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Gravitational wave observatory in Eastern WA breaks quantum limit. Why it matters

By Annette Cary, Tri-City Herald
Published: January 3, 2024, 7:27am

KENNEWICK — The LIGO Hanford Observatory near Richland is expected to detect 60% more cataclysmic cosmic events — like colliding neutron stars and black holes — thanks to a quantum limit breakthrough.

Since the observatory was turned back on in May after three years of upgrades, including adding new quantum squeezing technology, it can probe a larger volume of the universe.

“Now that we have surpassed this quantum limit, we can do a lot more astronomy,” said Lee McCuller, assistant professor of physics at the California Institute of Technology and a leader in the study published in the journal “Physical Review X.”

“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,” McCuller said in an explanation of quantum physics work at LIGO prepared by the Massachusetts Institute of Technology and Caltech.

In 2015 the Laser Interferometer Gravitational-wave Observatory at Hanford site land in Eastern Washington and its twin observatory in Louisiana detected gravitational waves for the first time, providing physical confirmation of Einstein’s theory of relativity a century earlier.

Since then it has detected gravitational waves from dozens of mergers between black holes as well as from collisions between stellar remnants called neutron starts.

Rather than detecting light like more conventional observatories, the twin LIGOs detect gravitational waves from past violent events in space as the waves move through the Earth.

At LIGO Hanford vacuum tubes extend for 2.5 miles at right angles across previously unused Hanford site shrub steppe land near the Tri-Cities, Wash. At the end of each tube, a mirror is suspended on glass fibers.

A high-power laser beam is split to go down each tube, bouncing off the mirrors at each end. If the beam is undisturbed, it will bounce back and recombine perfectly.

But if a gravitational wave is pulsing through the Earth, making one of the tubes repeatedly infinitesimally longer and the other infinitesimally shorter, the beam will not recombine as expected.

LIGO Hanford and its Louisiana twin can measure the stretching and the squeezing of the fabric of space-time on scales 10 thousand trillion times smaller than a human hair.

But other things than gravitational waves can influence the laser beam.

LIGO and quantum limit

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 limits the range of detections.

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LIGO’s laser is made of photons, each under the influence of vacuum fluctuations that can produce a crackle in the interferometer, limiting the range of detections.

That interference can be reduced by “squeezing” out quantum noise by creating a quantum state of light and injecting it into the vacuum tubes.

But quantum noise that is moved out of the time, or frequency, of laser light, is moved into the amplitude, or power, of the laser light.

But more powerful lasers could push LIGO’s heavy mirrors around, creating unwanted noise at the lower frequencies of gravitational waves. That masks the detectors’ ability to sense low-frequency gravitational waves.

The solution developed was controlling the relative phases of the light waves in such a way that the researchers can selectively move the quantum noise into different features of light — phase or amplitude — depending on frequency range of gravitational waves.

The quantum squeezing device was used in the previous operating run of Hanford LIGO. But now it has been improved with the addition of a 984-foot tunnel along one of the LIGO arms that allows the further reduction of quantum noise at more frequencies.

Dhruva Ganapathy, a graduate student at MIT and a lead-coauthor of the paper published in “Physical Review X,” said he’s most excited about the possibility of the twin LIGOs detecting more neutron star smashups with the help of frequency-dependent squeezing.

“With more detections, we can watch the neutron stars rip each other apart and learn more about what’s inside,” he said.

The work done for LIGO also may benefit science and technology in other areas.

“We can take what we have learned from LIGO and apply it to problems that require measuring sub-atomic distances with incredible accuracy,” McCuller said.

It could have ramifications for future quantum technologies such as quantum computers and other microelectronics as well as for fundamental physics experiments.

How to tour LIGO

Monthly Saturday tours are offered of the LIGO Hanford Observatory with the next one set for Jan. 13. They include an hour walking tour of indoor and outdoor locations and time in the LIGO Exploration Center, which has interactive exhibits and artifacts from LIGO’s earlier days.

Registration and a ticket are required for the free tours. Go to ligo.caltech.edu/WA/page/lho-public-tours.

Drop in visitors are welcome at the LIGO Exploration Center 9:30 a.m. to 4 p.m. Tuesdays through Fridays.

To reach LIGO, search for “LIGO Hanford Observatory” on Google Maps. Or drive northwest from Richland on Highway 240 and turn right on Hanford Route 10 and drive about five miles.

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