Quantum Measurement Could Improve Gravitational Wave Detection Sensitivity

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Minutes before dawn on Sept. 14, 2015, the Laser Interferometer Gravitational-wave Observatory (LIGO) became the first-ever instrument on Earth to directly detect a gravitational wave. This work, led by the LIGO Scientific Collaboration with prominent roles from MIT and Caltech, was the first confirmation of this consequence of Albert Einstein’s theory of general relativity — 100 years after he first predicted it. The groundbreaking detection represented an enormous step forward in the field of astrophysics. In the years since, scientists have striven to achieve even greater sensitivity in the LIGO detectors.

Image Caption: New technology allows LIGO researchers to model noise from quantum phenomena at room temperature. The green path shows the optical fiber that carries light into the chamber, and the red path shows where the light exits.

New research has taken investigators one step closer to this goal. Nergis Mavalvala, the Curtis and Kathleen Marble Professor of Astrophysics at MIT, postdoc Robert Lanza, graduate student Nancy Aggarwal, and their collaborators at Louisiana State University (LSU) recently conducted experiments that could help overcome a future limitation in Advanced LIGO. In their laboratory study, the team successfully measured a type of noise that will soon hold the LIGO instruments back from detecting gravitational waves with greater sensitivity.

Their study, reported recently in Nature, was the first to measure an important source of quantum noise at room temperature and at frequencies relevant to gravitational wave detectors. Funded by the National Science Foundation, this work could enable researchers to understand this limiting noise source and test ideas for circumventing it to further increase LIGO’s sensitivity to gravitational waves.

In addition to future applications for improving LIGO’s detection abilities, Mavalvala says these observations of quantum effects at room temperature could help scientists learn more about how quantum mechanics can disturb the precision of measurements generally — and how best to get around these quantum noise limits.

“This result was important for the gravitational wave community,” says Mavalvala. “But more broadly, this is essentially a room-temperature quantum resource, and that's something that many communities should care about.”

Sensitivity Upgrade

LIGO has undergone upgrades since its first gravitational wave searches in 2002; the currently operating version of the instrumentation is called Advanced LIGO following major upgrades in 2015. But to get LIGO to its maximum design sensitivity, Mavalvala says her team needs to be able to conduct experiments and test improvement strategies in the laboratory rather than on the LIGO instruments themselves. LIGO’s astrophysical detection work is too important to interfere with, so she and her collaborators have developed instruments in the lab that can mimic the sensitivity of the real thing. In this case, the team aimed to reproduce processes that occur in LIGO to measure a type of noise called quantum radiation pressure noise (QRPN).

In LIGO, gravitational waves are detected by using lasers to probe the motion of mirrors. The mirrors are suspended as pendulums, allowing them to have periodic motion similar to a mass on a spring. When laser beams hit the movable mirrors, the momentum carried by the light applies pressure on the mirror and causes them to move slightly.

“I like to think of it like a pool table,” says Aggarwal. “When your white cue ball strikes the ball in front of it, the cue ball comes back but it still moves the other ball. When a photon that was traveling forward then travels backwards, the momentum went somewhere; [in this case] that momentum went into the mirror.”

The quantum nature of light, which is made up of photons, dictates that there are quantum fluctuations in the number of photons hitting the mirrors, creating an uncertain amount of force on the mirrors at any given moment. This uncertainty results in random perturbations of the mirror. When the laser power is high enough, this QRPN can interfere with gravitational wave detection. At Advanced LIGO’s full design sensitivity, with many hundreds of kilowatts of laser power hitting 40-kilogram mirrors, QRPN will become a dominant limitation.

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