Using Einstein’s ‘spooky action at a distance’ to hear ripples in spacetime

Quantum entanglement could help improve gravitational wave detectors beyond the quantum limit. Cathal O’Connell explains.

The new technique will aid in the detection of gravitational waves caused by colliding black holes.
Henze / NASA

In new work that connects two of Albert Einstein’s ideas in a way he could scarcely have imagined, physicists have proposed a way to improve gravitational wave detectors, using the weirdness of quantum physics.

The new proposal, published in Nature Physics, could double the sensitivity of future detectors listening out for ripples in spacetime caused by catastrophic collisions across the universe.

When the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves in late 2015 it was the first direct evidence of the gravitational waves Einstein had predicted a century before.

Now it another of Einstein’s predictions – one he regarded as a failure – could potentially double the sensitivity of LIGOs successors.

The story starts with his distaste for quantum theory – or at least for the fundamental fuzziness of all things it seemed to demand.

Einstein thought the universe would ultimately prove predictable and exact, a clockwork universe rather than one where God “plays dice”. In 1935 he teamed up with Boris Podolsky and Nathan Rosen to publish a paper they thought would be a sort of reductio ad absurdum. They hoped to disprove quantum mechanics by following it to its logical, ridiculous conclusion. Their ‘EPR paradox’ (named for their initials) described the instantaneous influence of one particle on another, what Einstein called “spooky action at a distance” because it seemed at first to be impossible.

Yet this sally on the root of quantum physics failed, as the EPR effect turned out not to be a paradox after all. Quantum entanglement, as it’s now known, has been repeatedly proven to exist, and features in several proposed quantum technologies, including quantum computation and quantum cryptography.

Now we can add gravity wave detection to the list.

LIGO works by measuring the minute wobbling of mirrors as a gravitational wave stretches and squashes spacetime around them. It is insanely sensitive – able to detect wobbling down to 10,000th the width of a single proton.

At this level of sensitivity the quantum nature of light becomes a problem. This means the instrument is limited by the inherent fuzziness of the photons bouncing between its mirrors — this quantum noise washes out weak signals.

To get around this, physicists plan to use so-called squeezed light to dial down the level of quantum noise near the detector (while increasing it elsewhere).

The new scheme aids this by adding two new, entangled laser beams to the mix. Because of the ‘spooky’ connection between the two entangled beams, their quantum noise is correlated – detecting one allows the prediction of the other.

This way, the two beams can be used to probe the main LIGO beam, helping nudge it into a squeezed light state. This reduces the noise to a level that standard quantum theory would deem impossible.

The authors of the new proposal write that it is “appropriate for all future gravitational-wave detectors for achieving sensitivities beyond the standard quantum limit”.

Indeed, the proposal could as much as double the sensitivity of future detectors.

Over the next 30 years, astronomers aim to improve the sensitivity of the detectors, like LIGO, by 30-fold. At that level, we’d be able to hear all black hole mergers in the observable universe.

However, along with improved sensitivity, the proposed system would also increase the number of photons lost in the detector. Raffaele Flaminio, a physicist at the National Astronomical Observatory of Japan, points out in a perspective piece for Nature Physics, Flaminio that the team need to do more work to understand how this will affect ultimate performance.

“But the idea of using Einstein’s most famous (mistaken) paradox to improve the sensitivity of gravitational-wave detectors, enabling new tests of his general theory of relativity, is certainly intriguing,” Flaminio writes. “Einstein’s ideas – whether wrong or right – continue to have a strong influence on physics and astronomy.”

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Cathal O'Connell is a science writer based in Melbourne.