Continuous measurements and quantum trajectories in circuit QED
Abstract: Time evolution in quantum mechanics depends on the system's coupling to an external environment. A closed system retains its quantum information, evolving unitarily. By contrast, open quantum systems dissipate their information as they become entangled with the environment's fluctuations. Tracing out these external degrees of freedom causes superpositions to decay to classical mixed states. Alternatively, by monitoring and efficiently collecting the dissipated information, the environment behaves like a probe for quantum information, providing a noisy measurement record. Using this record, a shrewd physicist may conditionally update the system's state, resulting in a stochastic "quantum trajectory", driven by measurement back-action competing with unitary evolution. In this talk I will discuss experimental quantum trajectories of a superconducting qubit in a circuit QED environment. Here, photons act as information probes; adjusting their properties and interactions can result in a variety of measurement dynamics. The obtained trajectories agree with tomographic measurements and are a useful resource for quantum control, quantum thermodynamics, and even foundational questions.