Researchers Unveil Method to Detect Gravitational Waves in New Frequency Range

A team of researchers in the United Kingdom has proposed a novel method using a network of optical cavities to detect gravitational waves (GWs) at previously unexplored frequencies. This approach could enable astronomers to search for GWs in the milli-Hz frequency band, specifically within the range of 10⁻⁵ Hz–1 Hz. The advancement holds promise for uncovering signals from binary systems of white dwarfs, neutron stars, and stellar-mass black holes, many of which are expected to originate from within the Milky Way.

Gravitational waves were first detected a decade ago, and since then, the LIGO-Virgo-KAGRA detection network has identified waves from numerous black hole and neutron star mergers, primarily operating in the 10 Hz–30 kHz frequency range. Researchers have also recorded gravitational wave backgrounds at nanohertz frequencies through pulsar timing arrays. Nevertheless, the milli-Hz band has remained largely uncharted.

The new initiative, known as QSNET, was part of the UK’s Quantum Technology for Fundamental Physics (QTFP) programme. Giovanni Barontini, a researcher at the University of Birmingham, explained that QSNET aimed to develop a network of clocks for measuring the stability of fundamental constants. He noted, “This programme brought together physics communities that normally don’t interact, such as quantum physicists, technologists, high energy physicists, and astrophysicists.” Although the QTFP programme concluded in 2023, Barontini and his colleagues made significant progress in demonstrating how milli-Hz gravitational waves could be detected using optical cavities.

Inside an ultrastable optical cavity, light at specific resonant frequencies bounces continuously between a pair of mirrors. When this light is generated by a specific atomic transition, it can act as a highly accurate clock. Barontini stated, “Ultrastable cavities are a main component of modern optical atomic clocks. We demonstrated that they have reached sufficient sensitivities to be used as ‘mini-LIGOs’ and detect gravitational waves.”

The QSNET findings suggest that while the spacing between mirrors in an optical cavity does not change in response to passing gravitational waves, the phase of the light within the cavity is altered. Team member Vera Guarrera emphasized that “methods from precision measurement with cold atoms can be transferred to gravitational-wave detection.” By merging these techniques, the researchers envision compact optical resonators as credible probes for the milli-Hz range, complementing existing detection methods.

The proposed detector would consist of two optical cavities positioned at 90 degrees to each other, each operating at different frequencies, along with an atomic reference at a third frequency. The phase shift caused by a passing gravitational wave would manifest as a change in the interference patterns among the three frequencies.

The team advocates for the establishment of a global ground-based network of these detectors. According to Xavier Calmet from the University of Sussex, this network could not only detect gravitational waves but also pinpoint their sources in the sky. He stated, “This detector will allow us to test astrophysical models of binary systems in our galaxy, explore the mergers of massive black holes, and even search for stochastic backgrounds from the early universe.”

Barontini expressed hope that this work would inspire the creation of a global sensor network capable of scanning a new frequency window rich with sources, including many from our own galaxy. “By harnessing this existing technology,” he added, “we can open up a new era of discovery regarding gravitational waves in the milli-Hz range, possibly much sooner than many current projects.”

The research detailing these findings has been published in Classical and Quantum Gravity, marking a significant step forward in gravitational wave detection technology.