a, Pulse scheme. The system is adiabatically tuned from the initialization to the lower state. An additional π-pulse is used to rotate it to the upper state. b, Measured exponential decay of the lower (data shown in green; fit in dark green) and upper (data indicated in blue; fit in dark blue) state. The Bloch sphere in the inset shows the state of the Bloch vector at selected times, which are marked in the same colour in a.