Topic: "The Evolution of a Quantum Wave-function During Measurement: From Quantum Jumps to Feedback" (Video)
Abstract: The textbook picture of measurement in quantum mechanics describes an instantaneous process that transforms a quantum superposition into probabilistic classical outcomes. Since any realistic measurement protocol takes a finite amount of time, it is natural to ask: How does the quantum wavefunction evolve during the process of measurement? The theory of weak continuous measurement addresses this question by evolving the quantum wavefunction based on the partial information available in a given measurement time. In this talk, I will discuss experiments using superconducting electrical circuits to implement real-time monitoring of the quantum wavefunction as it evolves during a measurement.
Using microwaves, we continuously probe the state of a resonantly driven quantum two-level system—a superconducting quantum bit (qubit)—coupled to a cavity. The microwave signal, whose intensity sets the measurement strength, interacts with the cavity and is amplified using a near-noiseless parametric amplifier. Under strong measurement, we observe quantum jumps between the qubit states and study the quantum Zeno effect. In the weak measurement regime, the qubit evolution remains oscillatory but with a phase that diffuses slowly. We continuously track and correct this phase diffusion using feedback, causing the ensemble averaged oscillations to persist indefinitely—the first demonstration of quantum feedback in a solid-state system. These tools provide new techniques to track and control solid-state quantum systems and will also enable us to probe the boundary between the quantum and classical worlds.