Lee P. McCuller, Chris Whittle, D. Ganapathy, Kentaro Komori, M. Tse, Álvaro Fernández-Galiana, L. Barsotti, Peter K Fritschel, Myron E MacInnis, Fabrice Matichard, K. Mason, N. Mavalvala, Richard K Mittleman, Haocun Yu, Michael E. Zucker, M. Evans
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+."
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+."
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.
The astrophysical reach of current and future ground-based gravitational-wave detectors is mostly limited by quantum noise, induced by vacuum fluctuations entering the detector output port. The replacement of this ordinary vacuum field with a squeezed vacuum field has proven to be an effective strategy to mitigate such quantum noise and it is currently used in advanced detectors. However, current squeezing cannot improve the noise across the whole spectrum because of the Heisenberg uncertainty principle: when shot noise at high frequencies is reduced, radiation pressure at low frequencies is increased. A broadband quantum noise reduction is possible by using a more complex squeezing source, obtained by reflecting the squeezed vacuum off a Fabry-Perot cavity, known as filter cavity. Here we report the first demonstration of a frequency-dependent squeezed vacuum source able to reduce quantum noise of advanced gravitational-wave detectors in their whole observation bandwidth. The experiment uses a suspended 300-m-long filter cavity, similar to the one planned for KAGRA, Advanced Virgo, and Advanced LIGO, and capable of inducing a rotation of the squeezing ellipse below 100 Hz.
The era of gravitational-wave astronomy has begun, with the detection of 5 confirmed binary black holes and a binary neutron star coalescence by the Advanced Laser Interferometer Gravitational- wave Observatory (aLIGO), and later with the advanced Virgo detector. These detections have already revealed a wealth of discoveries across the fields of nuclear physics, general relativity and astrophysics. The work presented in this thesis is part of the ongoing effort to improve the sensitivity of ground-based interferometric gravitational-wave detectors. The sensitivity of aLIGO, and other interferometric detectors, is broadly limited by quantum noise. Improving on the quantum noise will increase the astrophysical range of the detectors, and improve the source parameter estimation. One way to reduce quantum noise is to inject audio- band squeezed vacuum states into the detection port. This technique has been demonstrated on the initial LIGO and GEO600 detectors. A squeezed light source for aLIGO must meet stringent requirements in terms of optical loss, phase noise, and scattered light. The squeezer must produce high levels of audio-band squeezing and operate under vacuum, to take advantage of the excellent existing isolation systems and to minimise optical loss. At design sensitivity, squeezed states whose quantum noise depends on frequency will be required. We have demonstrated an ultra-stable glass-based squeezed light source, the first experiment of this kind to operate under vacuum. The squeezer cavity is constructed quasi-monolithically, with optics and nonlinear crystal oven optically contacted to a breadboard base. The cavity is designed to have extremely low length noise, and to produce high levels of audio-band squeezing. We have measured 8.6 ± 0.9 dB of squeezing and infer the generation of 14.2 ± 1.0 dB after accounting for all known losses. The squeezer has demonstrated record phase noise performance of 1.3 mradRMS, dominated by sources other than cavity length noise. This exceeds the phase-noise requirement for a squeezer for aLIGO. A copy of this squeezer is currently being installed in a squeezing- ellipse rotation experiment to demonstrate frequency-dependent squeezing for aLIGO. Lessons learnt during the construction and operation of the in-vacuum squeezer have helped inform the design of a frequency-independent squeezed light source currently being installed at the LIGO sites. Future gravitational-wave detectors will continue to use interferometric techniques, and will be limited by quantum noise for the foreseeable future. To improve on thermal noise limits and interferometer power handling, future detectors look to cryogenic silicon as a test mass material. To take advantage of the desirable properties of silicon, including low scatter and absorption, a longer operating wavelength is required. The proposed LIGO Voyager upgrade has an operating wavelength in the 2 μ m region, with the specific wavelength to be determined. LIGO Voyager will require a squeezed light source in the 2 μ m region to reach its design sensitivity. We present the design, characterisation, and results of the first squeezed light source in the 2 μ m region. Laser and detector technologies at 2 μ m are less developed than their 1064 nm counterparts, causing significant technical challenges. We have measured 4.0 ± 0.2 dB of squeez- ing at 1984 nm, limited by loss due to detector quantum efficiency. Accounting for known losses in the system, we infer the generation of 10 dB of squeezing. This is an important demonstra- tion of quantum noise reduction for future detectors, and a pathfinder technology for the design choices of LIGO Voyager. So far we have found no reason why a 2 μ m interferometer should not be feasible.
The detection of the binary neutron star inspiral GW170817 by the Advanced Laser Interferometer Gravitational Wave Observatory (LIGO) and Advanced Virgo, followed by counterpart observations across a broad electromagnetic spectrum, marks the dawn of multi-messenger astronomy. The discovery rate and potential of gravitational-wave astronomy is limited by the detector's sensitivity. A key challenge in improving the sensitivity of gravitational-wave detectors is to reduce quantum noise which arises due to the quantum nature of light. A well established technique to reduce the quantum noise in the detector is by replacing the coherent vacuum fluctuations entering the readout port with a squeezed vacuum state. This technique has been successfully demonstrated at the GEO-600 and LIGO Hanford detector, and is now in routine operation in current gravitational wave detectors. This thesis presents research from three experiments to develop quantum noise reduction techniques for next-generation gravitational wave detector using non-classical quantum states such as squeezed states. Current proposals for future generation gravitational wave detectors, such as LIGO Voyager, plan to utilise cryogenic silicon instead of fused silica as the test mass material. This is to improve the power handling of the detector and reduce thermal noise. The switch to silicon test masses requires a change of laser wavelength to the 2 micron region. In the first experiment, we built and operated the first squeezed vacuum source at a wavelength of 1984 nm. With a stable squeezing ellipse phase control employed, 3.9 dB of squeezing was measured, predominately limited by losses due to the low quantum efficiency of the photodiode. The squeezed vacuum system presented is compatible with future detectors and is a key pathfinder experiment to inform the design of cryogenic silicon interferometers. The quantum nature of light results in quantum radiation pressure noise that limits the low frequency sensitivity of gravitational wave detectors. In collaboration with Louisiana State University, the second experiment demonstrates the reduction of quantum radiation pressure noise by injecting bright squeezed light into a centimetre-scale optomechanical cavity at room temperature. The experiment involves the development and integration of a low-frequency bright squeezed source to reduce the quantum radiation pressure noise by 1.2 dB at 20 kHz. The result also represents the lowest frequency bright squeezed state measured from a crystal-based squeezer. Quadrature rotation of the injected squeezed state is required to achieve broadband quantum noise reduction in gravitational wave detectors due to the quantum back-action effect. This requires the squeezed state to interact with a low loss optical filter with a low frequency bandwidth prior to injection and represents a significant technical challenge. The final experiment presented in this thesis demonstrates an alternate method where frequency-dependent squeezing with Einstein-Podolsky-Rosen (EPR) entangled states is generated. When integrated into a gravitational wave detector, the alternate method allows the detector to simultaneously serve as the optical filter, eliminating the need for the additional apparatus.
Quantum noise imposes a fundamental limitation on the sensitivity of interferometric gravitationalwave detectors like LIGO, manifesting as shot noise and quantum radiation pressure noise.Here, we present the first realization of frequency-dependent squeezing in full-scale gravitational-wave detectors, resulting in the reduction of both shot noise and quantum radiation pressure noise, with broadband detector enhancement from tens of hertz to several kilohertz.In the LIGO Hanford detector, squeezing reduced the detector noise amplitude by a factor of 1.6 (4.0 dB) near 1 kHz; in the Livingston detector, the noise reduction was a factor of 1.9 (5.8 dB).These improvements directly impact LIGO's scientific output for high-frequency sources (e.g., binary neutron star postmerger physics).The improved lowfrequency sensitivity, which boosted the detector range by 15%-18% with respect to no squeezing, corresponds to an increase in the astrophysical detection rate of up to 65%.Frequency-dependent squeezing was enabled by the addition of a 300-meter-long filter cavity to each detector as part of the LIGO Aþ upgrade.
In this Letter, we present the design and performance of the frequency-dependent squeezed vacuum source that will be used for the broadband quantum noise reduction of the Advanced Virgo Plus gravitational-wave detector in the upcoming observation run. The frequency-dependent squeezed field is generated by a phase rotation of a frequency-independent squeezed state through a 285 m long, high-finesse, near-detuned optical resonator. With about 8.5 dB of generated squeezing, up to 5.6 dB of quantum noise suppression has been measured at high frequency while close to the filter cavity resonance frequency, the intracavity losses limit this value to about 2 dB. Frequency-dependent squeezing is produced with a rotation frequency stability of about 6 Hz rms, which is maintained over the long term. The achieved results fulfill the frequency dependent squeezed vacuum source requirements for Advanced Virgo Plus. With the current squeezing source, considering also the estimated squeezing degradation induced by the interferometer, we expect a reduction of the quantum shot noise and radiation pressure noise of up to 4.5 dB and 2 dB, respectively.