Extended Data Fig. 8: S − T− spin echo pulse sequence and data.
a The double dot is prepared in a superposition of \(\left\vert {S}^{+-}\right\rangle\) and \(\left\vert {T}_{-}^{++}\right\rangle\) via a π/2 pulse at the S − T− avoided crossing on the third spin funnel. Specifically, we prepare \(\left\vert {\psi }^{{\prime} }\right\rangle =\frac{1}{\sqrt{2}}(\left\vert {S}^{+-}\right\rangle +i\left\vert {T}_{-}^{++}\right\rangle )\) for real, positive \({\Delta }_{2}^{sv}\). The state evolves near ϵ=9 mV, where the S − T− splitting is approximately 50 MHz and varies roughly linearly with ϵ, for a total time te + δt, during which we apply a π pulse at the avoided crossing to refocus the dephasing. After the evolution is complete, we apply another π/2 pulse before PSB readout. b Echo amplitude decay as a function of the total qubit evolution time te. To determine the amplitude at each point, we fit the inhomogeneously broadened decay for fixed te and varying δt [27,54]. We find \({T}_{2}^{* }\approx 210\) ns and \({T}_{2}^{echo}=3.6\mu s\).
