One limit on the performance of a quantum computer is unambiguous readout of data encoded in the qubit states. We analyze a method to measure the states of a coherently coupled dot device (qubit) using a single electron transistor. We derive a master equation to describe the unconditional evolution of the qubit when the measured records are averaged over, and a conditional stochastic master equation describes the conditional evolution of the qubit. The results from both descriptions are consistent.
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