Quantum limited optical phase detection in a high-power suspended interferometer

Peter K Fritschel, Gabriela I. González, Brian T. Lantz +2 more

Interferometric detection of gravitational waves at levels of astrophysical interest is expected to require measurement of optical phase differences of $\ensuremath{\le}{10}^{\ensuremath{-}10}$ rad. To achieve this sensitivity the measurement must be made at the quantum (shot noise) limit. We have built a laboratory-scale interferometer to investigate limits to sensing the phase at this level, and obtained, with 70 W of circulating power, a phase sensitivity limited by shot noise of $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\phantom{\rule{0ex}{0ex}}\mathrm{rad}/\sqrt{\mathrm{Hz}}$ at frequencies above 1.5 kHz. We describe the experiment and discuss the implications for the Laser Interferometer Gravitational-Wave Observatory (LIGO).

Rafał Demkowicz-Dobrzański, Marcin Jarzyna, Jan Kołodyński

Wajid Ali

General relativity predicts the existence of Gravitational Waves (GWs), ripples in spacetime that are now observable to highly sensitive interferometers. However, Quantum Noise (QN) poses a significant challenge for Gravitational Wave (GW) interferometers, influencing the entire bandwidth range (10-10,000 Hz) of the current GW detectors. At higher frequencies, Shot Noise (SN) predominates, while at lower frequencies, Radiation Pressure Noise (RPN) becomes the limiting factor, reducing both sensitivity and measurement precision. Squeezing techniques are implemented to address this issue by confining uncertainty within one quadrature of the light field. During O4, LIGO successfully implemented Frequency-Dependent Squeezing (FDS), while Virgo integrated and commissioned the FDS system without actively operating it during the science run. By employing a 300-meterlong detuned Filter Cavity (FC), the squeezing ellipse is rotated, allowing phase squeezing at high frequencies to reduce and amplitude squeezing at low frequencies to mitigate RPN, ultimately refining detection capabilities across the frequency spectrum. This doctoral thesis focuses on a promising alternative technique for future GW detectors, that achieves broadband QN reduction by exploiting Einstein–Podolsky–Rosen (EPR) quantum entanglement. This approach involves injecting two entangled, squeezed beams at different frequencies into the dark port of the interferometer, enabling it to act as both a FC and a GW detector. To experimentally investigate these techniques, the suspended Interferometer for Ponderomotive Squeezing (SIPS) platform adopts a large-scale GW detector layout featuring a Michelson configuration with Fabry-Perot (FP) arm cavities. To guarantee the extreme optical alignment necessary for quantum noise mitigation, a high-precision, real-time local control system was developed for the SIPS main optics. Using a dummy mirror suspended in a double-pendulum configuration with monolithic fibers, a custom real-time signal processing and feedback control architecture was developed. This subsystem utilizes a PXI hardware framework and position-sensitive detectors (Power Spectral Densitys (PSDs)) implemented in LabVIEW to actively damp the mechanical resonances of the suspensions. Extending these core activities toward future infrastructures, this work directly contributed to the realization and commissioning of the new PNRR ETIC-GALILEO quantum optics cleanroom facility in Genoa for the Einstein Telescope (ET). Within this infrastructure, preliminary optical studies were initiated for Einstein Telescope Low Frequency (ET-LF) low-loss Faraday isolators operating at 1550 nm. Concurrently, numerical simulations were performed using Finesse3 to model and investigate candidate excess noise sources currently limiting the sensitivity of Advanced Virgo Plus

Juan F. Hodelin

F de Marco, S. Di Pace, V. Sequino +1 more

A mode cleaner optical cavity is a resonator that can be tuned to transmit a desired transverse mode present in the incoming beam, suppressing all the others. We present the optical design of a suited input mode cleaner for a small-scale Michelson interferometer, called SIPS (Suspended Interferometer for Ponderomotive Squeezing). The physical goal is to feed SIPS with a beam with a Gaussian profile, which also ensures a highly focused beam along the propagation axis. The SIPS mode cleaner has been designed to have a maximum circulating power of 130W, and to deliver a transmitted power of at least 2.5W in input of SIPS interferometer. It guarantees a suppression factor of at least 20 dB for every tested higher order mode. This work is framed within the implementation of a table-top optical experiment to test the broadband Quantum Noise reduction in the SIPS sensitivity curve, via frequency-dependent squeezing achieved by employing Einstein-Podolsky-Rosen quantum entangled states of light.

Hans‐A. Bachor