MAGGIE: A Magnetic Gravitational Wave Induction Experiment

MAGGIE employs a search for high-frequency gravitational waves (HFGWs) that exploits their coupling to electromagnetism in the presence of a strong magnetic field. A passing GW in an external field induces an effective displacement current that generates a secondary magnetic flux, similarly to the WISPLC case. GravMAGA measures this flux with a purpose-built pickup geometry inside the 14 Tesla superconducting warm-bore solenoid at the Universität Hamburg.

The detector centers on a figure-eight (8-shape) pickup loop that breaks the azimuthal symmetry of the induced currents and converts the GW-driven flux into a measurable voltage. A symmetric “blind” loop provides a simultaneous background/veto channel, while an injection loop enables in-situ end-to-end calibration. Signals from the measurement and blind loops are amplified, band-filtered, and digitized in parallel, enabling real-time noise rejection.

The science target is broadband and transient HFGW signals around the 10 kHz–40 MHz band, including templates for primordial black-hole mergers and other short bursts. A GPU-accelerated matched-filter pipeline (time-domain) evaluates candidate events online using a template bank, while continuous monitoring tracks the voltage noise spectral density and environmental diagnostics. The expected experimental reach in terms of strain spectral noise density at the 40 MHz high-frequency end is projected to reach ∼4 × 10⁻¹⁶/√Hz for transient searches, and in terms of strain, projected to reach ∼10⁻¹⁹ for 1 year of continuous search. This allows MAGGIE to constrain currently unexplored regions of the HFGW parameter space.

Detector development for MAGGIE  is completed, and commissioning is now ongoing, while optimisation of magnetic shielding and ground isolation for improved noise rejection is being implemented. The modular loop holder and shielded readout allow straightforward future upgrades (e.g. additional windings, refined filtering, extended bandwidth). The warm–bore access of the 14 Tesla magnet simplifies installation and calibration, making the platform well-suited for iterative improvements and extended observing campaigns.

Figure 1: Schematic overview of the MAGGIE experiment. A 14T warm-bore solenoidal magnet houses three pickup loops: the measurement loop (ML, green), the signal injection loop (SIL, blue) used for calibration, and the blind loop (BL, red) used as an internal noise/systematics monitor. The ML and BL signals are amplified with high impedance low-noise amplifiers (LNAs), filtered with band-pass filters (BPFs) and digitized in a shielded enclosure by a synchronized two-channel analog-to-digital converter (ADC). Calibration tones are injected via the SIL using a signal generator (SG) referenced to a common 10 MHz clock.

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Figure 2: Projected strain sensitivity of MAGGIE for continuous signals assuming a 14T magnetic field, 10 windings per loop, loop area of 914 cm2, SNR = 3 and room temperature operation. The accessible frequency range spans 10 kHz–10 MHz.

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Figure 3: Projected strain-equivalent SND of MAGGIE for ×\times×-polarized transient GW signals from equal-mass PBH binaries evaluated for SNR=3\text{SNR}=3SNR=3 (purple area) and the matched-filter sensitivity with a gain factor of G≈7.72G\approx7.72G≈7.72 compared to the narrowband estimate. The corresponding PBH mass is obtained from the ISCO relation. Using matched filtering, the expected sensitivity (purple line) is increased by approximately one order of magnitude. .

Selected Presentations and Publications

• J. Jödicke, M. Maroudas, T.-S. Cezar, D. Horns, MAGGIE: A Gravitational-Wave Magnetic-Field Antenna (concept note, 2025).
• M. Maroudas, “Axion, ALP, and HFGW Searches Across Complementary Experimental Frontiers”, 20th Patras Workshop on Axions, WIMPs and WISPs, Lisbon, 24–26 Sept 2025. https://agenda.infn.it/event/46273/contributions/269303/<;/a>

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