Resonant loop interferometers for high-frequency gravitational waves

Abstract Gravitational waves at kilohertz and higher frequencies offer a unique probe of the early Universe at temperatures well beyond the reach of the cosmic microwave background, corresponding to energy scales ≳ 10 9 GeV. Existing detector concepts fall many orders of magnitude short of the big-bang nucleosynthesis (BBN) bound on the stochastic background in this regime. We propose a new interferometric architecture based on closed optical loops, in which the gravitational-wave–induced phase shift accumulates coherently over many traversals. This produces sharp, narrowband resonances whose predictable comb structure provides a distinct experimental signature. For a folded loop with parameters compatible with the Einstein Telescope infrastructure, and finesse values of order 500 , we project sensitivity that approaches and even surpasses the BBN bound up to tens of kilohertz after one year of integration. Such loop interferometers thus open a realistic and distinctive path toward exploring high-frequency stochastic gravitational-wave backgrounds.