Global detector network to search for high-frequency gravitational waves (GravNet): conceptual design

Abstract

We propose GravNet (Global detector network to search for high-frequency gravitational waves), a novel experimental scheme enabling the search for gravitational waves in the MHz to GHz frequency range. Such high-frequency gravitational waves could arise from a variety of phenomena connected to some of the most pressing and fundamental questions in modern cosmology. The GravNet concept is based on synchronous measurements of signals from multiple experimental measurement devices operating at geographically separated locations. While gravitational-wave-induced signatures may be present in the signal of a single detector, distinguishing them from instrumental or environmental noise is highly challenging. By analyzing correlations between signals from several distant detectors, the detection significance is substantially enhanced, while simultaneously enabling studies of the nature and origin of the gravitational-wave signal. In this work, we discuss the GravNet concept specifically in the context of cavities operated in strong magnetic fields, as these currently represent the most technically mature and experimentally advanced realization of the scheme. As part of this proposal, a first demonstration experiment using a non-superconducting cavity has been performed, providing the basis for the data-analysis strategies discussed in this work. Finally, we outline the prospects and future development of GravNet as a global network for high-frequency gravitational-wave searches.

Figures (10)

• Figure 1: Comparison of axion–photon conversion in a magnetic field (left) and gravitational-wave–to–photon conversion in a strong electromagnetic background (right), i.e. the quadratic interaction of EM fields $\gamma$ with GWs $h^{\mu\nu}$, illustrating the close analogy between the two processes exploited in cavity-based searches.
• Figure 2: The gravitational coupling $C_{\rm GW}^\times$ for an ideal TM$_{010}$ mode in a cylindrical cavity with fixed height $H=20$ cm and various radii $R$. A uniform magnetic field is applied along the cavity symmetry axis. The coupling is maximized when the GW propagation direction is perpendicular to the magnetic field (at polar angles $90,\ 270^\circ$).
• Figure 3: Magnet and cryostat systems used in the GravNet program. Left: Schematic cross section of the FLASH magnet, indicating the magnet bore, cryostat structure, cooling connections and vacuum system (credits Cesidio Capoccia). Right: BlueFors dilution refrigerator with the installed 9 GHz cavity system at LNF, illustrating the compact cavity assembly and associated cabling inside the cryostat.
• Figure 4: Left: The GW coupling $C_\mathrm{GW}^\times$ for different sky positions of the GW source. The detector is located at LNF at 0:00 AM, and the magnetic field is aligned with the local zenith. Right: The GW coupling of a cross-polarized gravitational wave to the TM$_{010}$ mode of the FLASH cavity.
• Figure 5: Left: NbN coated cavity studied in the Supax setup in Schmieden:2024wqp. Right: schematic drawing of the FLASH cavity prototype ($1/6$ scale compared to the original).