Novel superconducting qubit concepts for higher temperature quantum information processing*

The ability to operate quantum computers at higher temperatures vs. today’s transmon-based systems would greatly expand their range of applications.  This could be achieved by increasing resonance frequencies and/or utilizing collective modes 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 in which the superconducting inductor is linear, while a nonlinear capacitor is enabled by a nonlinear dielectric. Due to progress in 5/6G, some ferroelectric materials may enable operation of a nonlinear dielectric – superconductor qubit at hundreds of GHz. Other nonlinear dielectrics include quantum paraelectrics, such as SrTiO3 and KTaO3, and charge density wave materials. These are intriguing due to collective modes resulting from macroscopically occupied states. Additional concepts include half- or quarter-wavelength resonators employing nonlinear dielectrics. Possible signatures of collective behavior include coherent many-body Rabi oscillations or super-Rabi oscillations.