Squeezed vacuum source to enhance gravitational wave detection

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Vacuum technology developed by researchers at MIT and the Australian National University could double the sensitivity of the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), which made the first successful detection of gravitational waves last year.

The researchers reported on advances they have made to what is known as a squeezed vacuum source in the Optical Society's journal, Optica. Building this new squeezed vacuum source into the LIGO detector could help double its sensitivity, enabling the detection of far weaker gravitational waves, or waves from sources much further away than currently possible.

Scientists at LIGO announced the first-ever observation of gravitational waves earlier this year, a century after Albert Einstein predicted their existence in his general theory of relativity. Studying gravitational waves, which are incredibly faint ripples in space-time, can reveal important information about astrophysical events involving black holes and neutron stars.

Creating the squeezed vacuum source involved modifying a vacuum state, which is a quantum state with the lowest energy possible. ‘We captured part of this electromagnetic vacuum in an optical cavity by first building the experiment with laser beams and then making the squeezed vacuum state by dialling down the laser power until there is no light, and only the vacuum is left,’ explained Nergis Mavalvala, a leader of the MIT Kavli Institute for Astrophysics and Space Research team. ‘Then, everything we would have done to the light, we can do to the squeezed vacuum state.’

The researchers are planning to add their new squeezed vacuum source to Advanced LIGO in the next year or so. Once implemented, it will improve the sensitivity of the gravitational detectors, particularly at the higher frequencies important for understanding the composition of neutron stars. These extremely dense objects are the remnants of a supernova event, where the core of a star has collapsed at the end of its life, crushing the star’s mass into a sphere with a 5km radius.

‘Nobody knows exactly how the neutrons in these stars behave when you crush them into such a dense package,’ said Mavalvala. ‘These neutron stars sometimes collide with each other, and at the moment that they are ripping each other apart, you can study the properties of this nuclear matter by detecting gravitational waves that occur at higher frequencies.

‘We want to use Advanced LIGO detectors to sense the farthest gravitational wave or weakest gravitational wave possible,’ continued Mavalvala. ‘However, this is limited by the quantum fluctuations of the laser light, which create a certain level of noise. If a gravitational wave is weaker than that level of noise, then we can't detect it. Thus, we have a big impetus to decrease that noise, and we can do that using our squeezed vacuum source.’

Researchers from the California Institute of Technology and MIT conceived, built, and operate identical Advanced LIGO detectors in Livingston, Louisiana and Hanford, Washington. Each observatory uses a 2.5-mile long optical device known as an interferometer to detect gravitational waves coming from distant events, such as the collision of two black holes detected last year.

Laser light travelling back and forth down the interferometer’s two arms is used to monitor the distance between mirrors at each arm’s end. Gravitational waves will cause a slight, but detectable variation in the distance between the mirrors. Both detectors must sense the variation to confirm that change in distance between the mirrors was caused by gravitational waves, as oppose to seismic activity or other terrestrial effects.

The improved squeezed vacuum source builds on work conducted by researchers at Leibniz University of Hannover and the University of Hamburg, both in Germany. The new squeezed vacuum source exhibits about ten times less phase noise than previously reported sources. The researchers accomplished this by decreasing vibrations that can adversely affect the squeezed state and by making improvements to a system that corrects any remaining phase noise.

‘The best approach is to try to reduce the amount of intrinsic phase noise, but if you can't do that, you can measure how much it's jittering and then use feedback to correct it,’ said Eric Oelker, first author of the paper. ‘We used a variation of a correction scheme that has been employed before, but our version allowed us to increase the bandwidth of the feedback loops, suppressing the phase noise in a completely new way.’

The researchers say that the new squeezed vacuum source is almost ready to deploy in Advanced LIGO. In separate research, they have shown that they can also reduce optical losses that can degrade a squeezed vacuum state. ‘By combining the optical losses that we think we can achieve and this new lower phase noise result, we're aiming for a factor of two in improvements for Advanced LIGO,’ said Mavalvala. ‘We hope to achieve greater improvements in gravitational wave sensitivity than was previously thought possible.’ 

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Further information:

Advanced LIGO