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.

Host:  Lev Ioffe