Power dependent dynamics of the 2nd excited state of a Transmon qubit

Abstract

A qubit, the quantum counterpart of the classical binary digit, can be constructed based on a system with two possible quantum states. For instance, superconducting circuits have been employed to construct an artificial atom in which ground and excited states are used for two quantum states. The anharmonicity of an artificial atom distinguishes the higher-order excited states from two possible quantum states, although they can interact with the cavity as well. The interaction can be important when a large number of photons are allowed in the cavity and results in a nonlinear effect on a Transmon-cavity system. We studied power-dependent nonlinear behaviors of a Transmon-cavity system. The system exhibits dramatic shifts from a dispersive coupling at a low power regime to punch-out behaviors at high powers. This nonlinearity provides us high fidelity readouts for 2nd excited state as well as 1st excited state.