There is Room at the Top: Fundamental Quantum Limits for Detecting Ultra-high Frequency Gravitational Waves

The sky of astrophysical gravitational waves is expected to be quiet above $\sim 10{\rm kHz}$, which is the upper limit of characteristic frequencies of dynamical processes involving astrophysical black holes and neutron stars. Therefore, the ultrahigh ($\ge 10{\rm kHz}$) frequency window is particularly promising for detecting primordial gravitational waves, as isolating the contribution from the astrophysical foreground has always been a challenging problem for gravitational wave background de...