Applied Physics Seminar

The length of time that a quantum system can exist in a coherent superposition is determined by how strongly it interacts with its environment.  Unmonitored environmental fluctuations can be viewed as a source of noise, causing random evolution of the quantum system from an initially pure state into a statistical mixture. However, by accurately measuring the environment in real time, the quantum system can be maintained in a pure state and its time evolution described by a 'quantum trajectory' determined by the measurement outcome.   I will discuss how we use weak measurements to monitor a microwave cavity embedding a superconducting qubit and track the individual quantum trajectories of the system.   We analyze ensembles of trajectories to determine statistical properties such as the most likely path and most likely time connecting pre and post-selected quantum states. Furthermore, by introducing a qubit drive, we investigate the interplay between unitary state evolution and non-unitary measurement dynamics.  Our results reveal how quantum trajectories underlie the processes of measurement and decoherence and suggest new means for implementing "quantum steering" - harnessing action at a distance to manipulate quantum states through measurement.