Quantum Noise Limits High-Frequency Gravitational Wave Detection

This research introduces the quantum noise fraction (β) as a diagnostic tool for assessing the potential of quantum enhancement in gravitational wave detectors, particularly in the high-frequency (kHz-GHz) range. The analysis reveals that resonant mass detectors operating through tidal coupling are thermally dominated at frequencies below approximately 230 MHz at dilution temperatures. This thermal dominance limits the effectiveness of squeezing and entanglement techniques. The study identifies a thermal frontier, defined by ħω = k_B T ln 3, above which the quantum regime becomes accessible. A concrete realization is presented: a bulk acoustic wave resonator at 1 GHz and 10 mK, achieving a quantum noise fraction of 0.98. The proposed gravitational wave detector employs squeezed phononic states via circuit QED readout. While an array of such resonators with 10 dB mechanical squeezing reaches a certain sensitivity level, it remains significantly above the BBN bound on stochastic backgrounds at 1 GHz. This indicates that the sensitivity gap is predominantly classical in origin and that concurrent advances in classical detector parameters are required. The findings highlight the importance of addressing thermal noise limitations in the development of high-frequency gravitational wave detectors. Future research should focus on exploring novel materials and detector designs that minimize thermal noise and maximize the potential of quantum enhancement techniques.

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