"Anharmonic dressing of multi-level superconducting quantum circuits"

Martin P. Weides, Karlsruhe Institute of Technology, Germany

Abstract

The experimental realization of quantum detectors, simulators and computers requires well-controllable, interconnected anharmonic circuits as building blocks, with superconducting quantum circuits being a leading candidate for their implementation.

In this talk, we report on the quantum circuit research at KIT [1,2]. Superconducting anharmonic multilevel circuit coupled to a harmonic readout resonator were investigated spectroscopically. Multiphoton transitions via virtual energy levels up to the fifth excited state are observed, and higher-order excitations backaction on the readout device are analyzed quantitatively. Dressed states are dynamically probed by a weak tone, showing higher-order Rabi sidebands and associated Autler-Townes splittings involving up to five levels of the investigated anharmonic circuit.

In a second work, we developed a planar tunable qubit design with coherence times in the order of 10 µs and a simple fabrication process. Limiting loss channels (Purcell decay, incoherent dielectric defects, radiative loss) are systematically investigated. The fast tunability of the qubit transition frequency enables full tomographic control of the quantum circuit. Its gradiometric design allows for increasing the magnetic flux loop, i.e. the magnetic dipole moment of the qubit and the implementation of passive direct sigma_z coupling.

Such circuits provide, for instance, inductive interfacing to artificial or natural quantum systems such as spin-chains or ferromagnets to explore magnon-polariton dynamics.

[1] Braumüller et al., PRB B 91, 054523 (2015)

[2] Braumüller et al., in preparation