Although in introductory quantum mechanics courses, measurements are treated as instantaneous events, the correct description is much more subtle. Like any realistic physical process, measurements and decoherence occur over a finite time scale which can be resolved given sufficiently sensitive tools. The advanced measurement group (AMG) is dedicated to building quantum limited amplifiers which are capable of studying and exploiting the dynamics of a measurement. We also use these tools to explore the applications of this enabling technology.

One of the seminal results was the observation of quantum trajectories in superconducting circuits, which involved watching a superposition state collapse. The ability to monitor these dynamics also opens the possibility of acting on the state during the collapse, which led to an experiment in which feedback was used to protect the coherent dynamics of a qubit from decoherence. High-efficiency measurements also allowed us to implement a highly non-standard measurement which is capable of generating entanglement between two qubits that are too far apart to directly interact. We are currently working to further improve quantum limited amplifier technology and implementing enhanced measurement techniques based on quantum feedback, squeezed light and other non-classical systems, which allow us to surpass the standard precision limits of measurement.

High efficiency, quantum limited amplifiers enabled the observation of quantum trajectories, or the gradual collapse of a superposition under measurement. These dynamics are rich and have opened many avenues for further study. The above picture depicts a set of paths through Hilbert space when a system is driven and measured at the same time. An action principle developed by theory collaborators allowed for the prediction of the most likely path that the system takes.