Chen Wang (UMass) - The old mystery and the new battle: Dielectric two-level systems in the crosshairs of quantum computing 

Superconducting circuits are one of the leading technology platforms for the development of a practical quantum computer.  In superconducting quantum circuits, qubits are typically constructed from Al/AlOx/Al Josephson tunnel junctions and patterned superconducting thin films such as Al, Nb, Ta on crystalline Si or sapphire substrate, and operated with external microwave pulses.  Superconducting qubits enjoy advantages in their circuit design flexibilities, lithographic scalability, and operational clock speed, but must overcome decoherence from the solid-state environment.  In particular, noise from dielectric materials, including the Josephson junction, the bulk substrate, and various interfaces, have increasingly stood out as a limiting factor for both the lifetime and frequency stability of superconducting qubits.  Such dielectric dissipation and fluctuation have been attributed to the omni-presence of microscopic two-level systems (TLS), which have been modeled for glasses and amorphous materials since the 1970s. These TLS are present over an extremely broad frequency range (from kHz to GHz) with spectral densities that affect qubits in both discrete and continuum limits. Various improvements in qubit design and materials processing have led to substantial improvement of qubit coherence over the past two decades through mitigation of the TLS problem, but any atomistic understanding of these TLS in relevant materials remains elusive.  In this talk, I hope to give a birds-eye view of the phenomenological studies of dielectric loss and two-level systems in superconducting circuits, and explain how the dream of building a quantum computer has been intertwined with so-far the most sensitive probe and the most demanding quest of traditional but pristine materials.