Dynamic Decoupling and Noise Characterization in Superconducting Qubits

Superconducting qubits (quantum bits) are solid-state artificial atoms, comprised of Josephson tunnel junctions and superconducting interconnects and microwave resonators. When cooled to milli-Kelvin temperatures, these superconducting circuits exhibit quantum mechanical behavior, such as quantized states of flux, charge, or junction phase depending on design parameters. Such superconducting artificial atoms have already proven a useful vehicle for advancing the scientific community's general understanding of coherence in quantum mechanical systems, particularly in regimes not easily accessible with natural atoms and molecules. Moreover, superconducting qubits are promising candidates for quantum information science and technology applications, including quantum computing. The main limiting factor in using superconducting qubits is noise, The sources of noise in these systems will be studied and characterized. Established techniques from the related field of NMR (such as those used in Magnetic Resonance Imaging), for example, the targeted control sequences known as dynamical decoupling and two-dimensional NMR spectroscopy, will be adapted and extended. The underlying microscopic sources that destroy the quantum nature of these superconducting qubits will be identified and mitigated. While the future applications of quantum information science and technology are still being recognized, a broad social benefit from the technology itself (e.g., quantum sensors, simulation machines) is anticipated, as well as from the the ancillary spin-off technologies that will arise (e.g., materials, fabrication, control schema), and the young researchers who work to make them a reality. An educational feature of this work will be the access and participation by students in academic (MIT, U. Tokyo, Chalmers), corporate (NEC), and government (Lincoln Laboratory, RIKEN) research environments in the US, Japan, and Sweden. Via shared research and student internships, this will provide a culture that fosters young scientists with a global research perspective, capable of developing and leading interdisciplinary teams across institutional and international boundaries.

This work addresses the characterization, identification, and mitigation of noise sources in advanced, high-coherence superconducting qubits. One objective is to use control techniques such as dynamical decoupling to assess and mitigate noise in a new generation of these advanced qubits (2D and 3D transmons with high-Q materials, metastable flux qubit). In general, this is achieved using NMR-based techniques that are known to benchmark and elucidate microscopic noise generators in order to identify and mitigate the underlying sources of decoherence. The goal is to understand what is limiting their coherence times. The information can then be used to improve fabrication processes and materials. A second objective is further advance the noise characterization and mitigation toolset. The research will utilize a quadrature amplitude modulator and sequencer with arbitrary amplitude and phase to generate microwave pulse sequences, which will be applied to transmons and metastable flux qubits in a dilution refrigerator. Coherence characterization, including the measurement of standard coherence times, will be performed as a function of the qubit quantization axis. Randomized benchmarking, state tomography, and process tomography will be used to characterize gate fidelity. Decoherence will be mitigated through the application of dynamical decoupling pulse sequences. These techniques will also be used to measure the noise power spectral density. Two-dimensional NMR spectroscopy techniques will be used to assess the microscopic nature and origin of decoherence.