Final Report Summary - CAVITYQPD (Cavity quantum phonon dynamics)
Perhaps the strangest prediction of quantum theory is the phenomenon of entanglement, whereby two distant objects become intertwined in a manner that defies both classical physics and a common sense understanding of reality. Nonetheless, entanglement is now considered a cornerstone of quantum mechanics, and is the key resource behind a host of potentially transformative quantum technologies. Entanglement is however extremely fragile, and has previously only been observed with microscopic systems such as light or atoms, and recently with superconducting electric circuits. In our project, we managed to bring the motions of two distinct vibrating drumheads into an entangled quantum state. The objects in the experiment are truly massive and macroscopic when compared to the atomic scale: each circular drumhead is fabricated from metallic aluminium on a silicon chip, and has diameter similar to the width of a thin human hair. The vibrating bodies are made to interact via a superconducting microwave circuit. The electromagnetic fields in the circuit act to absorb thermal disturbances, leaving behind only the quantum-mechanical vibrations. Elimination of all forms noise is crucial in the experiments, and therefore they are carried out at very low temperatures close to the absolute zero at -273 C. Remarkably, this approach allows this unusual state to persist for long periods of time, in the present case up to half an hour. The result demonstrates that control over large mechanical objects is now at the level where exotic quantum states can be generated and stabilized. This opens the door to new kinds of quantum technologies and sensors, but could also enable studies of fundamental physics, such as the poorly understood interplay of gravity and quantum mechanics. In the project we also have devised various approaches for sensitive measurements using novel techniques involving nanodrums. Our measurements even go beyond the quantum limit. For the particle this would be possible by measuring only either the position or momentum, and completely discarding the information about the other property. For a light wave, accessing only part of the wave and discarding the information in the other part realizes an analogous measurement. It can be used in accessing tiny signals for example in quantum computing and perhaps also in measurement of gravitational waves.