Electromagnetic resonant systems, such as cavities or LC circuits, have
emerged as powerful detectors for probing ultralight boson dark matter and
high-frequency gravitational waves. However, the limited resonant bandwidth of
conventional single-mode resonators, imposed by quantum fluctuations,
necessitates numerous scan steps to cover broad unexplored frequency regions.
The incorporation of multiple auxiliary modes can realize a broadband detector
while maintaining a substantial signal response. The broadened sensitive width
can be on the same order as the resonant frequency, encompassing several orders
of the source frequency for heterodyne detection, where a background cavity
mode transitions into another. Consequently, our approach enables significantly
deeper exploration of the parameter space within the same integration time
compared to single-mode detection.Comment: 18 pages, 6 figure