Role of materials in quantum computing with superconducting circuits
Properties of materials often fundamentally change as they are cooled from room temperature to near-absolute zero Kelvin. Superconductors, in which conduction electrons condense into Cooper pairs, are useful in quantum computing due to their lack of electrical dissipation and the existence of a non-linear inductive element, the Josephson junction. However, unpaired electrons known as quasiparticles may tunnel across these junctions causing decoherence events in which quantum information is lost. These quasiparticles may be mitigated by clever materials engineerings, such as incorporating materials with different superconducting gaps or resistive materials with strong electron-phonon scattering. Additionally, planar superconducting circuits feature strong localized electric fields that may couple to material defects in the junctions or material interfaces, and spin fluctuations of these surface defects may cause additional noise detrimental to flux-tunable elements. Finally, the thermal properties of materials at low temperature are of considerable importance as the superconducting circuits must be well-thermalized with the cryostat to prevent decoherence and reduce the residual excited state population.