Imaging Photon Lattice States by Scanning Defect Microscopy

Microwave photons inside lattices of coupled resonators and superconducting qubits can exhibit surprising matter-like behavior. Realizing such open-system quantum simulators presents an experimental challenge and requires new tools and measurement techniques. Here, we introduce Scanning Defect Microscopy as one such tool and illustrate its use in mapping the normal-mode structure of microwave photons inside a 49-site Kagome lattice of coplanar waveguide resonators. Scanning is accomplished by moving a probe equipped with a sapphire tip across the lattice. This locally perturbs resonator frequencies and induces shifts of the lattice resonance frequencies which we determine by measuring the transmission spectrum. From the magnitude of mode shifts we can reconstruct photon field amplitudes at each lattice site and thus create spatial images of the photon-lattice normal modes

Evolution of 1/f1/f Flux Noise in Superconducting Qubits with Weak Magnetic Fields

The microscopic origin of $1/f$ magnetic flux noise in superconducting circuits has remained an open question for several decades despite extensive experimental and theoretical investigation. Recent progress in superconducting devices for quantum information has highlighted the need to mitigate sources of qubit decoherence, driving a renewed interest in understanding the underlying noise mechanism(s). Though a consensus has emerged attributing flux noise to surface spins, their identity and interaction mechanisms remain unclear, prompting further study. Here we apply weak in-plane magnetic fields to a capacitively-shunted flux qubit (where the Zeeman splitting of surface spins lies below the device temperature) and study the flux-noise-limited qubit dephasing, revealing previously unexplored trends that may shed light on the dynamics behind the emergent $1/f$ noise. Notably, we observe an enhancement (suppression) of the spin-echo (Ramsey) pure dephasing time in fields up to $B=100~\text{G}$. With direct noise spectroscopy, we further observe a transition from a $1/f$ to approximately Lorentzian frequency dependence below 10 Hz and a reduction of the noise above 1 MHz with increasing magnetic field. We suggest that these trends are qualitatively consistent with an increase of spin cluster sizes with magnetic field. These results should help to inform a complete microscopic theory of $1/f$ flux noise in superconducting circuits

Terahertz Second-Harmonic Generation in Extreme-Confinement Cavities

It remains a standing challenge to produce high-power electromagnetic sources operating in the spectral range of 0.1-10 THz (the “terahertz gap"), a frequency band for applications ranging from spectroscopy to security and high-speed wireless communications. In this thesis, we will analyze a method to produce coherent radiation spanning the THz gap by second-harmonic generation (SHG) in low-loss dielectric structures, starting from the ∼100 GHz range. For this purpose, we present hybrid THz-band dielectric cavity designs that combine (1) nonlinear materials enhanced by phonon resonances with (2) extreme field concentration in high-quality-factor resonators. An efficient device for THz SHG would enable cascaded parametric frequency converters extensible into the mid-IR spectrum and beyond.S.M

Contrast Enhancement Using Silicon Photonic Nonlinearities

We explore how to take advantage of nonlinear optical effects in silicon to perform contrast enhancement on pulsed photonic signals. Improving the contrast ratio of two-level photonic signals with low power contrast has implications for developing a cryogenic modulation system to be used in quantum computing applications. Integrated resonator-enhanced interferometers are promising candidates for implementing such functions because they operate by converting small nonlinear phase switching effects to large amplitude switching effects. Using both simulation and experiment, we study the resonator-enhanced Mach-Zehnder interferometer (REMZ) and the dual resonator enhanced asymmetric MZI (DREAM) structures on a silicon-on-insulator platform. The first-order optical effects of free carrier dispersion and free carrier absorption (influenced by the nonlinear carrier rate equation) and the third-order effects of Kerr and two-photon absorption are the critical physical phenomena underlying the structures’ nonlinear behavior. Coupled mode theory is used to analyze the dynamics of the microring circulating field, and simulations are carried out for the ring-enhanced Mach-Zehnder structures. An experiment is conducted on the DREAM device that confirms the qualitative accuracy of the model. Unlike in previous theoretical descriptions of nonlinear resonators, we discover that free carrier dispersion can dominate over the Kerr effect in silicon, and therefore cannot be neglected in the design of nonlinear silicon photonic devices