Quantum noise reduction in optical systems and the determination of quasi-distribution functions, in particular Wigner functions, to describe quantum optical states, have been central issues in quantum optics for a number of years. The generation of sources of squeezed light has opened very exciting possibilities of ultrahigh precision measurements beyond the standard quantum limit, by either interferometric or spectroscopic means. In the present issue, the paper by Mundarain and Orszag discusses the interferometric detection of ultrasmall signals, in particular gravitational waves, when one injects a squeezed state in the unused port of the interferometer. The authors find that the sensitivity of the system is very strongly affected by the quantum efficiency of the non-ideal photodetectors, when the squeezed signal is injected, as opposed to the ordinary vacuum case where the effects are rather small. This imposes strong conditions on the quality of those photodetectors. The standard quantum limit and the corresponding minimum detectable gravitational amplitude is also discussed in general, for short time measurements.
Quantum noise reduction in optical systems and the determination of quasi-distribution functions, in particular Wigner functions, to describe quantum optical states, have been central issues in quantum optics for a number of years. The generation of sources of squeezed light has opened very exciting possibilities of ultrahigh precision measurements beyond the standard quantum limit, by either interferometric or spectroscopic means. In the present issue, the paper by Mundarain and Orszag discusses the interferometric detection of ultrasmall signals, in particular gravitational waves, when one injects a squeezed state in the unused port of the interferometer. The authors find that the sensitivity of the system is very strongly affected by the quantum efficiency of the non-ideal photodetectors, when the squeezed signal is injected, as opposed to the ordinary vacuum case where the effects are rather small. This imposes strong conditions on the quality of those photodetectors. The standard quantum limit and the corresponding minimum detectable gravitational amplitude is also discussed in general, for short time measurements.
The first detections of gravitational waves have opened an exciting new field of astronomy. One of the most fundamental limitations for the sensitivity of current and future interferometric gravitational-wave detectors is imposed by the quantum nature of light: Quantum vacuum fluctuations entering the interferometer through the readout port will contribute to the detection noise, at high frequencies in the form of shot noise and at low frequencies by radiation pressure noise. A promising way to reduce this quantum noise is the injection of squeezed states of light that have a lower uncertainty in one quadrature than the vacuum state. The GEO 600 gravitational-wave detector demonstrated the use of squeezed light in 2010 and it is now the first detector to routinely apply squeezing to improve its sensitivity beyond the limits set by classical quantum shot noise. This thesis details the practical aspects of long-term stable and efficient squeezed-light integration in a large-scale gravitational-wave detector. Imperfections that can limit the amount of observable non-classical noise improvement, such as optical losses and phase fluctuations, were studied in detail and methods for their mitigation were developed. Novel control schemes for the active stabilisation of the squeezed light field's phase and alignment were one main focus of the investigations. At the same time, important experience was gathered in the operation of the squeezed light source over long timescales. Over the course of the thesis work, improvements were implemented that significantly increased the performance of the squeezed-light application. Squeezing was injected with an overall duty cycle of 88%, reaching a noise reduction of up to 4.4 dB, corresponding to a 40% lowered shot-noise level. This work has firmly established the practical application of squeezing as a mature technology. The gained knowledge will directly inform the implementation of squeezed light for all future gravitational-wave detectors.
We study the photon counting noise in optical interferometers used for gravitational wave detection. In order to reduce quantum noise a squeezed vacuum state is injected into the usually unused input port. Here, we specifically investigate the so-called “dark port case,” when the beam splitter is oriented close to 90° to the incoming laser beam, such that nearly all photons go to one output port of the interferometer, and only a small fraction of photons is seen in the other port (“dark port”). For this case it had been suggested that signal amplification is possible without concurrent noise amplification [R. Barak and Y. Ben-Aryeh, J. Opt. Soc. Am. B 25 , 361 (2008)]. We show that by injection of a squeezed vacuum state into the second input port, counting noise is reduced for large values of the squeezing factor, however the signal is not amplified. Signal strength only depends on the intensity of the laser beam.
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.
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.
During the recent years an international network of interferometric gravitational wave (GW) detectors has been commissioned and searches for various classes of GWsignals are ongoing.Although the observatories are beginning to reach astrophysically interesting sensitivity levels, extensive upgrades are aimed for the next decade bringing us into the era of gravitational wave astronomy.One approach to enhance the detector sensitivity is the injection of squeezed vacuum states of light replacing the vacuum fluctuations entering the detector through its antisymmetric port.In this thesis the unprecedented generation of squeezed vacuum states with a noise power 6.5 dB below vacuum noise within the entire detection bandwidth of ground-based GW-detectors (10 Hz -10 kHz) was experimentally demonstrated at a wavelength of 1064 nm.For the first time a coherent control scheme was employed for comprehensive phase control of the experimental setup.This scheme is directly applicable to future GW-detector topologies.In addition, parasitic interference induced by scattered light was found to be a dominant noise source degrading squeezed states in the audio band.The compatibility of squeezed field injection into a detuned operated dual-recycled Michelson interferometer was demonstrated in a table top experiment.A detected non-classical sensitivity improvement of up to 3.0 dB within the entire detection bandwidth (5 -15 MHz) was accomplished by optimal preparation of the injected squeezing by means of a detuned locked filter cavity.In a further experiment strongly squeezed vacuum states were generated in the MHz-regime.With a maximum measured quantum noise reduction of 11.5 dB below the shot noise limit a new benchmark was set.This was achieved by suppression of phase noise and a remarkable reduction of optical loss in the experimental setup.Based on the squeezed light sources developed in this thesis and the experimental results achieved, a conceptual design of a compact squeezed light source was devised, allowing for a high duty cycle sensitivity improvement of GW-detectors.
Current gravitational wave detectors, such as LIGO, Virgo and KAGRA, are laser interferometers. This technique consists of injecting a laser beam through an input port, which is then separated into two arms several kilometers long by a semi-reflecting mirror. At the end of each arm, a mirror reflects the beam back to the semi-reflecting mirror. The two beams then interfere and the power of the resulting beam, measured at the detection port, contains information about the relative length of the two arms. When a gravitational wave passes through the detector, this relative length varies in time. Several noise sources degrade the quality of the recorded signal, which affects the number of gravitational waves that can be observed. Currently, this sensitivity is limited at high-frequency by the quantum noise.An electromagnetic wave described by quantum mechanics has a classical amplitude and quantum fluctuations in amplitude and phase. A state with a null classical amplitude is called a vacuum state, but still has these quantum fluctuations. Thus, in the detector, although no light beam is introduced through the detection port, the vacuum state is present. It is then the quantum fluctuations of this vacuum state that are responsible for the quantum noise of the interferometer. In particular, the phase and amplitude fluctuations are respectively responsible for the quantum noise at high and low frequencies; the separation between these two regions occurs at about 100 Hz.The work presented in this thesis focuses on the injection of squeezed vacuum states into the Virgo detector via the detection port in order to lower the quantum noise. These states are characterized by the fact that their phase (amplitude) fluctuations are reduced, which allows reducing the quantum noise at high (low) frequency. As the Virgo detector is currently limited only by high-frequency quantum noise, only phase squeezed vacuum states were used during the last observation period O3. It was first necessary to ensure that these squeezed vacuum states had the proper geometric and phase parameters with respect to the interferometer beam such that they were not degraded. Control loops allowed these parameters to be maintained at their nominal values during O3, a year long period. The decrease in high-frequency quantum noise, but also the increase in low-frequency quantum noise, have been measured for the first time with the Virgo detector. These observations were in agreement with the measurements of the losses that affect the squeezed vacuum states.From O4 , quantum noise will also be limiting at low frequency. It will then be possible to use squeezed vacuum states having their amplitude fluctuations reduced at low frequency and their phase fluctuations reduced at high frequency. Thus, the total quantum noise can be reduced. Such states can be generated using an optical cavity called a filter cavity. The second part of the work presented in this thesis consisted in defining the optical parameters of the filter cavity that will be installed on Virgo for O4. To do this, it was first necessary to study the sources of degradation of the squeezed states of the vacuum. This then allowed defining the length of the filtering cavity and the set of parameters of the mirrors that compose it. Finally, the implementation of this filter cavity within the Virgo infrastructure was studied. This has shown that the use of compressed states will effectively reduce quantum noise over the entire sensitivity spectrum of Virgo.