Mirror thermal noise will be a main limitation for the sensitivities of the next-generation ground-based gravitational-wave detectors (Einstein Telescope and Cosmic Explorer) at signal frequencies around 100 Hz. Using a higher-order spatial laser mode instead of the fundamental mode is one proposed method to further mitigate mirror thermal noise. In the current detectors, quantum noise is successfully reduced by the injection of squeezed vacuum states. The operation in a higher-order mode would then require the efficient generation of squeezed vacuum states in this mode to maintain a high quantum noise reduction. In our setup, we generate continuous-wave squeezed states at a wavelength of 1064 nm in the fundamental and three higher-order Hermite-Gaussian modes up to a mode order of 6 using a type-I optical parametric amplifier. We present a significant milestone with a quantum noise reduction of up to 10 dB at a measurement frequency of 4 MHz in the higher-order modes and pave the way for their usage in future gravitational-wave detectors as well as in other quantum noise limited experiments.
Mirror thermal noise will be a main limitation for the sensitivities of the next-generation ground-based gravitational-wave detectors (Einstein Telescope and Cosmic Explorer) at signal frequencies around 100 Hz. Using a higher-order spatial laser mode instead of the fundamental mode is one proposed method to further mitigate mirror thermal noise. In the current detectors, quantum noise is successfully reduced by the injection of squeezed vacuum states. The operation in a higher-order mode would then require the efficient generation of squeezed vacuum states in this mode to maintain a high quantum noise reduction. In our setup, we generate continuous-wave squeezed states at a wavelength of 1064 nm in the fundamental and three higher-order Hermite-Gaussian modes up to a mode order of 6 using a type-I optical parametric amplifier. We present a significant milestone with a quantum noise reduction of up to 10 dB at a measurement frequency of 4 MHz in the higher-order modes and pave the way for their usage in future gravitational-wave detectors as well as in other quantum noise limited experiments.
The engineering of strongly squeezed vacuum states of light is a key technology for the reduction of quantum noise in gravitational wave detectors. We report on the observation of up to 12.0 dB squeezed vacuum states of light at the wavelength of 1064 nm in the frequency band from 10 Hz to 100 kHz. This is the strongest squeezing reported to date within this detection band. The squeezed states were generated in a half-monolithic, standing-wave cavity optical parametric amplifier, which was resonant for the fundamental and harmonic light fields. We chose appropriate reflectivities to obtain a significant reduction of the required pump power, which was 8.6 mW only. Our analysis revealed that the residual measurement phase noise was smaller than 3.5 mrad rms and that the squeezed light source provided up to 14 dB of squeezing for a downstream application. The experiment was electronically stabilized in all relevant degrees of freedom, demonstrating the applicability of the linear, doubly resonant cavity topology for current and future gravitational wave detectors.
Future generations of gravitational-wave detectors (GWD) are targeting an effective quantum noise reduction of 10 dB via the application of squeezed states of light. In the last joint observation run O3, the advanced large-scale GWDs LIGO and Virgo already used the squeezing technology, albeit with a moderate efficiency. Here, we report on the first successful 10 dB sensitivity enhancement of a shot-noise limited tabletop Michelson interferometer via squeezed light in the fundamental Gaussian laser mode, where we also implement the balanced homodyne detection scheme that is planned for the third GWD generation. In addition, we achieved a similarly strong quantum noise reduction when the interferometer was operated in higher-order Hermite-Gaussian modes, which are discussed for the GWD thermal noise mitigation. Our results are an important step toward the targeted quantum noise level in future GWDs and, moreover, represent significant progress in the application of nonclassical states in higher-order modes for interferometry, increased spatial resolution, and multichannel sensing.
Squeezed vacuum states of light show a reduction in quantum noise along one quadrature compared to the vacuum state. This characteristic is the key to improve interferometric measurements beyond their quantum noise limitation. In particular for ground-based gravitational wave detectors [1] the application of squeezed light led to a significant sensitivity improvement of up to 6 dB. Future gravitational wave detectors, like the Einstein Telescope [2] or Cosmic Explorer [3], aim for an even higher enhancement in the range of 10 dB with the injection of squeezed states of light into the dark port of their interferometers.
The generation of strongly squeezed vacuum states of light is a key technology for future ground-based gravitational wave detectors (GWDs) to reach sensitivities beyond their quantum noise limit. For some proposed observatory designs, an operating laser wavelength of 1550 nm or around 2 μm is required to enable the use of cryogenically cooled silicon test masses for thermal noise reduction. Here, we present for the first time the direct measurement of up to 11.5 dB squeezing at 1550 nm over the complete detection bandwidth of future ground-based GWDs ranging from 10 kHz down to below 1 Hz. Furthermore, we directly observe a quantum shot-noise reduction of up to (13.5±0.1) dB at megahertz frequencies. This allows us to derive a precise constraint on the absolute quantum efficiency of the photodiode used for balanced homodyne detection. These results hold important insight regarding the quantum noise reduction efficiency in future GWDs, as well as for quantum information and cryptography, where low decoherence of nonclassical states of light is also of high relevance.
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.
The ability to directly detect gravitational waves will open a completely new branch of astronomy to view the Universe, one that is inaccessible to electromagnetic-based astronomy. First generation ground-based interferometric gravitational-wave detectors have achieved strain sensitivities of order one part in ten to the twenty-one, at 100 Hz detection frequency. A new generation of detectors are under construction, designed to improve on the sensitivity of the first-generation detectors by a factor of 10. The quantum nature of light will broadly limit the sensitivity of these new instruments. This quantum noise will originate from the quantum vacuum fluctuations that enter the unused port of the interferometer. One of the most promising options for reducing the quantum noise impact and further increasing the sensitivity is applying quantum squeezed vacuum states. These squeezed states have lower noise in one quadrature than the vacuum state. By replacing the quantum vacuum fluctuations entering the interferometer with squeezed vacuum states, the quantum noise impact is reduced. This thesis firstly details the development of a squeezed light source that produces squeezed states applicable for enhancing interferometric gravitational-wave detectors. A doubly-resonant, travelling-wave bow-tie cavity squeezed light source is presented. For the first time, greater than 10 dB of quantum noise suppression across the gravitational-wave detection band is directly observed, and 11.6 dB of quantum noise suppression is observed above 200 Hz. This squeezing cavity design also has benefits with intrinsic isolation to backscattered light. Experiments that quantify this isolation are reported. The properties affecting squeezing magnitude and low-frequency squeezing measurement are discussed. In addition, a modified squeezing-ellipse-phase control technique for squeezed vacuum states is presented. This thesis secondly presents results from the LIGO Squeezed Light Injection Experiment, undertaken to test squeezed light injection into a 4 km interferometric gravitational-wave detector. The results of the experiment show the first measurement of squeezing-enhancement in a 4 km gravitational-wave detector, with 2.15 dB measured above 250 Hz. This represents the best sensitivity to gravitational waves yet achieved at these frequencies by any single gravitational-wave detector to date. An unknown area was whether the addition of a squeezer would introduce noise couplings that degrade the crucial low frequency sensitivity. The results demonstrate that injected squeezed states are compatible with low frequency gravitational-wave measurement. The characterisation of squeezing-injection optical losses and fluctuations of the squeezing angle are also reported. The knowledge and processes gained, from both the squeezed light source development work and the LIGO Squeezed Light Injection Experiment, will inform the design, planning and implementation of squeezed states in future gravitational-wave detectors.