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Measuring Quantum Back Action Could Improve Sensitivity of Gravitational-Wave Detectors

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BATON ROUGE, La., March 26, 2019 — Researchers at Louisiana State University (LSU) have defined a broadband, off-resonance measurement of quantum radiation pressure noise (QRPN) in the audio band, at frequencies relevant to gravitational-wave detectors. Their work could lead to methods to improve the sensitivity of gravitational-wave detectors by developing techniques to mitigate imprecision in back action measurements, thus increasing the chances of detecting gravitational waves.

Measuring quantum back action, Louisiana State University.
LSU Department of Physics & Astronomy associate professor Thomas Corbitt and his team present a broadband, off-resonance measurement of quantum radiation pressure noise in the audio band, at frequencies relevant to gravitational wave detectors. Courtesy of Elsa Hahne, LSU.

The researchers developed devices, housed in miniature models of detectors like LIGO, that made it possible to observe quantum effects at room temperature. The devices consist of low-loss, single-crystal microresonators pioneered by Crystalline Mirror Solutions (CMS). These microresonators enabled the realization of an optical interferometer in which the ultimate sensitivity of the system is limited by QRPN. 

The micromirrors manufactured by CMS were used to develop a testbed optical cavity that can now be employed to study techniques to mitigate fundamental noise processes such as QRPN. The testbed will be key for realizing improvements in the performance and reach of future gravitational wave observatories.

Each mirror pad is the size of a pin prick and is suspended from a cantilever. When a laser beam is directed at one of the mirrors, the beam that is reflected induces enough fluctuating radiation pressure to bend the cantilever structure, causing the mirror pad to vibrate and create noise. The noise spectrum obtained by the team shows effects due to QRPN to be between about 2 kHz and 100 kHz. 

To minimize the uncertainty caused by the measurement of discrete photons and maximize the signal-to-noise ratio, gravitational wave interferometers use high-power lasers. These high-power beams increase position accuracy, but do so at the expense of back action in the form of QRPN. Advanced LIGO and other second- and third-generation interferometers could be limited by QRPN at low frequencies when running at their full laser power, but the work of the team, which also includes collaborators at MIT, Crystalline Mirror Solutions, and the University of Vienna, offers clues as to how scientists can work around this limitation when measuring gravitational waves.

“Given the imperative for more sensitive gravitational wave detectors, it is important to study the effects of quantum radiation pressure noise in a system similar to Advanced LIGO, which will be limited by quantum radiation pressure noise across a wide range of frequencies far from the mechanical resonance frequency of the test mass suspension,” said professor Thomas Corbitt.

Quantum Back Action Measurement Could Improve Sensitivity of Gravitational-Wave Detectors  BATON ROUGE, La., March x, 2019 &mdash; Researchers at Louisiana State University (LSU) have defined a broadband, off-resonance measurement of quantum radiation pressure noise (QRPN) in the audio band, at frequencies relevant to gravitational wave detectors. The noise spectrum obtained by the team shows effects due to QRPN between about 2 kilohertz and 100 kilohertz. The results of the research could lead to methods to improve the sensitivity of gravitational-wave detectors by developing techniques to mitigate the imprecision in back action measurements, thus increasing the chances of detecting gravitational waves. The researchers developed devices, housed in miniature models of detectors like LIGO, that made it possible to observe quantum effects at room temperature. The devices consist of low-loss, single-crystal microresonators. Each mirror pad is the size of a pin prick and is suspended from a cantilever. When a laser beam is directed at one of the mirrors, the beam that is reflected causes enough fluctuating radiation pressure to bend the cantilever structure, causing the mirror pad to vibrate and create noise. Gravitational wave interferometers use high-power lasers in order to minimize the uncertainty caused by the measurement of discrete photons and maximize the signal-to-noise ratio. These high-power beams increase position accuracy but do so at the expense of back action in the form of QRPN. Advanced LIGO and other second and third generation interferometers will be limited by QRPN at low frequencies when running at their full laser power; but the LSU team, which includes collaborators at MIT, offers clues as to how scientists can work around this when measuring gravitational waves. “Given the imperative for more sensitive gravitational wave detectors, it is important to study the effects of quantum radiation pressure noise in a system similar to Advanced LIGO, which will be limited by quantum radiation pressure noise across a wide range of frequencies far from the mechanical resonance frequency of the test mass suspension,” said professor Thomas Corbitt. Proposals to improve the sensitivity of gravitational-wave detectors exist; but until now, said the researchers, no platform has allowed for experimental tests of these ideas.  “This breakthrough opens new opportunities for testing noise reduction,” said Pedro Marronetti, a physicist and National Science Foundation program director. “The relative simplicity of the approach makes it accessible by a wide range of research groups, potentially increasing participation from the broader scientific community in gravitational wave astrophysics.” The research was published in <I>Nature</I> (https://doi.org/10.1038/s41586-019-1051-4). https://www.nature.com/articles/s41586-019-1051-4 kw: research & technology, education, Louisiana State University, LIGO, gravitational wave detector, metrology, lasers, Americas, optics, quantum, sensors & detectors, astronomy, quantum optics, single photons, quantum metrology CAPTION 1 LSU Department of Physics & Astronomy associate professor Thomas Corbitt and his team present a broadband, off-resonance measurement of quantum radiation pressure noise in the audio band, at frequencies relevant to gravitational wave detectors. Courtesy of Elsa Hahne, LSU.  Professor Thomas Corbitt in his lab, setting up a complex sequence of lasers. Courtesy of Elsa Hahne, LSU.


Professor Thomas Corbitt in his lab, setting up a sequence of lasers. Courtesy of Elsa Hahne, LSU.

Proposals to improve the sensitivity of gravitational-wave detectors exist, but until now, the researchers said, no platform has allowed for experimental tests of these ideas.

“This breakthrough opens new opportunities for testing noise reduction,” said Pedro Marronetti, a physicist and National Science Foundation program director. “The relative simplicity of the approach makes it accessible by a wide range of research groups, potentially increasing participation from the broader scientific community in gravitational wave astrophysics.”

The research was published in Nature (https://doi.org/10.1038/s41586-019-1051-4). 

 


Photonics.com
Mar 2019
GLOSSARY
metrology
The science of measurement, particularly of lengths and angles.
quantum
Smallest amount into which the energy of a wave can be divided. The quantum is proportional to the frequency of the wave. See photon.
astronomy
The scientific observation of celestial radiation that has reached the vicinity of Earth, and the interpretation of these observations to determine the characteristics of the extraterrestrial bodies and phenomena that have emitted the radiation.
quantum optics
The area of optics in which quantum theory is used to describe light in discrete units or "quanta" of energy known as photons. First observed by Albert Einstein's photoelectric effect, this particle description of light is the foundation for describing the transfer of energy (i.e. absorption and emission) in light matter interaction.
Research & TechnologyeducationLouisiana State UniversityLIGOgravitational wave detectormetrologylasersAmericasopticsquantumSensors & Detectorsastronomyquantum opticssingle photonsquantum metrologyCrystalline Mirror SolutionsUniversity of ViennaEurope

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