Quantum Back-Action of an Individual Variable-Strength Measurement
Michael Hatridge, Shyam Shankar, Mazyar Mirrahimi, Flavius Schackert, K. Geerlings, T. Brecht, Katrina Sliwa, Baleegh Abdo, Luigi Frunzio, S. M. Girvin, R. J. Schoelkopf, Michel Devoret · Science · 2013
Measuring a quantum system can randomly perturb its state. The strength and nature of this back-action depend on the quantity that is measured. In a partial measurement performed by an ideal apparatus, quantum physics predicts that the system remains in a pure state whose evolution can be tracked perfectly from the measurement record. We demonstrated this property using a superconducting qubit dispersively coupled to a cavity traversed by a microwave signal. The back-action on the qubit state of a single measurement of both signal quadratures was observed and shown to produce a stochastic operation whose action is determined by the measurement result. This accurate monitoring of a qubit state is an essential prerequisite for measurement-based feedback control of quantum systems.