In 2015, the Laser Interferometer Gravitational-Wave Observatory, or LIGO, made history when it made the first direct detection of gravitational waves, or ripples in space and time, produced by a pair of colliding black holes. Since then, the U.S. National Science Foundation (NSF)-funded LIGO and its sister detector in Europe, Virgo, have detected gravitational waves from dozens of mergers between black holes as well as from collisions between a related class of stellar remnants called neutron stars. At the heart of LIGO’s success is its ability to measure the stretching and squeezing of the fabric of space-time on scales 10 thousand trillion times smaller than a human hair.
As incomprehensibly small as these measurements are, LIGO’s precision has continued to be limited by the laws of quantum physics. At very tiny, subatomic scales, empty space is filled with a faint crackling of quantum noise, which interferes with LIGO’s measurements and restricts how sensitive the observatory can be. Now, writing in the journal Physical Review X, LIGO researchers report a significant advance in a quantum technology called “squeezing” that allows them to skirt around this limit and measure undulations in space-time across the entire range of gravitational frequencies detected by LIGO.
This new “frequency-dependent squeezing” technology, in operation at LIGO since it turned back on in May of this year, means that the detectors can now probe a larger volume of the universe and are expected to detect about 60 percent more mergers than before. This greatly boosts LIGO’s ability to study the exotic events that shake space and time.
The results also have ramifications for future quantum technologies such as quantum computers and other microelectronics as well as for fundamental physics experiments.
The laws of quantum physics dictate that particles, including photons, will randomly pop in and out of empty space, creating a background hiss of quantum noise that brings a level of uncertainty to LIGO’s laser-based measurements. Quantum squeezing, which has roots in the late 1970s, is a method for hushing quantum noise or, more specifically, for pushing the noise from one place to another with the goal of making more precise measurements.
The term squeezing refers to the fact that light can be manipulated like a balloon animal. To make a dog or giraffe, one might pinch one section of a long balloon into a small precisely located joint. But then the other side of the balloon will swell out to a larger, less precise size. Light can similarly be squeezed to be more precise in one trait, such as its frequency, but the result is that it becomes more uncertain in another trait, such as its power. This limitation is based on a fundamental law of quantum mechanics called the uncertainty principle, which states that you cannot know both the position and momentum of objects (or the frequency and power of light) at the same time.
Since 2019, LIGO’s twin detectors have been squeezing light in such a way as to improve their sensitivity to the upper frequency range of gravitational waves they detect. But, in the same way that squeezing one side of a balloon results in the expansion of the other side, squeezing light has a price. By making LIGO’s measurements more precise at the high frequencies, the measurements became less precise at the lower frequencies.
Now, LIGO’s new frequency-dependent optical cavities — long tubes about the length of three football fields — allow the team to squeeze light in different ways depending on the frequency of gravitational waves of interest, thereby reducing noise across the whole LIGO frequency range.
News Source: MIT
