Speaker
Description
Gravitational-wave detectors (GWDs) are fundamentally limited by quantum noise at kilohertz frequencies, reducing their sensitivity to binary neutron star mergers at the 0.9–5 kHz band. While externally injected squeezed vacuum is routinely used to suppress quantum noise, its bandwidth is fundamentally constrained. To overcome this limitation, a promising approach is to combine a lengthened signal recycling cavity (SRC) with internal squeezing, which generates and manipulates squeezed vacuum within the SRC. This enables broadband quantum noise reduction at kHz and sub-kHz frequencies. To support the experimental realisation of this concept, we developed and optimised a coupled-cavity squeezing system designed to demonstrate the underlying optical configuration.
The main experimental system consists of a test cavity, a squeezing cavity, and a balanced homodyne detection (HD) system. The test cavity emulates the twin Michelson arm cavities of a gravitational-wave detector, while the squeezing cavity emulates the SRC and generates a 1064-nm squeezed vacuum state through parametric down-conversion in a $\chi^{(2)}$ nonlinear crystal pumped by a 532 nm field. The squeezed vacuum is manipulated by the coupling between the two cavities, before interfering with a local oscillator beam and being characterised by balanced homodyne detection, enabling measurement of the squeezed quantum state.
A key contribution of this work is the implementation of independent cavity locking schemes that minimise optical loss while maintaining stable operation. Previous proof-of-principle demonstrations [1] have achieved 3.3 dB of measured squeezing, limited primarily by 36.8% total optical loss. In this work, the optical system has been redesigned to reduce total loss to approximately 16.8%. Simulations predict that this improvement will enable approximately 5 dB of observable squeezing, representing a significant step towards experimentally validating broadband quantum noise reduction for next-generation gravitational-wave detectors.
[1] Junker et. Al., Phys. Rev. Lett. 134, 243603 (2025)
| I am the presenting author | Yes |
|---|