Gravitational Waves
Why High-Frequency Gravitational Waves Matter
LIGO, Virgo, and KAGRA have opened gravitational wave astronomy by detecting waves from compact objects such as black holes and neutron stars. Those detectors are most sensitive to lower-frequency signals from stellar-mass systems. Gravitational waves above roughly 10 kHz are not expected from the standard compact-object mergers already observed, so a signal in this band would point toward more exotic physics.
Possible high-frequency sources include low-mass primordial black hole mergers and superradiance around compact objects. Primordial black holes are especially interesting because they could have formed in the early universe and remain one of the few dark matter candidates that is not a new particle. A detector sensitive to high-frequency gravitational waves therefore probes a different side of dark matter physics than a traditional particle search.
Gravitational Wave Searches with Lumped-Element Detectors
The connection between axions and gravitational waves comes from how they modify Maxwell's equations in the presence of a strong magnetic field. In the axion case, the axion-photon interaction, often described in this detection context as the inverse Primakoff effect, makes the axion behave like a tiny oscillating effective current. A gravitational wave can produce an analogous electromagnetic response through the inverse Gertsenshtein effect, the gravitational-wave version of conversion in a magnetic field. As the wave passes through the magnet, it perturbs the electromagnetic fields in a way that behaves like an effective current, producing a real oscillating magnetic field at the gravitational wave frequency.
Lumped-element detectors can search for this signal by measuring the changing magnetic flux through a pickup loop. The pickup converts the gravitational-wave-induced magnetic field into a tiny current, which is then read out with a SQUID or other quantum sensor. This is not a laser interferometer shrunk down to lab scale. It is a lumped-element electromagnetic detector, using the same basic experimental language as low-mass axion searches.
The ABRA-GW search demonstrated this idea using a modified ABRACADABRA-10 cm axion detector, a toroidal lumped-element experiment. The axion channel used a circular pickup loop, matched to the azimuthally symmetric axion-induced field, while the gravitational wave channel used a figure-eight pickup better matched to the dipole-like magnetic pattern produced by a gravitational wave. Because the two pickup shapes couple weakly to each other, the experiment could search for axions and high-frequency gravitational waves at the same time without either search spoiling the other.
CAL-Pathfinder applies this idea in a solenoidal geometry. For the axion search, the detector uses a coaxial pickup inside a strong solenoidal magnet. For the gravitational wave search, that coaxial pickup is replaced by a wire loop placed near the region of large magnetic field gradient at the end of the solenoid and read out in broadband with a SQUID. This lets the same experimental platform explore low-mass axion dark matter and a high-frequency gravitational wave window beyond the reach of conventional interferometers.
Sources
- LIGO Lab — "What are Gravitational Waves?"
- Pappas et al., "High-Frequency Gravitational Wave Search with ABRACADABRA-10 cm" — arXiv:2505.02821