Abstract: Error correction can only work with superconducting qubits if qubit errors are lowered. Surface loss from thin oxides is currently a dominant error mechanism. Formulas for useful qubit geometries are presented to predict surface loss, which can be used to optimize the qubit layout. A significant fraction of surface loss comes from the small wire that connects the Josephson junction to the qubit capacitor. Tapering this wire is shown to significantly lower its loss, as well as etching the underlying silicon to create free-standing wires.

Abstract: Large algorithms can be run on a quantum computer only if quantum error correction is used to lower logical qubit errors. The energy deposited by cosmic-ray muons produces a quasiparticle “heat” pulse that causes the qubits to decay in energy quickly, with errors correlated in space and time, so that error correction fails. Metal layers comprising normal metal and/or small-gap superconductors channel this energy away from the qubit into benign structures so that qubit performance is not degraded. These structures are designed according to the electron-phonon interactions and constraints from electromagnetic radiation to make large reductions in the induced errors so that error correction works properly.