PhD defense by Junxin Chen
Titel: Quantum Correlations Generated by a Soft-Clamped Membrane-in-the-Middle System
Abstract: Optomechanics is a rapidly developing field, exploring the subtleties of the interaction between light and mechanical oscillators. Thanks to the advances in system design and fabrication, quantum effects have become significant in a range of optomechanical systems. On the mechanical side, mechanical oscillators consisting of billions of atoms have recently been prepared in the motional ground state, squeezed state, single excitation (Fock) state and entangled state, which shows the great potential of massive quantum state preparation and control. By combining mechanical oscillators and qubits, researchers have also achieved complicated manipulation of mechanical quantum states. On the optical side, through the interaction with mechanics, ponderomotive squeezing, and optical-optical entanglement have been realized, which demonstrate the power of optomechanical systems as a quantum media to control optical states. Apart from preparation of quantum states of light and mechanics, optomechanics provides a platform for ultra-sensitive displacement and force measurements, which is key in many sensing applications like gravitational wave detection and force sensing by magnetic resonance force microscopy (MRFM). A quantum limited measurement with high signal-to-noise ratio is also a prerequisite for measurement-based quantum control of mechanical systems, like feedback cooling to the quantum ground state. To realize this measurement, the experimental apparatus needs to enable high detection efficiency and strong measurement, simultaneously, which is a demanding requirement. Existing optomechanical systems either have strong interaction, but low detection efficiency or a noisy detection chain, or have high detection efficiency, but cannot sustain enough optical power to get strong interaction. Combining these two features is so far a unique advantage of our membrane-in-the-middle system (MIM) with ultra-coherent soft-clamped membrane, which will be introduced in this thesis. With this system, we have acquired displacement measurement record with record breaking quality, only 33% above the Heisenberg Limit, a fundamental limit for displacement measurement. With this high quality measurement record, we achieved the first feedback cooling of a mechanical mode to its quantum ground state, which integral to other measurement-based quantum control protocols. High detection efficiency and strong measurement is also a key for observing effects of optomechanically induced quantum correlations. Ultilizing this correlation, we demonstrate the first displacement and force sensing below the Standard Quantum Limit (SQL), a milestone in quantum metrology, which opens the gate to the world of sub-SQL sensing. By having two optical fields interacting with the same mechanical oscillator, we also demonstrate ponderomotive entanglement between the two optical fields, which provides a entanglement generator compatible with various condensed matter based quantum systems. This is a necessary building block for optical-microwave entanglement, a useful resource for quantum networks and the quantum internet, eventually.