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New research in Â鶹ÒùÔºical Review Letters suggests that superconducting magnets used in dark matter detection experiments could function as highly precise gravitational wave detectors, thereby establishing an entirely new frequency band for observing these cosmic ripples.

This concept expands on the initial Weber bar architecture from the 1960s, in which Joseph Weber proposed detecting gravitational waves using massive metal cylinders that would respond through mechanical resonance.

Although Weber's technique succeeded at certain resonant frequencies, it experienced reduced sensitivity outside these restricted frequency bands.

This study extends this concept, demonstrating that DC magnets can function as magnetic Weber bars, potentially detecting gravitational waves in the previously challenging kilohertz to megahertz frequency range.

Â鶹ÒùÔº spoke to co-author Dr. Sebastian Ellis from the University of Geneva about the research, which he conducted with Valerie Domcke from CERN and Nicholas L. Rodd from Lawrence Berkeley National Laboratory.

"What we recognized was that while the Weber Bar concept works very well if the gravitational wave frequency is very near to a resonant mode of the bar itself, it doesn't work as well off-resonance," Ellis explained to Â鶹ÒùÔº. "You can think of it as an instrument that plays well on-key but sounds horrible off-key."

The new magnetic approach addresses this fundamental limitation by leveraging the enormous magnetic energy stored in superconducting magnets, which far exceeds the electric energy available in traditional Weber bar readout systems.

How magnetic fields interact with gravitational waves

The detection mechanism relies on a clever two-step interaction between gravitational waves and magnetic fields.

A gravitational wave passing through a superconducting magnet induces microscopic vibrations across the entire structure, analogous to the barely perceptible motion of LIGO's mirrors.

"As a gravitational wave passes over and through the magnet, it causes a vibration of the whole structure since the effect of the wave is similar to that of a mechanical force acting on the object," Ellis explained.

"This vibration leads to deformations of the structure containing the wire through which the current flows, which generates a magnetic field."

These deformations create an oscillating magnetic field component that researchers can detect using extraordinarily sensitive quantum sensors called SQUIDs (Superconducting Quantum Interferometric Devices).

A pickup loop (which acts as a magnetic antenna) placed near the magnet's end can capture these minute magnetic field changes, translating gravitational wave signals directly into electromagnetic readings.

The approach offers several key advantages over traditional methods.

Unlike conventional Weber bars that require complex mechanical-to-electromagnetic signal conversion, magnetic Weber bars produce intrinsically electromagnetic signals. This removes a significant source of interference and complication while delivering broadband sensitivity over an extensive frequency spectrum.

Using dark matter experiments to hunt for gravitational waves

The research specifically highlights powerful magnets being constructed for axion dark matter experiments, including DMRadio and ADMX-EFR (Axion Dark Matter eXperiment—Extended Frequency Range).

These experiments feature enormous superconducting magnets that could simultaneously search for both dark matter and gravitational waves.

"The primary advantage of the magnets that will be used for axion dark matter experiments is their enormous magnetic energy. They have very powerful magnetic fields, and they're also very large," Ellis noted.

"As we pointed out in our paper, it is the (electro-)magnetic energy that dominates the off-resonance sensitivity of a Weber bar, whether it is magnetic or traditional."

The researchers estimated that the sensitivity of these MRI magnets would be somewhat lower than LIGO's peak performance. However, it would operate across a much broader frequency range, from a few kilohertz to about 10 megahertz.

Importantly, this would make it more sensitive than LIGO at frequencies above a few kilohertz, opening up an entirely new detection window.

New cosmic windows

This frequency range represents largely uncharted territory for gravitational wave astronomy.

The research arose from recognizing that existing and planned axion experiments possessed exactly the right infrastructure for gravitational wave detection.

"Our idea arose when we realized that planned and existing experiments targeting a dark matter candidate known as the axion had very large, powerful magnets that could be used simultaneously to search for gravitational waves," Ellis said.

"We hoped that being able to search for two signals rather than one would augment the scientific case for performing these experiments."

Converting this concept into working detectors will require overcoming significant technical hurdles, particularly in isolating the instruments from environmental vibrations that could mimic gravitational wave signals.

"The device needs to be extremely well isolated from environmental vibrations," Ellis noted.

"This requirement is very similar to the one faced by LIGO, and by traditional Weber Bars such as the 2-ton bar AURIGA. The fact that they were able to successfully isolate their devices makes us optimistic."

The team is now expanding their collaboration and studying specific gravitational wave signals that could be detected with operational magnetic Weber bars. They're also exploring advanced quantum sensing techniques beyond SQUIDs that could further enhance sensitivity.

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