Tuning microwave losses in superconducting resonators

The performance of superconducting resonators, particularly cavities for particle accelerators and micro cavities and thin film resonators for quantum computations and photon detectors, has been improved substantially by recent material treatments and technological advances. As a result, the niobium cavities have reached the quality factors Q similar to 10(11) at 1-2 GHz and 1.5 K and the breakdown radio-frequency (rf) fields H close to the dc superheating field of the Meissner state. These advances raise the questions of whether the state-of-the-art cavities are close to the fundamental limits, what these limits actually are, and to what extent the Q and H limits can be pushed by the materials nano structuring and impurity management. These issues are also relevant for many applications using high-Q thin film resonators, including single-photon detectors and quantum circuits. This topical review outlines basic physical mechanisms of the rf nonlinear surface impedance controlled by quasiparticles, dielectric losses and trapped vortices, as well as the dynamic field limit of the Meissner state. Sections cover methods of engineering an optimum quasiparticle density of states and superfluid density to reduce rf losses and kinetic inductance by pairbreaking mechanisms related to magnetic impurities, rf currents, and proximity-coupled metallic layers at the surface. A section focuses on mechanisms of residual surface resistance, which dominates rf losses at ultra low temperatures. Microwave losses of trapped vortices and their reduction by optimizing the concentration of impurities and pinning potential are also discussed.

Automated summary

The performance of superconducting resonators, including particle accelerators, quantum computations, and photon detectors, has been significantly improved by recent material treatments and technological advances. This review discusses the current limits and potential for further improvement, focusing on nanostructuring and impurity management. The physical mechanisms of surface impedance, including quasiparticles, dielectric losses, and trapped vortices, are explained, along with methods for reducing losses and increasing performance. The review also addresses residual surface resistance and microwave losses of trapped vortices, offering strategies for optimizing impurity concentration and pinning potential.