Quantum effects are generally considered small and fragile. Usually we can only detect them when things are small and remain close to absolute zero, and are flooded by non-quantum effects outside of those conditions. Mainly. In the Wednesday edition of Nature, the researchers report that quantum effects can be detected on some very large objects: the 40-kg mirrors of the Laser Interferometer Gravitational Wave Observatory, or LIGO.
The document details how the researchers were able to detect noise in LIGO’s mirrors caused by quantum fluctuations in the light reflecting off them. And by adding some specially prepared light, the researchers limited that noise, allowing for greater sensitivity in detecting gravitational waves.
Put on the grip
There are many sources of noise in LIGO hardware. The key hardware is inside a vacuum chamber, but we can’t actually remove all the stray molecules from colliding with it. The mirrors are kept at room temperature, so there is some thermal noise that always interferes with our measurements. And then there is quantum noise. LIGO is based on mirrors separated by kilometers that reflect laser beams back and forth multiple times. And those lasers are made up of photons that obey the rules of quantum mechanics.
In this case, the problem is light measurement, which forms an interference pattern that will subtly change if the laser beams have been altered by a passing gravitational wave. But that same light is also influenced by quantum fluctuations in space itself. These fluctuations create a source of noise that limits the precision of the instrument’s measurements and therefore limits our ability to detect gravitational wave events.
The key to limiting this noise is the Heisenberg Uncertainty Principle, which dictates that there are limits to how well we can know the properties of quantum objects. Therefore, we cannot determine the position of these photons beyond a certain limit, as they constantly fluctuate around a range of values.
But there is a way to avoid Heisenberg. Certain quantum properties can be correlated, in which case uncertainty limits the combined values of these properties. If you accept a lot of noise in one of the properties, you can get much more precision in your measurements than the other. Famous, this applies to location and momentum – if you want to know more precisely where a particle is, you can do so by sacrificing precision over its momentum.
But location and momentum aren’t the only properties that can be correlated. In this case, the researchers relied on the correlations between the amplitude of the light waves and their phase. By manipulating one, they could lessen uncertainty in the other.
Vacuum juicer
Those correlations naturally occur in light within the LIGO instrument. When it bounces back and forth between the two mirrors, it exerts a force called radiation pressure on the mirrors. This process induces a correlation between the amplitude and the phase of the photons that have been inside the instrument. The phase, in turn, is critical to creating the interference pattern that records the passage of gravitational waves.
To use this correlation to squeeze out quantum noise, the researchers manipulated the amplitude of light using what’s called a squeezed vacuum. In this state, the average light amplitude is zero, but the phase and amplitude can still be manipulated, allowing one of these properties to be squeezed out. The compressed vacuum was directed toward the instrument, where it interacted with a critical mirror, one that also interacted with the compressed photons leaving the instrument.
As a control, the researchers ran the setup without the squeezed vacuum, which provided a reference reading of noise from all sources. They then injected the compressed vacuum into the instrument, attempting varying degrees of compression of its amplitude and phase. This resulted in lower total noise, and their measurements showed that the noise was sensitive to the compression properties. By altering these properties, they were able to change the minimum to different noise frequencies, increasing LIGO’s sensitivity to specific classes of events.
This result is significant in several ways. On the one hand, the differences in noise depended on the interactions between light and a 40-kilogram mirror that resided at room temperature. This does not mean that the mirror was behaving like a quantum object or that it had become entangled in light. But it does mean that we can measure the effect that quantum interactions have on the physical motion of a large object sitting at room temperature.
This has a couple of interesting consequences. For one thing, this is the first time that researchers have managed to escape the limit set in a system by quantum uncertainty in a configuration like this. It also means that researchers have altered the quantum properties of light without destroying the information it contains.
While both achievements are significant from the perspective of understanding quantum mechanics, noise reduction is also critical to operating the equipment. “During the third LIGO / Virgo observation run, the compression angle in LIGO is configured to optimize the sensitivity of the detectors to gravitational waves from binary neutron star mergers,” the authors write. “This is one of the factors that has allowed Advanced LIGO to go from detecting approximately one astrophysical event per month in observation runs 1 and 2 to approximately one astrophysical trigger per week in the third LIGO / Virgo observation run.”
Nature, 2019. DOI: 10.1038 / s41586-020-2420-8 (About DOIs).
