Quantum Enhancement of the Zero-Area Sagnac Interferometer Topology for Gravitational Wave Detection

Abstract

Only a few years ago, it was realized that the zero-area Sagnac interferometer topology is able to perform quantum nondemolition measurements of position changes of a mechanical oscillator. Here, we experimentally show that such an interferometer can also be efficiently enhanced by squeezed light. We achieved a nonclassical sensitivity improvement of up to 8.2 dB, limited by optical loss inside our interferometer. Measurements performed directly on our squeezed-light laser output revealed squeezing of 12.7 dB. We show that the sensitivity of a squeezed-light enhanced Sagnac interferometer can surpass the standard quantum limit for a broad spectrum of signal frequencies without the need for filter cavities as required for Michelson interferometers. The Sagnac topology is therefore a powerful option for future gravitational-wave detectors, such as the Einstein Telescope, whose design is currently being studied.All currently operating interferometric gravitational- GEO [3] and TAMA [4]) are Michelson interferometers. Their purpose is to measure the position changes (displacements) of quasi free-falling mirrors thereby revealing changes of space-time curvature, i.e. gravitational waves. The current detectors are aiming for the first direct observation of gravitational waves. Future detectors will aim for establishing gravitational wave astronomy which will require a considerable increase of the detectors' displacement sensitivity. The Einstein Telescope (ET) [5] is an on-going European design study project for such a gravitational wave detector. An important issue for future detectors is a reduction of the quantum measurement noise (photon shotnoise) and quantum back-action noise (quantum fluctuations in the radiation pressure acting on the mirrors) for a given laser power. This way, the standard quantum limit (SQL) can eventually be surpassed allowing for quantum-non-demolition (QND) displacement measurements. Future detectors will most likely be operated at cryogenic temperatures in order to reduce thermally excited motions of the mirror surfaces and optical absorption will set an upper limit to the laser power inside the interferometer. For Michelson interferometers a nonclassical reduction (squeezing) of the quantum measurement noise can be achieved by injecting squeezed light [6][7][8][9]. Squeezing of back-action noise is also possible but turned out to be experimentally challenging because in Michelson interferometers the back-action noise depends on Fourier frequency. Long-baseline narrow-band filter cavities are required to compensate for the frequency dependence [10] which complicates the topology and introduces additional optical loss.

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