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Quantum control of levitated massive mechanical systems: a new approach for gravitational quantum physics

Periodic Reporting for period 4 - QLev4G (Quantum control of levitated massive mechanical systems: a new approach for gravitational quantum physics)

Reporting period: 2019-12-01 to 2020-05-31

Quantum physics and general relativity are probably the most successful and well-tested theories of modern science. At the same time, their fundamental concepts are so dramatically different that there is even disagreement on the most obvious questions such as “how does a mass in a quantum superposition state gravitate?“. Achieving progress on such foundational questions requires experiments at the interface between quantum physics and gravity, of which to date only a few of exist. The main objective of QLev4G is to establish quantum control of levitated massive objects as a new paradigm system for such experiments and to enter a hitherto inaccessible parameter regime of large mass and long quantum coherence. This includes the development of methods and technologies that are necessary for realizing such quantum states of matter.
The project is built on the enormous recent success in quantum control of the motion of solid-state mechanical resonators, which has emerged over the last decade as a new branch of interdisciplinary research in quantum and solid-state physics. In conclusion, QLev4G has successfully transferred this methodology to optically and magnetically levitated systems to achieve (i) exceptional sensitivity to weak gravitational forces, hence enabling measurements of gravity between sub-millimeter objects; (ii) unprecedented levels of decoupling from the environment, thereby opening up a new route for long-lived quantum coherence of genuinely massive systems.
From a bottom-up perspective the QLev4G team has been developing new ways to achieve full quantum control over levitated solid-state systems using both cavity-enhanced light-matter interaction and real-time optimal feedback control, allowing us to realize for the first time the quantum ground state of motion of an isolated, massive solid-state object being trapped in a room temperature environment. In parallel, QLev4G has pioneered levitation and feedback-control of micron-scale superconductors at ultracold temperatures. We have also been implementing several new protocols for preparing non-Gaussian motional states of solids, and we have been developing protocols for quantifying and extracting quantum entanglement from optomechanical interactions. From a top-down perspective we have been successfully decreasing the size of gravitational source masses in table-top experiments and have realized for the first time gravitational coupling between 2 millimeter-sized objects. These results have been disseminated through high-impact publications, as well as through scientific and public presentations. In conclusion, our project results represent a significant advance compared with the state of the art when the project started.
All results described above go significantly beyond the current state of the art by operating in a parameter regime or under conditions that have previously not been achieved.
Most prominently, with respect to quantum physics QLev4G has realized for the first time the motional quantum ground state of an isolated, massive solid-state object containing hundreds of millions of atoms in a room temperature environment. With respect to gravitational physics, QLev4G has measured the gravitational field of a 1mm sized solid-state object, by far the smallest gravitational source mass in experiments to measure gravity.
An optically trapped nanoparticle cooled to its quantum ground state of motion in an optical cavity