Reading-out the state of a qubit:: an analysis of the quantum measurement process

Quantum measurements have to be performed in order to read out the final state of qubits in quantum computers. For a Josephson charge qubit [Phys. Rev. Lett. 79 (1997) 2371; Nature 386 (1999) 305] this can be accomplished by coupling a single-electron transistor (SET) capacitively to the qubit. As long as no transport voltage is applied to the SET, no dissipative currents flow, and the presence of the SET merely renormalizes (weakly) the parameters of the qubit. To perform a measurement, the transport voltage is turned on. The measurement process makes use of the high sensitivity of the current through the SET (the meter) to the voltage applied to the middle island. Since different states of the qubit differ in charge, the voltage felt by the SET, and the macroscopic, measurable current through it, depend on the state of the qubit. (C) 2001 Elsevier Science B.V. All rights reserved.