Professor Alexander N. Korotkov
University of California, Riverside

Continuous quantum measurement of solid-state qubits

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? (Particular realization is a “charge” qubit measured by a quantum point contact or a single-electron transistor.) While for ensemble of qubits the measurement simply leads to decoherence, the evolution of a single qubit is significantly different: it depends on the noisy detector output and may be fully coherent, though non-unitary. The theory describing such evolution has been developed relatively recently and provides a number of experimentally testable predictions. As an example, we will discuss how to maintain nondecaying Rabi oscillations in a qubit by using a quantum feedback loop. Other experimental proposals include qubit entanglement by measurement, quantum nondemolition squeezing of a nanoresonator, and undoing of a weak quantum measurement of a qubit. The first experiment verifying the coherent non-unitary evolution of a superconducting qubit due to continuous measurement has been recently realized.