Non-projective measurement of solid-state qubits:
collapse and uncollapse
Alexander N. Korotkov
The starting point of the talk is a
simple question: what happens to a solid-state qubit in the process of its
continuous measurement by a detector? While for an ensemble of qubits the
measurement simply leads to decoherence, a single qubit state changes in
accordance with gradually acquired information, and there is no decoherence due
to measurement if a good detector is used. The theory of such measurement
allows us to understand what is "inside" the quantum collapse and
provides a number of experimentally testable predictions. As an example, it
shows that partial collapse due to a weak quantum measurement can be undone, fully
restoring ("uncollapsing") an arbitrary initial state. Another
potentially useful application is quantum feedback control of a qubit. So far
three experiments on non-projective collapse of solid-state qubits have been
realized: partial collapse of a superconducting phase qubit, uncollapse of a phase qubit, and persistent Rabi
oscillations in a superconducting charge qubit.