1/f Noise and Dephasing from Surface Magnetic States in SQUIDs and Superconducting Qubits
Low-frequency 1/f flux noise is a dominant source of dephasing in the Josephson phase and flux qubits. The flux noise inferred from recent qubit experiments is consistent with the noise measured more than 20 years ago in a series of experiments performed on dc Superconducting QUantum Interference Devices (SQUIDs) cooled to millikelvin temperatures. The noise was observed to be “universal", that is, only weakly dependent on a wide range of parameters such as superconducting materials, SQUID loop geometry and inductance, and temperature.
Recent work by our group has revealed the presence of a high density of unpaired spins at the surfaces of superconducting thin films; it is now believed that these spins are the source of the noise, although the microscopic noise mechanism is not completely understood. Here we describe experiments on SQUIDs and Josephson phase qubits designed to shed light on the underlying noise mechanism, and we report on efforts to develop novel materials with reduced levels of noise. Recently we have shown that encapsulation of the SQUID washer in PECVD-grown silicon nitride leads to a reduction in the flux noise power by more than an order of magnitude. We discuss prospects for incorporating the novel dielectric interfaces into high-performance phase qubit circuits.