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Classical and Quantum Gravity

Analysis of a sub-shot-noise power recycled Michelson interferometer

Kirk McKenzie, B. C. Buchler, D. A. Shaddock, Ping Koy Lam, David E. McClelland

Published Feb 12, 2004• 8 citations
Publisher

Abstract

The sensitivity of interferometric gravitational wave detectors is ultimately limited by the 'quantum noise' of light. In this paper we compare results from a bench-top experiment and a theoretical model which show how squeezed states of light may be used to modify the quantum noise behaviour of a power recycled Michelson interferometer. We develop a simple theoretical model of the experiment and find close agreement of theoretical and experimental results. We measure quantum noise suppression of 2.3 dB and demonstrate the lock stability of the experiment for long periods.

DOI
10.1088/0264-9381/21/5/098

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The sensitivity of interferometric gravitational wave detectors is ultimately limited by the 'quantum noise' of light. In this paper we compare results from a bench-top experiment and a theoretical model which show how squeezed states of light may be used to modify the quantum noise behaviour of a power recycled Michelson interferometer. We develop a simple theoretical model of the experiment and find close agreement of theoretical and experimental results. We measure quantum noise suppression of 2.3 dB and demonstrate the lock stability of the experiment for long periods.
We report on the experimental combination of three advanced interferometer techniques for gravitational wave detection, namely, power recycling, detuned signal recycling, and squeezed field injection. For the first time, we experimentally prove the compatibility of especially the latter two. To achieve a broadband nonclassical sensitivity improvement, we applied a filter cavity for compensation of quadrature rotation. The signal-to-noise ratio was improved by up to 2.8 dB beyond the coherent state's shot noise. The complete setup was stably locked for arbitrary times and characterized by injected single-sideband modulation fields.
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.
The quantum noise of the light field is a fundamental noise source in interferometric gravitational wave detectors. Injected squeezed light is capable of reducing the quantum noise contribution to the detector noise floor to values that surpass the so-called Standard-Quantum-Limit (SQL). In particular, squeezed light is useful for the detection of gravitational waves at high frequencies where interferometers are typically shot-noise limited, although the SQL might not be beaten in this case. We theoretically analyze the quantum noise of the signal-recycled laser interferometric gravitational-wave detector GEO600 with additional input and output optics, namely frequency-dependent squeezing of the vacuum state of light entering the dark port and frequency-dependent homodyne detection. We focus on the frequency range between 1 kHz and 10 kHz, where, although signal recycled, the detector is still shot-noise limited. It is found that the GEO600 detector with present design parameters will benefit from frequency dependent squeezed light. Assuming a squeezing strength of -6 dB in quantum noise variance, the interferometer will become thermal noise limited up to 4 kHz without further reduction of bandwidth. At higher frequencies the linear noise spectral density of GEO600 will still be dominated by shot-noise and improved by a factor of 10^{6dB/20dB}~2 according to the squeezing strength assumed. The interferometer might reach a strain sensitivity of 6x10^{-23} above 1 kHz (tunable) with a bandwidth of around 350 Hz. We propose a scheme to implement the desired frequency dependent squeezing by introducing an additional optical component to GEO600s signal-recycling cavity.
Twin-Signal-Recycling (TSR) builds on the resonance doublet of two optically coupled cavities and efficiently enhances the sensitivity of an interferometer at a dedicated signal frequency. We report on what we believe to be the first experimental realization of a TSR Michelson interferometer and also its broadband enhancement by squeezed light injection. The complete setup was stably locked, and a broadband quantum noise reduction of the interferometers shot noise by a factor of up to 4 dB was demonstrated. The system was characterized by measuring its quantum noise spectra for several tunings of the TSR cavities. We found good agreement between the experimental results and numerical simulations.
It has long been thought that the sensitivity of laser interferometric gravitational-wave detectors is limited by the free-mass standard quantum limit, unless radical redesigns of the interferometers or modifications of their input or output optics are introduced. Within a fully quantum-mechanical approach we show that in a second-generation interferometer composed of arm cavities and a signal recycling cavity, e.g., the LIGO-II configuration, (i) quantum shot noise and quantum radiation-pressure-fluctuation noise are dynamically correlated, (ii) the noise curve exhibits two resonant dips, (iii) the standard quantum limit can be beaten by a factor of 2, over a frequency range $\ensuremath{\Delta}f/f\ensuremath{\sim}1,$ but at the price of increasing noise at lower frequencies.