Speaker
Description
We present a bidirectional internal squeezing scheme for gravitational-wave detectors and show that it saturates the most stringent known lower bounds on quantum noise from internal optical dissipation. The scheme uses two optical parametric amplification stages inside the signal-extraction cavity that act on intra-cavity fields propagating in opposite directions. Thereby, most vacuum fields entering the interferometer are squeezed, while the signal and internal vacuum fields are amplified so that loss in the readout path adds no further noise. We show that the resulting signal-referred quantum noise spectral density is independent of the arm-cavity input and signal-extraction transmissivities, opening design freedom to mitigate technical noise and radiation-pressure effects. We derive these results analytically, compare them with other internal squeezing and amplification schemes, and validate the full quantum-noise spectrum through numerical simulations. We also assess realistic implementation, including the dissipation added through the internal squeezing implementation and transverse mode mismatch, and find that `mode healing' in the signal-extraction cavity can suppress mismatch losses. These results identify bidirectional internal squeezing as a possible upgrade path for LIGO, and the scheme may also benefit future gravitational-wave detectors and other interferometry experiments.