Speaker
Description
Quantum squeezed light is a key technology in gravitational-wave detection, enabling improved sensitivity in current and future detectors. Achieving its full benefit requires controlling optical decoherence beyond nominal loss and phase noise. A particularly important mechanism is hyperloss: spatial mode mismatch at interferometric interfaces coherently couples the squeezed field into higher-order modes that can re-couple downstream. Because the fundamental and higher-order modes accumulate different propagation phases, the re-coupling mixes in the strongly anti-squeezed quadrature, producing phase-sensitive decoherence that can be far stronger than predicted by the usual “mismatch ≈ incoherent loss” model. At mismatch levels relevant to next-generation detectors, this can severely reduce observable quantum enhancement.
We experimentally demonstrate hyperloss for the first time using two filter-cavity interfaces: 8% mismatch at each cavity converts 5.8 dB of squeezing into an effectively thermal state with ≈1.5 dB excess noise above shot noise. Because the effect is coherent, it is controllable: correlations can be recovered by tuning the differential spatial-mode phase (e.g., Gouy/propagation phase). We demonstrate this recovery experimentally, not only eliminating hyperloss but even suppressing mismatch-induced degradation, with 15% geometric mismatch behaving like only ≈2.8% effective loss.
Hyperloss has major implications for future detectors, such as Einstein Telescope or Cosmic Explorer, influencing their sensitivity, design, and the technological advancements required to mitigate decoherence. We discuss these implications and highlight the directions of future research.