Using optical techniques, we are studying the quantum mechanical properties of nanomechanical structures. In particular, we are interested in developing the tools for quantum-limited transduction of motion enabling the preparation and measurement of highly non-classical states of a mechanical system, understanding fundamental limits to dissipation and decoherence in acoustic resonators, and engineering phononic circuits consisting of arrays of acoustic resonators interconnected by waveguides.
Phononic Engineering of Quantum Acoustic Elements
In optics, geometric structuring at the nanoscale has become a powerful method for modifying the electromagnetic properties of a bulk material, leading to metamaterials capable of manipulating light in unprecedented ways. In the most extreme case, photonic bandgaps can emerge in which light is forbidden from propagating, dramatically altering the emission of light from within such materials. More recently, a similar phononics revolution in the engineering of acoustic waves has led to a variety of new devices, from thermal crystals for controlling the flow of heat to phononic topological insulators for scattering-free transport of acoustic waves.
Phononic bandgap structures, similar to their electromagnetic counterparts, can be used to modify the emission or scattering of phonons. These ideas have recently been explored in quantum optomechanics and electromechanics experiments to greatly reduce the mechanical coupling to the thermal environment through acoustic radiation. To date, however, far less attention has been paid to the impact of geometry and phononic bandgaps on acoustic material absorption. Fundamental limits to sound absorption in solids are known to result from the anharmonicity of the host crystal lattice. At the very lowest lattice temperatures (<10K), where Landau-Rumer damping due to phonon-phonon interactions has dropped off, a residual damping emerges due to material defects. These two-level system (TLS) defects, typically found in amorphous materials, correspond to a pair of nearly-degenerate local atomic arrangements in the solid which can have both an electric and an acoustic transition dipole, and couple to both electric and strain fields. Recent theoretical analysis shows that TLS interactions with acoustic waves can be dramatically altered in a structured material. Recent experiments in the Painter group involving a microwave-frequency nano-acoustic cavity with a phononic bandgap shield have now measured cavity-phonon lifetime exceeding one second, corresponding to a record quality factor exceeding Q > 30 billion.
These recent observations indicate that the advanced methods of nanofabrication and phononic engineering can provide a new toolkit to explore quantum acoustodynamics in solid-state materials. Continued studies of the behavior of TLS in engineered nanostructures may lead to, among other things, new approaches to modifying the behavior of quasi-particles in superconductors, mitigation of decoherence in superconducting and color center qubits, and even new coherent TLS-based qubit states in strong coupling with an acoustic cavity. Perhaps most intriguing is the possibility of creating a hybrid quantum architecture consisting of acoustic and superconducting quantum circuits (see our research page on this topic), where the small scale, reduced cross-talk, and ultralong coherence time of quantum acoustic devices may provide significant improvements in connectivity and performance of current quantum hardware.
While our vision of phononic engineering is quite broad, our near-term efforts are focused on developing phononically engineered acoustic devices in conjunction with color-center and transmon qubits. This involves the study of phononic shields for reducing decoherence in both types of qubits, as well as phononic crystal cavities and waveguides for controlling acoustic interactions between the different qubit types and between the qubits and their phonon bath(s). We will emphasize the study of decoherence effects in silicon (Si) phononic bandgap acoustic resonators, which as described below, we plan to develop as a microwave-frequency quantum memory for SQCs. Current results indicate that residual decoherence in the OMC cavities stem from fluctuations in the excited state population of surface TLS defect states. Mitigation of surface defects, through a combination of surface etching, reconstruction, and passivation will be explored. In addition, we are exploring the integration of phononic bandgap shields around the different elements of SOI-based transmon qubits, where coherence times have been limited to a few microseconds.