CAREER: Optomechanical Sensors Leveraging Quantum Noise

University Of Pittsburgh, Pittsburgh PA

Investigators

Abstract

Mechanical sensors with unprecedented sensitivity will provide a means to look for extremely tiny forces that are undetectable by current methods. Such small signals can arise from cosmological origins, such as certain hypothetical types of dark matter which must exist in great abundance to hold our galaxy together but cannot be seen directly with normal astronomical observation. Another source as-of-yet undetected signals is from proposed effects which act to spoil exotic quantum mechanical behaviors such as an object existing simultaneously in two places at once. Observing or putting limits on the existence of either of these signals will add to the fundamental understanding of the nature of our universe. At much smaller scales, ultrasensitive mechanical detectors could feel the magnetic forces from individual atoms and nuclei, performing nanoscale magnetic resonance imaging (MRI) for biologically important molecules or semiconductor electronic devices that cannot be imaged via conventional techniques. As mechanical sensors are devised with lower and lower noise, devices are running up against fundamental quantum noise limits. This project will develop methods to side-step these quantum limits and improve searches for small mechanical signals representing new physical phenomena. To this end, ultralow noise mechanical sensors with new optical probing techniques will be developed to enhance measurement sensitivity in the face of quantum noise. This project will also provide educational research training for undergraduate students, preparing the next generation of scientists and engineers to tackle real-world problems. Students will take a real open-ended research project in optical and mechanical sensors from concept and design all the way through fabrication and testing in rapid enough succession to allow for multiple iterations facilitated by modern computer-aided design, simulation, and manufacturing tools. Understanding quantum noise limits in optically probed mechanical systems goes back to the advent of quantum mechanics, with Heisenberg’s microscope thought experiment illustrating a fundamental trade-off between position sensitivity and backaction (i.e. random momentum kicks from recoiling photons obscuring the motion of an object being observed with light). Many methods to evade the deleterious effects of this quantum backaction have been proposed, most of which require complex optical or mechanical configurations with demanding constraints on classical noise, loss, and stability. Here, the goal is to demonstrate a similar quantum advantage in a much simpler and ubiquitous precision optical measurement technique – the optical lever, measuring the angular deviation of light reflecting off a tilting surface. Simple modifications to the standard setup yield an increase in the quantum information from the measurement and a reduction in the sensitivity to backaction-induced motion that would otherwise mask small signals. This technique will be applied to ultralow mechanical dissipation, high-tension vibrating string mechanical resonators that can be functionalized for sensing strain, gravity, and magnetic fields beyond standard quantum limits. Further, this string optomechanical sensor will be anchored to a macroscopic test mass and the quantum limits of measuring strain induced by test mass motion will be investigated. Measurements of this system will put meaningful constraints on beyond-standard-model interactions including exotic quantum decoherence models and ultralight scalar dark matter. In the longer term, the techniques developed here will find applications in the quantum-enhanced detection of gravitational waves, nanoscale force microscopy, and in mechanically mediated storage and transduction of quantum states for quantum information processing. 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.