We discuss the development and expected performance of a table-top experiment for measuring quantum fluctuations in a suspended-test-mass interferometer. The ultimate goal of this experiment is to perform measurements of the quantum noise-limited displacement sensitivity of the interferometer at the level of the standard quantum limit (5×10−19 m/sqrt(Hz) at 100 Hz). The case is made for an experiment that will show the feasibility of preparing a macroscopic quantum-limited system within a regular laboratory environment. This will be achieved by suspending a cryogenically cooled, high-finesse, optical cavity via a multi-stage suspension. We motivate the utility of our experiment in providing an increased understanding of the nature of quantum fluctuations in current and future gravitational wave detectors, as well as opening an avenue for research into aspects of macroscopic quantum mechanics and quantum gravity. We explain how these requirements drove our design specification. Given the extremely small displacements in question, particular attention is devoted to discussing noise mitigation, with a focus on the suppression of seismic and thermal noise.
We discuss the development and expected performance of a table-top experiment for measuring quantum fluctuations in a suspended-test-mass interferometer. The ultimate goal of this experiment is to perform measurements of the quantum noise-limited displacement sensitivity of the interferometer at the level of the standard quantum limit (5×10−19 m/sqrt(Hz) at 100 Hz). The case is made for an experiment that will show the feasibility of preparing a macroscopic quantum-limited system within a regular laboratory environment. This will be achieved by suspending a cryogenically cooled, high-finesse, optical cavity via a multi-stage suspension. We motivate the utility of our experiment in providing an increased understanding of the nature of quantum fluctuations in current and future gravitational wave detectors, as well as opening an avenue for research into aspects of macroscopic quantum mechanics and quantum gravity. We explain how these requirements drove our design specification. Given the extremely small displacements in question, particular attention is devoted to discussing noise mitigation, with a focus on the suppression of seismic and thermal noise.
We present our development of a table-top quantum sensor with femtometer sensitivity within the acoustic frequency band. Our sensor utilises the expertise developed within the gravitational-wave community, particularly in the control of high-finesse, suspended interferometers, and the mitigation of seismic and thermal noise. We construct a system consisting of a cryostat-mounted four-stage suspension supporting two cavities with optical finesse in excess of 350,000. We use active isolation of the entire cryostat system to achieve an order-of-magnitude of reduction of the seismic noise prior to the passive isolation of the internal suspension. We achieve the femtometer sensitivity using a custom-made phase lock loop. With further enhancements to the readout electronics and suspension characteristics, we aim to reach the quantum radiation pressure noise. This sensor holds the potential for future applications in demonstrating macroscopic entanglement and testing semi-classical and quantum gravity theories.
Interferometric sensors provide an excellent opportunity for studying novel aspects of quantum mechanics. In this paper, we present the updated design for a suspended, cryogenically cooled table-top interferometer that can operate at the standard quantum limit of sensitivity. In this mode of operation, we will be able to probe aspects of macroscopic entanglement, quantum correlations, and semi-classical and quantum gravity models. We present the up-to-date experimental progress as well as the results of ongoing investigations.
Gravitational waves manifest as a time varying straining of space: they arise from the accelerating motions of large bodies of mass and propagate across the universe at the speed of light as ripples in the fabric of space-time, a fleetingly weak effect so far eluding direct detection. The detection of gravitation waves is expected to yield a rich vein to observational astronomy, complementing existing electromagnetic surveys and revealing a hitherto unexplored range of phenomena. First generation interferometric gravitational wave detectors, notably Enhanced LIGO, achieved strain sensitivities of one part in ten to power twenty-one per square-root-Hertz at 100 Hz with an expected detection rate of 2-3 events per year. Commissioning of a new generation of Advanced LIGO interferometric detectors has concluded recently with a resultant ten-fold sensitivity improvement. Overall their potential event detection space has increased by a factor of 1000. The quantum nature of light within these detectors now limits their sensitivity over most of their frequency range. This quantum noise limit is driven by the vacuum quadrature fluctuations propagated through their open detection ports and represents a fundamental noise floor to their strain sensitivity. This thesis addresses two distinct approaches to quantum noise improvement for future upgrades to advanced detectors. The first addresses the issue of quantum noise by adopting a quantum non-demolition approach to detector readout variables, the so-called `speed-meter’ design. Such a modified instrument samples test mass momentum, a quantity for which time separated measurements commute and are therefore not bound by Heisenberg-like limits. A novel polarisation-folded sloshing cavity type speed-meter is proposed where readout fields are stored and delayed in the orthogonal polarisation of the interferometer’s arms cavities. Here frequency dependence is selected to cancel position like measurements so that only test mass momentum information remains. A quantum noise propagation model is developed and a sensitivity performance is demonstrated that beats the standard quantum limit below 100 Hz over a broad range of frequencies. A second approach to achieve quantum noise enhancement in advanced detectors involves injection of quadrature-squeezed states in the place of vacuum. This dissertation details the development of a prototype squeezed vacuum source suitable to the demanding enhancement requirements for an Advanced LIGO squeezing installation. The construction of a doubly resonant, bow-tie cavity source is presented. This employs a novel monolithic all-glass cavity construction and is vacuum compatible. This design demonstrates the viability of building a cavity using optical contacting as a construction technique for attaching mounting prisms to highly polished fused-silica breadboards. Such a design can be expected to have excellent length noise stability, provide low intrinsic phase noise and would be suitable to mount on seismic isolation stages within the LIGO vacuum envelope. Further, the travelling wave cavity design should provide excellent 50 dB intrinsic backscatter isolation. We demonstrate the first operation of such a complex non-linear device under vacuum, producing 8.6 dB of measured vacuum squeezing down to 10 Hz across the advanced LIGO ‘audio-band’ detection range.
We have developed an experiment of ultrasensitive interferometric measurement of small displacements based on a high-finesse Fabry-Perot cavity. We have observed the internal thermal noise of mirrors and fully characterized their acoustics modes with a sensitivity of 3 × 10 −20 m/Hz 1/2 . This unique sensitivity is a step towards the first observation of the radiation pressure effects and the resulting standard quantum limit in interferometric measurements. Our experiment may become a powerful facility to test quantum noise reduction schemes such as the use of squeezed light or quantum locking of mirrors. As a first result, we present the observation of a cancellation of radiation pressure effects in our cavity. This back-action noise cancellation was first proposed within the framework of gravitational-wave detection with dual resonators, and may drastically improve the sensitivity of such measurements.
Laser interferometers are quantum instruments. This chapter presents the quantum theory of laser interferometer gravitational wave detectors. We show the basics for analysing the quantum noise in the detector, and for deriving the associated standard quantum limit (SQL) for the sensitivity. By providing different perspectives on the origin of the SQL, we illustrate the motivations behind different approaches for surpassing the SQL. Introduction The most difficult challenge in building a laser interferometer gravitational wave (GW) detector is isolating the test masses from the rest of the world (e.g. random kicks from residual gas molecules, seismic activities, acoustic noises, thermal fluctuations, etc.) whilst keeping the device locked around the correct point of operation (e.g. pitch and yaw angles of the mirrors, locations of the beam spots, resonance condition of the cavities, and darkport condition for the Michelson). Once all these issues have been solved, we arrive at the issue that we are going to analyse in this chapter: the fundamental noise that arises from quantum fluctuations in the system. A simple estimate (following the steps of Braginsky, 1968) will already lead us into the quantum world – as it will turn out, the superb sensitivity of gravitational wave detectors will be constrained by the standard quantum limit (SQL), which relates to the fundamental heisenberg uncertainty principle . Further improvements of detector sensitivity beyond this require us to manipulate the quantum coherence of light to our advantage. In this chapter, we will introduce how to analyse GW detectors quantum mechanically, and will describe several advanced configurations to surpass the SQL.
We are developing a small-scale interferometer with monolithic suspension of test masses (SIPS), that will be sensitive to the quantum radiation pressure noise in the audio frequency band of gravitational wave detectors. In the same time, a table-top experiment for the frequency dependent squeezing generation, through the Einstein Podolsky Rosen (EPR) principle, is under construction. SIPS interferometer, being designed to achieve the quantum radiation pressure noise limit exploiting high finesse optical cavities, turns out to be a suitable test bench for the EPR technique, before a possible integration in Virgo for broadband quantum noise reduction. In this work, we describe the concept of the SIPS experiment, the most important noises affecting this setup, and we briefly present the current status and the plan for the integration with the EPR squeezing experiment.