Squeezing Improves LIGO’s Sensitivity to Quantum Noise

A new instrument called a quantum vacuum squeezer is helping LIGO — the Laser Interferometer Gravitational-Wave Observatory — to detect gravitational wave signals nearly every week. Just a year ago, LIGO was picking up gravitational waves about once a month. LIGO comprises two identical detectors, one located in Hanford, Wash., and the other in Livingston, La.

The squeezer was designed, built, and integrated with LIGO’s detectors by an international research team from MIT, Caltech, and the Australian National University (ANU). It suppresses the quantum noise that can infiltrate the LIGO detectors, extending the detectors’ range by about 15%. Combined with an increase in LIGO’s laser power, this means the detectors can pick out a gravitational wave generated by a source in the universe out to about 140 megaparsecs, or more than 400 million light-years away.

To detect a gravitational wave, scientists send a laser beam down each arm of the L-shaped detector. At the end of each arm is a mirror. Each beam bounces off its respective mirror and travels back up the arm. To identify a gravitational wave signal, scientists measure the time it takes for the beam to travel back up the arm to its point of origin. If a gravitational wave passes through the detector, it should shift the position of one or both mirrors, which should, in turn, affect the arrival time of each laser beam back to its point of origin.

Researchers install a new quantum squeezing device into one of LIGO’s gravitational wave detectors. Courtesy of Lisa Barsotti.

The signals that LIGO detects are so tiny that quantum fluctuations can potentially muddy or completely mask incoming signals of gravitational waves. “LIGO’s laser is made of photons,” researcher Maggie Tse said. “Instead of a continuous stream of laser light, if you look close enough it’s actually a noisy parade of individual photons, each under the influence of vacuum fluctuations. Whereas a continuous stream of light would create a constant hum in the detector, the individual photons each arrive at the detector with a little ‘pop.’” Professor Nergis Mavalvala described this quantum noise “like a popcorn crackle in the background that creeps into our interferometer and is very difficult to measure.”

The theory of quantum squeezing, first proposed in the 1980s, puts forth the idea that quantum vacuum noise can be represented as a sphere of uncertainty along two axes: phase and amplitude. If, for example, the sphere is squeezed in a way that constricts it along the amplitude axis, this would shrink the uncertainty in the amplitude state of a vacuum (the constricted part), while increasing the uncertainty in the phase state (the distended part).

A close-up of the quantum squeezer that has expanded LIGO’s expected detection range by 50%. Courtesy of Maggie Tse.

At the heart of the LIGO squeezer is an optical parametric oscillator — a bow tie-shaped device that holds a crystal within a configuration of mirrors. When the researchers direct a laser beam to the crystal, the crystal’s atoms facilitate interactions between the laser and the quantum vacuum in a way that rearranges their properties of phase versus amplitude, creating a new, “squeezed” vacuum that then continues down each of the detector’s arms as it normally would. This squeezed vacuum has smaller phase fluctuations than an ordinary vacuum, allowing scientists to better detect gravitational waves.

As LIGO seeks to detect farther, fainter signals, quantum noise has become more of a limiting factor. “The measurement we’re making is so sensitive that the quantum vacuum matters,” researcher Lisa Barsotti said. In addition to increasing LIGO’s ability to detect gravitational waves, the new quantum squeezer could help scientists better extract information about the sources that produce these waves.

The research was published in Physical Review Letters (www.doi.org/10.1103/PhysRevLett.123.231107). A commentary on the research was published in Physics.