Séminaire de l'IQ : Philippe Lewalle

Generating and Stabilizing Entanglement with Time–Continuous Fluorescence Monitoring Philippe Lewalle, group of Andrew N. Jordan, University of Rochester Quantum theory posits that measurements are invasive: It describes an interactive process, mediating all knowledge of quantum properties, in which an observer probes a quantum system and draws infer- ences about its behavior based on the outcomes they receive. Methods to track a quantum system, using time–continuous weak measurements, have emerged over the last three decades as part of the larger land- scape of methods concerned with quantum control, communication, and computation. The stochastic quantum trajectories arising from continuous monitoring have now been realized in a variety of experi- mental contexts. After some general discussion of these methods, I will focus my talk on the entanglement of two qubits via continuous measurement of their spontaneous emission. Entanglement is a key resource for many quantum information and communication protocols, and many methods consequently exist to generate and protect it. It has also been at the center of many conceptual debates about the scope and in- terpretations of quantum theory, and remains of foundational interest. I will describe the entanglement dynamics of quantum jump trajectories arising from continuous photon counting, as well as those of diffusive trajectories arising from homodyne monitoring. These processes generate equivalent entangle- ment yield on average, despite qualitatively different dynamics in their respective individual realizations. In these and related schemes, the ability of an apparatus to erase information allowing the source of any signal to be determined (i.e. erasure of “which path” information) is critical in allowing measurements to become entangling. Using insights into the monitored two–qubit dynamics, we can then explore feed- back control strategies, with the goal of increasing the two–qubit entanglement yield and lifetime. Specif- ically, I will illustrate how to generate “entanglement echoes” via feedback and measurement un-doing, in order to trap the two–qubit dynamics in limit cycles around Bell states.