysmic sources millions to billions of light years away. It comprises
two twin detectors, one in Hanford, Washington, and the other in Livingston,
Louisiana. Each detector is an L-shaped interferometer made up of two
4-kilometer-long tunnels, at the end of which hangs a 40-kilogram mirror.
To detect a gravitational wave, a laser located at the input of the LIGO
interferometer sends a beam of light down each tunnel of the detector, where
it reflects off the mirror at the far end, to arrive back at its starting
point. In the absence of a gravitational wave, the lasers should return at
the same exact time. If a gravitational wave passes through, it would
briefly disturb the position of the mirrors, and therefore the arrival times
of the lasers.
Much has been done to shield the interferometers from external noise, so
that the detectors have a better chance of picking out the exceedingly
subtle disturbances created by an incoming gravitational wave.
Mavalvala and her colleagues wondered whether LIGO might also be sensitive
enough that the instrument might even feel subtler effects, such as quantum
fluctuations within the interferometer itself, and specifically, quantum
noise generated among the photons in LIGO's laser.
"This quantum fluctuation in the laser light can cause a radiation pressure
that can actually kick an object," McCuller adds. "The object in our case is
a 40-kilogram mirror, which is a billion times heavier than the nanoscale
objects that other groups have measured this quantum effect in."
Noise squeezer
To see whether they could measure the motion of LIGO's massive mirrors in
response to tiny quantum fluctuations, the team used an instrument they
recently built as an add-on to the interferometers, which they call a
quantum squeezer. With the squeezer, scientists can tune the properties of
the quantum noise within LIGO's interferometer.
The team first measured the total noise within LIGO's interferometers,
including the background quantum noise, as well as "classical" noise, or
disturbances generated from normal, everyday vibrations. They then turned
the squeezer on and set it to a specific state that altered the properties
of quantum noise specifically. They were able to then subtract the classical
noise during data analysis, to isolate the purely quantum noise in the
interferometer. As the detector constantly monitors the displacement of the
mirrors to any incoming noise, the researchers were able to observe that the
quantum noise alone was enough to displace the mirrors, by 10-20 meters.
Mavalvala notes that the measurement lines up exactly with what quantum
mechanics predicts. "But still it's remarkable to see it be confirmed in
something so big," she says.
Going a step further, the team wondered whether they could manipulate the
quantum squeezer to reduce the quantum noise within the interferometer. The
squeezer is designed such that when it set to a particular state, it
"squeezes" certain properties of the quantum noise, in this case, phase and
amplitude. Phase fluctuations can be thought of as arising from the quantum
uncertainty in the light's travel time, while amplitude fluctuations impart
quantum kicks to the mirror surface.
"We think of the quantum noise as distributed along different axes, and we
try to reduce the noise in some specific aspect," Yu says.
When the squeezer is set to a certain state, it can for example squeeze, or
narrow the uncertainty in phase, while simultaneously distending, or
increasing the uncertainty in amplitude. Squeezing the quantum noise at
different angles would produce different ratios of phase and amplitude noise
within LIGO's detectors.
The group wondered whether changing the angle of this squeezing would create
quantum correlations between LIGO's lasers and its mirrors, in a way that
they could also measure. Testing their idea, the team set the squeezer to 12
different angles and found that, indeed, they could measure correlations
between the various distributions of quantum noise in the laser and the
motion of the mirrors.
Through these quantum correlations, the team was able to squeeze the quantum
noise, and the resulting mirror displacement, down to 70 percent its normal
level. This measurement, incidentally, is below what's called the standard
quantum limit, which, in quantum mechanics, states that a given number of
photons, or, in LIGO's case, a certain level of laser power, is expected to
generate a certain minimum of quantum fluctuations that would generate a
specific "kick" to any object in their path.
By using squeezed light to reduce the quantum noise in the LIGO measurement,
the team has made a measurement more precise than the standard quantum
limit, reducing that noise in a way that will ultimately help LIGO to detect
fainter, more distant sources of gravitational waves.
Reference:
Quantum correlations between light and the kilogram-mass mirrors of LIGO,
Nature (2020).
DOI: 10.1038/s41586-020-2420-8
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Physics