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Gravitational waves and precise clocks: What quantum squeezing can achieve

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Gravitational waves and precise clocks: What quantum squeezing can achieve

When two black holes spiral toward each other and then collide, they shake spacetime and create ripples that can spread across hundreds of millions of light-years. Since 2015, scientists have been directly observing these gravitational waves to answer fundamental questions about our cosmos – including the origin of heavy elements such as gold and the speed at which the universe is expanding.

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But detecting gravitational waves has so far been difficult. By the time they reach Earth and the twin detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US states of Louisiana and Washington, the waves have almost disappeared and are barely detectable. LIGO’s detectors must detect movements on the order of a ten-thousandth the width of protons to have any chance. LIGO has so far been able to confirm at least 90 gravitational wave detections, but the physicists working there want to prove significantly more. The system therefore needs to be made much more sensitive – a major technical challenge.

“The problem with these detectors is that every time you try to improve them, you can actually make the detection performance worse because they are so sensitive,” says Lisa Barsotti, a physicist at the Massachusetts Institute of Technology. Still, Barsotti and her colleagues have recently overcome this challenge, developing hardware that allows LIGO to detect far more black hole mergers and neutron star collisions. The device belongs to a growing class of instruments that use so-called quantum squeezing – a way for researchers to use the actually problematic uncertainty principle from quantum mechanics for practical applications.

Physicists usually describe objects in the quantum domain using probabilities. For example, an electron is not located “here or there”, but can be found in any possible location with a certain probability. Strictly speaking, it is only in a certain place if you measure whether it is there. These probabilities can be manipulated through quantum squeezing. Researchers are therefore increasingly using it to exert more control over the measurement process and thus dramatically improve the precision of quantum sensors such as LIGO.

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“For precision sensing applications where the goal is to detect very weak signals, quantum squeezing can be a pretty big win,” says Mark Kasevich, a physicist at Stanford University. He himself uses the phenomenon to develop more precise magnetometers, gyroscopes and clocks for navigation applications. The Canadian start-up Xanadu uses the technology in its quantum computers. Last fall, the US research agency DARPA announced the “Inspired” program, which aims to develop quantum squeezing technology in the form of a single chip. There are already practical applications for the process.

The key concept behind quantum squeezing is the phenomenon of Heisenberg’s uncertainty principle. In a quantum mechanical system, this principle sets a fundamental limit on the accuracy of measuring two properties of an object simultaneously. No matter how good the measuring device is, it is always subject to a fundamental inaccuracy that is part of nature itself. Physicists call these properties “complementary properties.” For example, if you want to accurately measure the speed of a particle, you have to sacrifice the accuracy of determining its location and vice versa. “Physics places limits on experiments and especially on precision measurements,” says John Robinson, physicist at quantum computing startup QuEra.

But you can also use the process by allowing more uncertainty about the physical properties that you don’t want to measure. This “squeezing” allows researchers to increase the precision of the desired measurement. Theoretical physicists suggested as early as the 1980s that the uncertainty in measurements could be compensated for. Since then, experimental physicists have developed the ideas further; Over the last decade and a half, experiments that previously had to be spread out over entire tables have resulted in practical devices. The big question now is which applications will benefit from this. “We’re just beginning to understand what this technology could become,” says Kasevich. “Then hopefully our imagination will grow and help us figure out what it’s really good for.”

LIGO is paving the way to answering this question – in the form of improved detectors that can measure extremely small distances. The observatory detects gravitational waves using L-shaped machines capable of detecting tiny movements along its four-kilometer-long “arms.” On each of these giant machines, the researchers split a laser beam into two segments and send a beam down each of the arms, which is reflected by a series of mirrors. In the absence of a gravitational wave, the peaks and valleys of the individual light waves should completely cancel each other out as the rays recombine. However, when a gravitational wave passes through, the arms are alternately stretched and compressed, so the split light waves are slightly out of phase.

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But the resulting signals are so weak that they could be drowned out by the noise of the so-called quantum vacuum, the universe’s inevitable background noise caused by the particles oscillating in and out. The quantum vacuum results in a sort of background flicker of light entering LIGO’s arms. This light hits the mirrors and shifts them on the same minimal scale as the gravitational waves that LIGO is actually trying to detect.

Barsotti’s team can’t easily get rid of this background flicker, but through quantum squeezing they can control it to a limited extent. To do this, the team installed a 300-meter-long cavity in each of the two L-shaped LIGO detectors. Using lasers, they can create an artificial quantum vacuum within it, manipulating the physical conditions to increase the degree of control over how bright the flicker can be or how randomly it occurs over time. However, detecting higher frequency gravitational waves is more difficult when the rhythm of the flickering is more random, while lower frequency gravitational waves can be drowned out when the background light appears brighter. In their artificial vacuum, the “noisy” particles still appear in the measurements, but in a way that no longer disrupts the detection of gravitational waves as much. “You can change the vacuum by manipulating it so that it is useful to yourself,” explains Barsotti.

There was a decades-long development process before this innovation was ready: In the 2010s, LIGO gradually integrated increasingly sophisticated forms of quantum squeezing, based on theoretical ideas from the 1980s. With the latest quantum squeezing innovation installed on the detector last year, researchers now expect that gravitational waves can be detected up to 65 percent more frequently than before.

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