Abstract
Speedmeters are known to be quantum non-demolition devices and, by potentially providing sensitivity beyond the standard quantum limit, become interesting for third generation gravitational wave detectors. Here we introduce a new configuration, the sloshing-Sagnac interferometer, and compare it to the more established ring-Sagnac interferometer. The sloshing-Sagnac interferometer is designed to provide improved quantum noise limited sensitivity and lower coating thermal noise than standard position meter interferometers employed in current gravitational wave detectors. We compare the quantum noise limited sensitivity of the ring-Sagnac and the sloshing-Sagnac interferometers, in the frequency range, from 5 Hz to 100 Hz, where they provide the greatest potential benefit. We evaluate the improvement in terms of the unweighted noise reduction below the standard quantum limit, and by finding the range up to which binary black hole inspirals may be observed. The sloshing-Sagnac was found to give approximately similar or better sensitivity than the ring-Sagnac in all cases. We also show that by eliminating the requirement for maximally-reflecting cavity end mirrors with correspondingly-thick multi-layer coatings, coating noise can be reduced by a factor of approximately 2.2 compared to conventional interferometers.
Highlights
The most sensitive position-sensing interferometers [1,2,3] are predicted to closely approach the standard quantum limit (SQL) within the few years and will be limited by quantum noise over much of the observing band [4]
We have disregarded all further detail of classical noise except that above 5 Hz it is assumed to lie somewhat below the standard quantum limit up to ∼100 Hz and below the expected quantum noise at higher frequencies, such that the potential offered by quantum non-demolition (QND) methods can be realised in terms of a more sensitive detector
The readout of the third generation interferometers is most likely to be performed by the balanced homodyne detector (BHD) that allows to measure an arbitrary quadrature of the outgoing light by means of measuring the difference photocurrent, iζout, of the two photodiodes of the BHD where the desired quadrature is chosen by means of changing the relative phase shift ζ between the local oscillator beam and the signal one: iζout ∝ bcout cos ζ + bosut sin ζ ≡ HζT ⋅ b, Hζ ≡ ⎡⎣⎢csoinsζζ ⎤⎦⎥ . (9)
Summary
The most sensitive position-sensing interferometers [1,2,3] are predicted to closely approach the standard quantum limit (SQL) within the few years and will be limited by quantum noise over much of the observing band [4]. We have disregarded all further detail of classical noise except that above 5 Hz it is assumed to lie somewhat below the standard quantum limit up to ∼100 Hz and below the expected quantum noise at higher frequencies, such that the potential offered by QND methods can be realised in terms of a more sensitive detector This has consequences, discussed below, for our approach to optimising the parameters of the speedmeters. When this technique is applied, squeezed light, or more precisely, squeezed vacuum generated in an external system of non-linear optics, is employed to reduce the measured quantum noise While this may result in improved sensitivity, we do not expect that it would be a strong differentiator among the configurations under consideration.
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