Physicists measure quantum "backward action" in the audio band at room temperature

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Listen to the quantum vacuum — Thomas Corbitt, Associate Professor in the Department of Physics and Astronomy of the Louisiana State University, and his team of researchers present the first non-resonance broadband measurement of quantum radiation radiation sound in the audio band, at frequencies corresponding to the gravitational wave detectors, as reported in the Nature journal scientific report. Credit: Elsa Hahne, SJM

Since the historical discovery of gravitational waves from two black holes colliding at more than a billion light years ago was made in 2015, physicists are advancing knowledge on the limits of accuracy measures that will help improve the next generation of tools and technologies used by gravitation. scientists of the wave.

Associate Professor Thomas Corbitt of LSU's Department of Physics and Astronomy and his team of researchers present the first non-resonant broadband measurement of quantum radiation pressure in the audio band, at frequencies corresponding to gravitational wave detectors, as reported today in the scientific journal Nature. The research was funded by the National Science Foundation, or NSF, and the results suggest methods to improve the sensitivity of gravitational wave detectors by developing techniques to mitigate inaccuracy in measurements, called "action". in return, "increasing the chances of detecting gravitational effects. waves.

Corbitt and researchers have developed physical devices to observe and hear quantum effects at room temperature. It is often easier to measure quantum effects at very cold temperatures, as this approach brings them closer to the human experience. Housed in miniature models of detectors such as LIGO, or the gravitational-wave gravitational observatory with laser interferometer, located in Livingston, Louisiana, and Hanford, Washington, these devices consist of micro – low loss single crystal resonators, each pad the size of a pin prick, hanging on a cantilever. A laser beam is directed towards one of these mirrors and, once the reflected beam, the fluctuating radiation pressure is enough to bend the structure cantilever, which causes the vibration of the mirror pad, this which creates noise.

Gravitational wave interferometers use as much laser power as possible to minimize the uncertainty generated by measuring discrete photons and to optimize the signal-to-noise ratio. These higher power beams increase the accuracy of the position but also the action in return, which corresponds to the uncertainty of the number of photons reflected by a mirror that corresponds to a fluctuating force due to the pressure of radiation exerted on the mirror, causing mechanical movement. Other types of noise, such as thermal noise, generally dominate the quantum radiation pressure noise, but Corbitt and his team, including MIT collaborators, have sorted them out. Advanced LIGOs and other second- and third-generation interferometers will be limited by low-frequency quantum radiation pressure noise when operating at full laser power. Corbitt's paper in Nature offers clues as to how researchers can work around this problem when measuring gravitational waves.

"Given the need for more sensitive gravitational wave sensors, it is important to study the effects of quantum radiation pressure noise in a system similar to Advanced LIGO, which will be limited by noise. quantum radiation pressure over a wide frequency range away from the mechanical stress, the resonant frequency of the tested mass suspension, "said Corbitt.

Former academic advisor to Corbitt and lead author of Nature paper, Jonathan Cripe, graduated from LSU with a Ph.D. in physics last year and is now a postdoctoral fellow at the National Institute of Standards and Technology:

"On a daily basis, at LSU, while working on the design of this experiment and the micro-mirrors and putting all the optics on the table, I did not really think about the impact of future results," says Cripe. "I focused on each step and took things one day at a time. [But] now that we have finished the experiment, it is really amazing to step back and think about the fact that quantum mechanics – something that seems out of the world and that is removed from the daily human experience – is the main driver of the movement of a mirror visible by the human eye. The quantum vacuum, or "nothingness", can have an effect on something you can see. "

Pedro Marronetti, physicist and program director of the NSF, says that it can be difficult to test new ideas to improve gravitational wave detectors, particularly to reduce noise that can only be measured in a large scale interferometer:

"This breakthrough opens up new opportunities for testing noise reduction," he said. The relative simplicity of the approach makes it accessible to a wide range of research groups, potentially increasing the participation of the broader scientific community in gravitational wave astrophysics. "

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More information:
Jonathan Cripe et al, Measurement of quantum back action in the audio band at room temperature, Nature (2019). DOI: 10.1038 / s41586-019-1051-4

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