LIGO reveals quantum correlations at work in mirrors weighing tens of kilograms
Physicists working on the LIGO gravitational-wave observatory in the US have shown that quantum-scale correlations can leave their mark on macroscopic objects weighing tens of kilograms. The team explored the interplay between the interferometer’s laser beam and its huge test masses, showing that the instrument’s quantum noise could be reduced below an intrinsic limit. This, the researchers say, could boost the future rate of discoveries with such observatories.
Gravitational waves are light speed disturbances in space–time that are generated by massive objects accelerating somewhere in space. They can be observed by monitoring the interference between two laser beams propagating at right angles to one another, given that the waves’ passage through the Earth lengthens the path of one beam very slightly compared to the other.
The sensitivity of these instruments is limited by Heisenberg’s uncertainty principle, which stipulates a minimum combined uncertainty in an object’s position and momentum. To have sufficient sensitivity to detect the minute distance changes caused by a passing gravitational wave, an observatory’s laser beams must each bounce many times between a pair of suspended mirrors before they meet and interfere. But the photons exert pressure on the mirrors as they bounce off them, causing the mirrors to deflect and the laser path-length to change very slightly. “The light measures position yet disturbs momentum, thus imposing the Heisenberg limit,” says LIGO group member Lee McCuller of the Massachusetts Institute of Technology.
Standard quantum limit
In practice, the interferometer’s sensitivity has a minimum determined by the discrete, random nature of this and another quantum-mechanical process – the arrival time of photons at the photoelectric detector. Normally, the best that can be achieved comes as a trade-off between the uncertainty in these two quantities called the standard quantum limit. However, that limit can in theory be beaten if there is a correlation between these uncertainties, which are known as shot noise and quantum radiation pressure noise.
The latest work provides the first experimental proof that the standard quantum limit can be beaten in a gravitational-wave observatory. The research was carried out by Haocun Yu, McCuller and other members of the LIGO collaboration on one half of the LIGO observatory – a pair of 4 km-long interferometer arms located in Livingston, Louisiana.
To make their measurements, Yu and colleagues used the interferometer in two different modes. In both, the laser light was subject to ever-present vacuum fluctuations that create uncertainties in measurements of its phase and amplitude – giving rise to shot noise and radiation noise. But in the first mode those vacuum fluctuations were entirely natural, and on average the two sources of noise were equally large. In the second, in contrast, the fluctuations were manipulated so that one noise source was suppressed while the other expanded – creating a “squeezed” vacuum state.
Classical noise
Using five hours’ worth of data collected last year, the researchers plotted the variation in the uncertainty of the interferometer’s distance measurement over a range of frequencies in the output signal. To deduce the detector’s total quantum noise, they subtracted from this distribution classical noise sources – such as thermal fluctuations in the mirror coatings – which they had quantified in a reference measurement. They then compared the resulting data against model predictions.
Reporting its results in Nature, the LIGO group says that its work marks two important milestones in quantum measurement. One, it says, is having directly observed that radiation noise contributes to the motion of the interferometer’s mirrors – each of which weighs 40 kg. This, they write, indicates that an effect brought about by Heisenberg’s uncertainty principle “persists even at human scales”.
The researchers’ second key finding is having shown that when using squeezed vacuum states the resulting quantum noise does indeed drop below the standard limit at frequencies of about 30–50 Hz. This, they say, proves the existence of quantum correlations between the laser beam and the mirrors.
Room temperature result
Writing a commentary piece to accompany the paper, Valeria Sequino of the University of Naples and Mateusz Bawaj of the University of Perugia in Italy point out that the LIGO group is not the first to have reduced quantum noise below the standard limit. But they note that much previous work, which did not involve gravitational-wave observatories, required cryogenic conditions to reduce thermal noise. One impressive aspect of the latest research, they say, is the fact it was carried out at room temperature.
Sequino and Bawaj also point out that LIGO and the Virgo observatory in Italy already use squeezed vacuum states to enhance the sensitivity of their interferometers at high frequencies. But in an e-mail to Physics World, they explain that the quantum correlations introduce a “frequency dependent squeezing”. This suppresses the noise source that creates the biggest problem in a certain region of the spectrum – meaning phase noise above 100 Hz and amplitude noise below it. And they add that since this squeezing simultaneously boosts the other type of noise in each region, the uncertainty principle remains intact.
However, they stress that this improvement in broadband detection has not yet been achieved – noting that the LIGO group obtained its result by “a software subtraction of classical noise”. Reducing this noise in practice will require further work, they say.
FROM PHYSICSWORLD.COM 3/7/2020
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