C. Wellstood'
title: Multilevel effects in the Rabi oscillations of a Josephson phase qubit
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Introduction {#SIntro}
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Over the past decade, a variety of qubits based on superconducting Josephson junctions has been proposed and experimentally realized.[@Makhlin01a; @Devoret04a; @You05a; @Wendin07a; @Zagoskin07a] Among them is the phase qubit, which can take the form of a single current-biased junction,[@Ramos01a; @Yu02a] a flux-biased rf superconducting quantum interference device (SQUID),[@Simmonds04a] a low inductance dc SQUID,[@Claudon04a] a large inductance dc SQUID where one of the junctions is thought of as the qubit,[@Martinis02a] or a vortex in an annular junction.[@Wallraff03c] The behaviors of the devices are similar, as they all employ a large area junction, where the dynamics of the quantum mechanical phase difference across the junction (or the orientation of the vortex in the last case) are determined by a tilted washboard-like potential. The two lowest states in a well of this potential serve as the qubit basis. Among the possible benefits of the phase qubit are its relative insensitivity to charge and flux noise and its ability to operate over a wide range of parameters. Recent demonstrations include fast readout,[@Claudon04a] Rabi oscillations,[@Yu02a; @Martinis02a; @Claudon04a; @Strauch06a; @Lisenfeld07a] and simple capacitive coupling of two qubits as 