# Superconducting Qubits and Gates

Superconducting Qubits and Devices

Superconducting qubits based on Josephson junctions are a promising platform for quantum computation, reaching quality factors of over one million. Such high quality factors enable the investigation of decoherence mechanisms with high accuracy. An intrinsic decoherence process originates from the coupling between the qubit degree of freedom and the quasiparticles that tunnel across Josephson junctions. Interestingly, the quasiparticle-induced decoherence rate can be modulated by a magnetic flux, due to the interference between processes involving electron-like and hole-like quasiparticles [1]. Theoretical predictions for quasiparticle effects have been verified in recent experiments with various qubit designs, from the single-junction transmon [2] to the multi-junction fluxonium. Quasiparticles can also affect the behavior of other devices, such as Cooper pair pumps and nanorefrigerators.

[1]. see, e.g., G. Catelani et al., Phys. Rev. Lett. 106, 077002 (2011)
[2]. H. Paik et al., Phys. Rev. Lett. 107, 240501 (2012)

### Superconductivity – Fundamentals and Applications

The theoretical description of conventional superconductors has a long history, but there are still open questions, whose answer can also have important consequences for practical applications. For example, superconducting cavities for particle accelerators are operated in a metastable Meissner state, and the fundamental limitations on such a state are under investigation [1]. For metallic films in parallel magnetic fields, a detailed understanding of quantum superconducting fluctuations [2] has made possible the extension to high fields of techniques used to measure e.g. electron polarization in ferromagnetic metals and exchange fields induced by ferromagnetic insulators.

[1]. G. Catelani and J. P. Sethna, Phys. Rev. B 78, 224509 (2008)
[2]. M. Khodas, A. Levchenko, and G. Catelani, Phys. Rev. Lett. 108, 257004 (2012), and references therein