Jun 17, 2019 | Updated: 11:38 AM EDT

Physicists Are Now Listening to the Quantum Vacuum

Mar 26, 2019 09:07 AM EDT

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The Louisiana State University Department of Physics & Astronomy associate professor Thomas Corbitt and his team of researchers now present the first broadband, off-resonance measurement of quantum radiation pressure noise in the audio band, at frequencies relevant to gravitational wave detectors. The results hint at methods to improve the sensitivity of gravitational-wave detectors by developing techniques to mitigate the imprecision in measurements called "back action," thus increasing the chances of detecting gravitational waves.

Corbitt and researchers have developed physical devices that make it possible to observe and hear, quantum effects at room temperature. Housed in miniature models of detectors like LIGO, or the Laser Interferometer Gravitational-Wave Observatory, located in Livingston, La., and Hanford, Wash., these devices consist of low-loss, single-crystal micro-resonators-each a tiny mirror pad the size of a pinprick, suspended from a cantilever. A laser beam is directed at one of these mirrors, and as the beam is reflected, the fluctuating radiation pressure is enough to bend the cantilever structure, causing the mirror pad to vibrate, which creates noise.

Gravitational wave interferometers use as much laser power as possible in order to minimize the uncertainty caused by the measurement of discrete photons and to maximize the signal-to-noise ratio. Other types of noise, such as thermal noise, usually dominate over quantum radiation pressure noise, but Corbitt and his team, including collaborators at MIT, have sorted through them.

"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," Corbitt said.

Corbitt's former academic advisee Jonathan Cripe, a graduate from LSU with a Ph.D. in physics said, "Day-to-day at LSU, as I was doing the background work of designing this experiment and the micro-mirrors and placing all of the optics on the table, I didn't really think about the impact of the future results," Cripe said. "I just focused on each individual step and took things one day at a time. [But] now that we have completed the experiment, it really is amazing to step back and think about the fact that quantum mechanics-something that seems otherworldly and removed from the daily human experience is the main driver of the motion of a mirror that is visible to the human eye. The quantum vacuum, or 'nothingness,' can have an effect on something you can see." 

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