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 spacetime 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 spacetime 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.
"We can't control nature, but we can control our detectors," says Lisa Barsotti, a senior research scientist at MIT who oversaw the development of the new LIGO technology, a project that originally involved research experiments at MIT led by Professor of Physics Matt Evans (PhD '02) and Professor of Astrophysics Nergis Mavalvala. The effort now includes dozens of scientists and engineers based at MIT, Caltech, and the twin LIGO observatories in Hanford, Washington, and Livingston, Louisiana.
"A project of this scale requires multiple people, from facilities to engineering and optics-basically the full extent of the LIGO Lab with important contributions from the LIGO Scientific Collaboration. It was a grand effort made even more challenging by the pandemic," Barsotti says.
"Now that we have surpassed this quantum limit, we can do a lot more astronomy," explains Lee McCuller, assistant professor of physics at Caltech and one of the leaders of the new study. "LIGO uses lasers and large mirrors to make its observations, but we are working at a level of sensitivity that means the device is affected by the quantum realm."
