CLOSEtoQG · Cryogenic on-chip Levitated Optomechanics for a Spin Entanglement witness to Quantum Gravity

This proposal addresses one of the key challenges of modern physics: understanding the interface between quantum mechanics and general relativity. Recently, an experimental test was proposed that can directly witness the need to unify the two theories: observing quantum entanglement between objects that only interact through the gravitational field. A successful test would prove the existence of superpositions of space-time and have far-reaching implications on how we understand our world.So far, no experimental platform exists that can meet the challenging central requirement for this test: A picogram-scale mass in a micrometre-scale spatial superposition with a second-scale coherence time. Here I propose to build such a platform.The objectives of the research are to trap and levitate a picogram mass, cool its centre-of-mass motion to the quantum ground state, couple its motion to a controllable qubit system and, finally, produce and measure a spatial superposition of the mass.Considering all requirements, I identify the unique combination of techniques necessary to achieve this:- diamagnetic levitation at cryogenic temperatures using on-chip superconducting coils;- on-chip superconducting quantum interference device (SQUID)-resonator based sideband cooling;- coupling to solid-state spin qubits.Combining recent microfabrication techniques for chip-based confinement of micro-particles, high-Q resonant circuits and microscopic diamond membranes with spins, it is now possible to realize this system in the lab. My extensive experience with spins and nanomechanical systems as well as microfabrication and low-noise cryogenic measurements, and Leiden University?s infrastructure for vibration-isolated cryogenics supports CLOSEtoQG?s objectives.The research would represent a major step towards a spin-based entanglement witness of quantum gravity. Moreover, each sub-objective can benefit applications in force sensing and magnetic resonance force microscopy.