Abstract: We present measurements of coherence and successive decay dynamics of higher energy levels of a superconducting transmon qubit. By applying consecutive pi pulses for each sequential transition frequency, we excite the qubit from the ground state up to its fourth excited level and characterize the decay and coherence of each state. We find the decay to proceed mainly sequentially, with relaxation times in excess of 20 us for all transitions. We also provide a direct measurement of the charge dispersion of these levels by analyzing beating patterns in Ramsey fringes. The results demonstrate the feasibility of using higher levels in transmon qubits for encoding quantum information.

In a separate experiment, we present an improved superconducting qubit, leveraging design elements from both charge- and flux-based qubits to achieve a device with large frequency tunability, large anharmonicity, and energy relaxation and dephasing times in excess of 40 μs. We show that the qubit relaxation times are limited by a combination of Ohmic charge noise and 1/f-type flux noise, a noise source that previously has been considered mainly in the context of dephasing. Furthermore, by mapping out the noise power spectral density seen by the qubit, we uniquely identify thermal shot noise of residual photons in the readout resonator as the dominant source of dephasing in our system, a result that applies to any qubit modality where the read out is implemented by a transverse dispersive coupling of the qubit to a resonator.  By implementing the CPMG dynamical-decoupling protocol, we are able mitigate to the adverse influence of the photon shot noise, and improve T₂^Echo ~ 40 us to reach T₂^CPMG ~ 80 μs ~ 2*T₁.