We report on the demonstration of broadband squeezed laser beams that show a frequency-dependent orientation of the squeezing ellipse. Carrier frequency as well as quadrature angle were stably locked to a reference laser beam at $1064\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. This frequency-dependent squeezing was characterized in terms of noise power spectra and contour plots of Wigner functions. The latter were measured by quantum state tomography. Our tomograph allowed a stable lock to a local oscillator beam for arbitrary quadrature angles with $\ifmmode\pm\else\textpm\fi{}1\ifmmode^\circ\else\textdegree\fi{}$ precision. Frequency-dependent orientations of the squeezing ellipse are necessary for squeezed states of light to provide a broadband sensitivity improvement in third-generation gravitational-wave interferometers. We consider the application of our system to long-baseline interferometers such as a future squeezed-light upgraded GEO 600 detector.
We report on the demonstration of broadband squeezed laser beams that show a frequency-dependent orientation of the squeezing ellipse. Carrier frequency as well as quadrature angle were stably locked to a reference laser beam at 1064nm. This frequency-dependent squeezing was characterized in terms of noise power spectra and contour plots of Wigner functions. The latter were measured by quantum state tomography. Our tomograph allowed a stable lock to a local oscillator beam for arbitrary quadrature angles with \ifmmode±\else\textpm\fi1\ifmmode∘\else\textdegree\fi precision. Frequency-dependent orientations of the squeezing ellipse are necessary for squeezed states of light to provide a broadband sensitivity improvement in third-generation gravitational-wave interferometers. We consider the application of our system to long-baseline interferometers such as a future squeezed-light upgraded GEO 600 detector.
The first detection of gravitational waves by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015 launched the era of gravitational-wave astronomy. The quest for gravitational-wave signals from objects that are fainter or farther away impels technological advances to realize ever more sensitive detectors. Since 2019, one advanced technique, the injection of squeezed states of light, is being used to improve the shot-noise limit to the sensitivity of the Advanced LIGO detectors, at frequencies above ∼50 Hz. Below this frequency, quantum backaction, in the form of radiation pressure induced motion of the mirrors, degrades the sensitivity. To simultaneously reduce shot noise at high frequencies and quantum radiation pressure noise at low frequencies requires a quantum noise filter cavity with low optical losses to rotate the squeezed quadrature as a function of frequency. We report on the observation of frequency-dependent squeezed quadrature rotation with rotation frequency of 30 Hz, using a 16-m-long filter cavity. A novel control scheme is developed for this frequency-dependent squeezed vacuum source, and the results presented here demonstrate that a low-loss filter cavity can achieve the squeezed quadrature rotation necessary for the next planned upgrade to Advanced LIGO, known as "A+."
Quantum noise is one of the limiting factors in laser-interferometric gravitational-wave (GW) detectors. The application of squeezed states in these interferometers allows the reduction of quantum noise in one quadrature. Due to opto-mechanical coupling in a GW detector the squeezed quadrature needs to be rotated within the spectrum to achieve a broadband noise reduction. So far, the implementation of additional filter cavities is considered that allow for the optimal, frequency-dependent rotation of the squeezed quadrature. However, these cavities need to have low loss, a length in the order of \unit[100]{m} and must be situated in the vacuum system, making them cost-intensive. In 2017, Ma and coworkers proposed a scheme for the broadband quantum-noise reduction without the need of additional filter cavities. It was shown by Brown et al. that a similar scheme can be used to broadband-enhance interferometers with a detuned signal-recycling cavity. Here, we performed a proof-of-principle experiment of the proposal on a table-top-scale. Squeezed states were produced detuned to the carrier field of a \unit[2.5]{m}-linear cavity and read out in a bichromatic homodyne detection. The frequencies of the lower and upper local oscillator were at entangled sidebands of the squeezed field. Depending on the relations between the involved frequencies, we can address both variants of the proposal. We show, that the frequency-dependences of the resulting noise spectra fit to a theoretical model we derived from the theory used by Ma et al. With this work we set the path towards an implementation of these schemes in a GW-detector prototype, where the compatibility of the approach with a low-frequency suspended interferometer can be tested. Moreover, we used the same setup to show nonclassical interferometer enhancement at low frequencies by high-frequency squeezed states. Here, a heterodyne readout scheme was implemented to avoid limiting noises at low frequencies. The application of squeezed states centered around the local oscillator frequency yielded an improvement in signal-to-noise ratio of $\unit[3.4]{dB}\pm\unit[0.3]{dB}$. Additionally, I designed, built and characterized a compact source of squeezed vacuum-states at \unit[1064]{nm} with a footprint of just $\unit[0.8]{m^2}$. I show measurements of squeezed states from this source with a reduction of quantum noise of $\unit[10.7]{dB}\pm\unit[0.2]{dB}$ below the vacuum noise and present a noise reduction in the frequency range from \unit[70]{kHz} to \unit[65]{MHz}.
Abstract Ground-based interferometric gravitational wave detectors are highly precise sensors for weak forces, limited in sensitivity across their detection band by quantum fluctuations of light. Current and future instruments address this limitation by injecting frequency-dependent squeezed vacuum into the detection port, utilizing narrow-band, low-loss optical cavities for optimal rotation of the squeezing ellipse at each signal frequency. This study introduces a novel scheme employing the principles of quantum teleportation and entangled states of light. It allows achieving broadband suppression of quantum noise in detuned signal recycled-Fabry-Perot--Michelson interferometers, which is the baseline design of the low-frequency detector within the Einstein Telescope xylophone detector, without requiring additional filter cavities or modifications to the core optics of the main interferometer.
Quantum vacuum fluctuations impose strict limits on precision displacement measurements, those of interferometric gravitational-wave detectors among them. Introducing squeezed states into an interferometer's readout port can improve the sensitivity of the instrument, leading to richer astrophysical observations. However, optomechanical interactions dictate that the vacuum's squeezed quadrature must rotate by 90° around 50 Hz. Here we use a 2-m-long, high-finesse optical resonator to produce frequency-dependent rotation around 1.2 kHz. This demonstration of audio-band frequency-dependent squeezing uses technology and methods that are scalable to the required rotation frequency and validates previously developed theoretical models, heralding application of the technique in future gravitational-wave detectors.
Abstract Quantum noise is limiting the sensitivity of ground based gravitational wave detectors both at high frequency, in the form of shot noise, and low frequency, in the form of radiation pressure noise. In the last observing run, the injection of frequency independent squeezing improved Virgo and LIGO sensitivities at high frequency, slightly worsening the performance at low frequency. A broadband quantum noise reduction can be achieved using frequency dependent squeezing, i. e. rotating the vacuum squeezed ellipse below 100 Hz by reflecting the squeezed vacuum off a Fabry–Perot cavity, called filter cavity. The frequency-dependent squeezed quadrature rotation with rotation frequency around a few tens of Hz has been proven at National Astronomical Observatory of Japan (NAOJ) and at Massachusetts Institute of Technology (MIT). The experiment at NAOJ uses the former TAMA facility, a 300 meter long filter cavity, reproducing in scale the ones planned to be installed in Virgo and LIGO. Once the frequency dependent squeezing is produced, it has to be injected into the interferometer. This interface is not trivial, since it requires the installation of additional benches and a 285 meter long cavity (in Advanced Virgo Plus) and also to couple the rotating squeezed vacuum with the detector. An important issue which can worsen the performance of frequency dependent squeezing or directly the interferometer sensitivity is the stray light. To avoid the propagation of additional stray light, we traced the ghost beam on squeezing benches, inside linking tubes and inside the filter cavity and several diaphragms and baffles will be installed to limit this problem.
Albert Einstein's General Theory of Relativity predicts the existence of ripples in space- time, the so-called gravitational waves (GWs). In the past two decades, an international array of ground-based, kilometer-scale Michelson interferometers has been set up aiming at a rst observation. These Earth-bound GW detectors target signals at audio frequen- cies in a band of ca. 10 Hz { 10 kHz. At Fourier frequencies above 1 kHz, their sensitivity is limited by quantum noise. This shot-noise arises from the zero-point uctuations of the electromagnetic eld, the so-called vacuum uctuations , which enter the interferometer through its anti-symmetric signal port. Future generations of GW observatories will even be limited by quantum noise almost over their entire detection band [ ET ]. A `classical' solution to further improve the signal-to-shot-noise ratio (SNR) of the observatories consists in an increase of the circulating light power. A higher optical load inside the interferometer will, however, lead to thermally-induced beam distortion as well as to an increased radiation pressure noise level. This will eectively result in an upper limit for the optical light power employable in future GW observatory generations. An alternative approach to improve the SNR is based on the injection of so-called squeezed states of light. Such a light eld has a characteristic non-classical noise distribution in the eld quadratures. Injected from the signal port, the squeezed state replaces the vacuum state, thereby reducing the interferometer's quantum noise. The rst part of this thesis discusses the setup of a squeezed-light laser for the German- British GW observatory GEO 600. During characterization of the device, a noise reduc- tion of up to 9.6 dB below the vacuum noise level was observed in the frequency band of 10 Hz {10 kHz, setting a new benchmark for squeezing observed at audio frequencies. Corrected for detection losses, this value furthermore corresponds to more than 11 dB of squeezing available for injection into the dark port of GEO 600. Situated on a 1 : 15 m 1 : 35 m optical breadboard, the device features transportability and can be operated independently of the GW interferometer. A key feature of the squeezed-light laser is its unprecedented long-term stability. The longest characterization measurements lasted for 20 h with an overall duty cycle of more than 99 %. In the second part of this thesis, the implementation of squeezed light into GEO 600 is presented along with the results achieved. The observatory shot noise was reduced by up to 3.4 dB, corresponding to an improvement in sensitivity by a factor of up to 1.5. For isotropically distributed GW sources, this corresponds to an increase in detection probability by 1.5 3 3.4. This makes GEO 600 the rst large-scale GW interferometer with a non-classically enhanced sensitivity. Based on the performance of the squeezed- light laser, a sensitivity improvement by a factor of two, corresponding to an equivalent optical power increase by a factor of four, seems feasible in the near future.