Gravitational wave detection equipment can measure distortions in the fabric of space-time as small as one trillionth of the width of a human hair - small enough to hear the interference of particles entering and exiting. Now, the LIGO facility has surpassed this quantum limit by "squeezing" laser light, thereby increasing gravitational wave detection capabilities by about 60%.
When supermassive objects like black holes collide, the energy released is enough to send ripples through reality. Einstein first predicted these gravitational waves more than a century ago, but it wasn't until 2015 that scientists finally detected them directly for the first time.
The facility responsible for this major detection is the Laser Interferometer Gravitational-Wave Observatory (LIGO), which works by shooting laser light into two long tunnels, bouncing off mirrors, and then measuring how the light returns. By controlling for other effects and looking carefully, the detectors can sense tiny distortions in the laser beam—less than the width of a proton—that indicate a gravitational wave has passed through. In the years since, LIGO and other detectors have captured dozens of gravitational wave signals.
But there are limits to the sensitivity of these facilities, determined by the laws of quantum physics itself. Although a vacuum (including the vacuum in LIGO laser tubes) is often thought of as completely empty space, this is impossible to achieve. Quantum fluctuations mean that particles constantly appear, live for a fraction of a second, and then disappear again. This weak quantum noise interferes with LIGO's observations and puts hard limits on the observations.
Now, LIGO scientists have found and demonstrated a way to achieve the breakthrough using a technique called quantum squeezing. This method exploits the uncertainty principle, which states that the more precisely you know one feature of an object, the less precise you will be about other features. The most common example is a particle bouncing around in a box - if you can accurately measure its position at a particular time, you know less about its momentum, and vice versa.
In this case, scientists manipulated the uncertainty principle to get more information out of LIGO's lasers by adjusting two properties of light: phase and amplitude. Special crystals added to the pipeline during a 2019 upgrade "squeeze" the phase of light so that photons arrive at the sensor at a more predictable time. Of course, this would also reduce the certainty of the amplitude, meaning the laser would cause the mirror to vibrate, masking any low-frequency gravitational waves it might detect.
To solve this problem, a new instrument was installed on LIGO called a frequency-dependent squeeze cavity. As the name suggests, its working principle is to squeeze light of different frequencies with different properties to achieve the best of both worlds. For the most precise detection of gravitational waves, scientists need more certainty about the amplitude at lower frequencies and the phase at higher frequencies, and this system can now do that.
"Before, we had to choose where we wanted LIGO to be more precise," said Rana Adhikari, author of the study. "Now, we can cut the cake and celebrate. We've long known how to write down the equations to achieve this, but only now have we not known whether we can actually achieve it. It's like science fiction."
By pushing past this quantum limit, the increased accuracy will allow LIGO to detect 60% more gravitational wave events than before, the team said. LIGO's partner observatory Virgo in Italy is also expected to start using frequency-dependent squeezing by the end of next year.
The research team describes the work in the video below.