Role of metastable charge states in a quantum-dot spin-qubit readout*

Readout of a singlet-triplet qubit requires a spin-to-charge conversion mechanism, typically employing the spin blockade phenomenon, by which each spin state is mapped to a unique charge state followed by a charge state measurement using an electric field sensor such as a quantum point contact.\footnote{Petta et al., Science 327, 669 (2010).} We investigate alternative mechanisms for spin-to-charge conversion involving metastable excited charge states made possible by an asymmetry in the tunneling rates to the leads.\footnote{Studenikin et al., Appl. Phys. Lett. 101, 233101 (2012).} This technique is used to observe Landau-Zener-St\"uckelberg oscillations of the S-T$_{+}$ qubit within the (1,0) region of the charge stability diagram.\footnote{Mason et al., Phys. Rev. B 92, 125434 (2015).} The oscillations are $\pi$ phase shifted relative to those detected using the standard technique and display a non-sinusoidal waveform due to the increased relaxation time from the metastable state.\footnote{Granger et al., Phys. Rev. B 91, 115309 (2015).}