  Charge-based qubits have been proposed as fundamental elements for quantum
computers. One commonly proposed readout device is the single-electron
transistor (SET). SETs can distinguish between localized charge states, but
lack the sensitivity to directly distinguish superposition states, which have
greatly enhanced coherence times compared with position states. We propose
introducing a third dot, and exploiting energy dependent tunnelling from the
qubit into this dot (bias spectroscopy) for pseudo-spin to charge conversion
and superposition basis readout. We introduce an adiabatic fast passage-style
charge pumping technique which enables efficient and robust readout via charge
shelving, avoiding problems due to finite SET measurement time.
