A group of scientists from the Niels Bohr Institute (NBI) at the University of Copenhagen will soon start developing quantum-optical enhancements to dramatically improve the sensitivity of gravitational-wave detectors such as the Laser Interferometer Gravitational-Wave Observatory (LIGO).1
In July of 2017, Eugene Polzik and his team at Quantop (Quantum Optics) at NBI reported on a way to "fool" Heisenberg’s uncertainty principle, which in one form says that one cannot simultaneously know the exact position and the exact speed of an object.2 This has to do with the fact that observations conducted by irradiating an object with light inevitably will lead to the object being moved in random directions by photons. This phenomenon is known as quantum back action (QBA); these random movements put a limit to the accuracy with which measurements can be carried out at the quantum level. Polzik and his team were able to show in 2017 that it is, to a large extent, actually possible to neutralize QBA.
QBA is the very reason why laser-based gravitational-wave detectors "are not as accurate as they could possibly be," as Polzik says.
Neutralizing quantum back action
In essence, it is possible to neutralize QBA if the light used to observe an object is initially sent through a "filter" consisting of a cloud of 100 million cesium atoms hermetically sealed into a glass cell, which in the experiment was 1 cm long, 0.33 mm wide, and 0.33 mm high. The principle behind the "filter" is exactly what Polzik and his team are aiming to incorporate in gravitational-wave detectors.
In classical theory, one can optimize measurements of gravitational waves by switching to higher-power laser light than the gravitational-wave detectors in both Europe and USA are currently operating with, such as LIGO. However, according to quantum mechanics, that is not an option, says Eugene Polzik. "Switching to stronger laser light will just make a set of mirrors in the detectors shake more because QBA will be caused by more photons," he explains. "These mirrors are absolutely crucial, and if they start shaking, it will in fact increase inaccuracy."
Instead, the NBI scientists will send the laser light by which the gravitational-wave detectors operate through a custom version of the cesium-atom filter, says Eugene Polzik. "And we should be able to show proof of concept within approximately three years," he notes. "Our calculations show that we ought to be able to improve the precision of measurements carried out by the gravitational wave detectors by a factor of two. And if we succeed, this will result in an increase by a factor of eight of the volume in space which gravitational-wave detectors are able to examine at present."
Polzik will spearhead the development of the custom equipment. The research, which is supported by the EU, the Eureka Network Projects and the US-based John Templeton Foundation with grants totaling DKK 10 million, will be carried out in Eugene Polzik's lab at NBI.
1. F. Ya. Khalili and E. S. Polzik, Physical Review Letters (2018); https://link.aps.org/doi/10.1103/PhysRevLett.121.031101.
2. Christoffer B. Møller et al., Nature (2017); http://dx.doi.org/10.1038/nature22980.