Hybrid Quantum Systems for Higher Temperature Quantum Information Processing

The ability to operate superconducting quantum computers at higher temperatures would greatly expand their utility and range of applications. This could be achieved by increasing resonance frequencies and/or utilizing collective modes that are less noisy and more robust against decoherence. We discuss several nonlinear resonator concepts in which the roles of linear and nonlinear elements are reversed vs. the transmon. The simplest version is a nonlinear LC resonator with a linear superconducting inductor and a nonlinear capacitor employing a nonlinear dielectric material. Due to progress in tunable dielectrics for 6G, some ferroelectric composites may enable operation of a nonlinear dielectric - superconductor qubit at hundreds of gigahertz. Other nonlinear dielectric materials include quantum paraelectrics and charge density wave materials. These are of interest due to robust collective modes resulting from macroscopically occupied states. Other proposed nonlinear resonators employing nonlinear dielectrics include quarter- and half-wavelength resonators. Voltage tunability is a potential feature of the proposed concepts.