Multilevel effects in the Rabi oscillations of a Josephson phase qubit

TL;DR

This study investigates multilevel effects in Rabi oscillations of a Nb/AlOx/Nb Josephson phase qubit, revealing how higher excited states influence qubit dynamics at high power through experiments and simulations.

Contribution

It provides a detailed analysis of multilevel effects in Josephson phase qubits using combined experimental measurements and theoretical modeling.

Findings

Identification of resonance shifts due to higher levels

Observation of Rabi frequency suppression at high power

Detection of leakage to higher excited states

Abstract

We present Rabi oscillation measurements of a Nb/AlOx/Nb dc superconducting quantum interference device (SQUID) phase qubit with a 100 um^2 area junction acquired over a range of microwave drive power and frequency detuning. Given the slightly anharmonic level structure of the device, several excited states play an important role in the qubit dynamics, particularly at high power. To investigate the effects of these levels, multiphoton Rabi oscillations were monitored by measuring the tunneling escape rate of the device to the voltage state, which is particularly sensitive to excited state population. We compare the observed oscillation frequencies with a simplified model constructed from the full phase qubit Hamiltonian and also compare time-dependent escape rate measurements with a more complete density-matrix simulation. Good quantitative agreement is found between the data and simulations, allowing us to identify a shift in resonance (analogous to the ac Stark effect), a suppression of the Rabi frequency, and leakage to the higher excited states.