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Control and tomography of phonons in cavity optomechanical nanostructures

Phonons are the quanta of mechanical motion. One possible toolbox for manipulating phonons is provided by the field of cavity-optomechanics, in which light and motion interact via radiation pressure. Of particular importance to applications are cavity-optomechanical nanostructures, whose properties are entirely engineered and which can be integrated on the surface of a chip. In such platforms, new possibilities for the control of phonons emerge. Two key questions arise in these systems where phonons carry information: How can we control the propagation of phonons, and how can we use light to prepare and verify quantum states of mechanical systems? In this thesis, I present three main results using cavity optomechanical nanostructures. First, I will show that phonons confined to a resonator can be made to deterministically couple to an external channel. We propose, and demonstrate via cryogenic measurements, that a particularly simple symmetry-breaking perturbation in nanopatterned silicon allows for the efficient transduction of localized phonons into de-localized phonons. Second, we design and experimentally demonstrate a single-mode phononic wire. Our design incorporates a patterned silicon film with a complete two-dimensional acoustic bandgap. We show through optical measurements that only a single propagating mode is supported for GHz frequency phonons within the bandgap. Low-loss propagation over millimeter length scales is observed. In addition, methods to generate optically induced non-linearities for phonons are discussed. Finally, I describe recent results from our experiment combining photon counting and continuous measurement of mechanical position to perform quantum state tomography. By heralding our measurement on the detection of a single inelastically scattered photon, a single phonon is added to a nanomechanical oscillator. Despite the large thermal occupancy of the oscillator at room temperature, the addition, or subtraction, of a single phonon produces a distinctly non-Gaussian state of motion. The results open up new possibilities for preparing and verifying macroscopic quantum mechanical states using light

Rishi N. Patel
Stanford University
Publication Date
Type of Dissertation
Ph.D. Applied Physics