Technology Demonstration for Mid-Frequency Gravitational-Wave Detector

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This award supports the construction and demonstration of sensors that will be able to measure extremely small variations in the local gravitational field. The main motivation is to make it possible to build a new type of gravitational wave detector that will be able to detect gravitational-wave signals in a so-far little-explored frequency range, lower than what NSF's LIGO (and all other ground-based detectors) can sense, but higher than what the future space-based detector LISA will cover, thus enabling a new window to further explore physics and astrophysics with gravitational waves. Detecting signals in that middle range is supremely challenging, requiring extremely low-noise sensors and readout electronics that are virtually immune from vibrations. Superconducting materials and quantum-interference-device amplifiers should be able to meet those challenges if used in a configuration that compares multiple "test masses" whose tiny motions are compared directly using super-cooled circuits. This project is to construct an assembly with two test masses and confirm that it has the expected stability and sensitivity in the target frequency range. As an added benefit, it is anticipated that this sensor technique could also be adapted to rapidly sense the subtle gravitational changes from shifting faults deep underground, enabling faster early warning of earthquakes before the seismic waves reach the surface and thereby potentially saving lives and reducing damage to infrastructure.The sensor design for this project builds on the Maryland Gravitation Experiment group's experience with levitating superconducting test masses and sensing differential displacements using cryogenic circuits at liquid helium temperature (4 K). Direct differential sensing of displacements relative to a common rigid frame measures gravity gradients. A tetrahedral arrangement of test masses is sufficient to sense all tensor components of variations of the local gravitational field; this design is called Mini-SOGRO. This project will construct an assembly with two test masses, using a design that is expected to achieve Q factors greater than 10^6 in the differential sensing mode at frequencies below 0.1 Hz using capacitive sensing with superconducting capacitance bridges, SQUID amplifiers and lock-in amplifier readout. A key feature of the new design is the use of multiple curved ridges on the test mass surface, strategically aligned with coils, to produce a "negative spring" that will mitigate the intrinsic nonlinearity of the levitation (and sensing) fields on the test masses and thereby make it possible to achieve very low resonant frequency. The Q will be measured and the common-mode rejection, i.e. robustness to vibrations of the housing, assessed.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.