Quantum effects are generally viewed as small and fragile. Usually we can only recognize them when things are tiny and stay close to absolute zero and they are flooded by non-quantum effects outside of these conditions. Most of time. In Wednesday̵
The paper describes how researchers could detect noise in LIGO’s mirrors caused by quantum fluctuations in the light they reflected. By adding specially prepared light, the researchers were able to limit this noise and thus increase the sensitivity when detecting gravitational waves.
Put on the squeeze
There are many sources of noise in the LIGO hardware. The key hardware is in a vacuum chamber, but we can’t really stop all the stray molecules from bumping into it. The mirrors are kept at room temperature, so there are some thermal disturbances that always affect our measurements. And then there is quantum noise. LIGO is based on mirrors that are separated by kilometers and reflect laser beams back and forth several times. And these laser beams consist of photons that comply with the rules of quantum mechanics.
In this case, it is a matter of measuring the light, which forms an interference pattern that shifts slightly when the lasers are changed by a passing gravitational wave. The same light is also influenced by quantum fluctuations in the room itself. These fluctuations create a noise source that limits the accuracy of the instrument’s measurements, and thus our ability to detect gravitational wave events.
The key to limiting this noise is the Heisenberg uncertainty principle, which prescribes 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, since they constantly fluctuate around a range of values.
But there is a way around Heisenberg. Certain quantum properties can be correlated. In this case, the uncertainty limits the combined values of these properties. If you accept a lot of noise in one of the properties, you can make the measurements of the other much more precisely. As is well known, this applies to location and momentum: If you want to know more precisely where a particle is located, you can do this by sacrificing the precision of its momentum.
However, location and momentum are not the only properties that can be correlated. In this case, the researchers relied on correlations between the amplitude of the light waves and their phase. Manipulating one could reduce uncertainty in the other.
These correlations naturally occur in the light of the LIGO instrument. As it bounces back and forth between the two mirrors, it exerts a force called radiation pressure. This process induces a correlation between the amplitude and phase of the photons that were in the instrument. The phase is in turn crucial for the generation of the interference pattern, which registers the passage of gravitational waves.
To use this correlation to suppress quantum noise, the researchers manipulated the light amplitude using a so-called compressed vacuum. In this state, the average amplitude of the light is zero, but the phase and amplitude can still be manipulated, which can squeeze one of these properties. The compressed vacuum was directed into the instrument, where it interacted with a critical mirror – one that also interacted with the compressed photons that emerged from the instrument itself.
As a control, the researchers performed the setup without the compressed vacuum that provided a basic measurement of the noise from all sources. Then they injected the compressed vacuum into the instrument and tried to compress its amplitude and phase to different degrees. This resulted in a lower overall noise, and their measurements showed that the noise was sensitive to the properties of the squeeze. By changing these properties, they were able to shift the minimum to different noise frequencies and thus increase the sensitivity of LIGO for certain event classes.
This result is important in many ways. First, the noise differences were dependent on the interactions between light and a 40-kilogram mirror at room temperature. This does not mean that the mirror behaves like a quantum object or is involved with the light. However, this means that we can measure the effect that quantum interactions have on the physical movement of a large object at room temperature.
This has some interesting consequences. For one thing, it is the first time that researchers have managed to escape the limits of a system imposed by quantum uncertainty in this way. This also means that the researchers have changed the quantum properties of light without destroying the information it contains.
While these two accomplishments are important from an understanding of quantum mechanics, reducing noise is also critical to the operation of the equipment. “During the third observation run by LIGO / Virgo, the pinch angle in LIGO is adjusted so that the sensitivity of the detectors to gravitational waves from binary neutron star fusions is optimized,” the authors write. “This is one of the factors that has enabled Advanced LIGO to go from approximately one astrophysical event per month when observing 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).