Rajamani Vijayaraghavan, Berkeley
Abstract
How does a quantum system evolve during measurement? The textbook picture of instantaneous, projective measurements only describes the situation before and after, but since any realistic experiment takes a finite amount of time, it is natural to talk about the evolution during the process. Surprisingly, using weak measurements it is possible to completely determine the evolution of the quantum state and look inside the so-called wavefunction collapse.
A key requirement for such experiments is a quantum-limited detector which maximizes the information extracted and I will describe our implementation using superconducting electrical circuits. In particular, I will discuss the continuous monitoring of a resonantly driven quantum two level system—a superconducting quantum bit (qubit)—using a near-noiseless parametric amplifier. When measured strongly, 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. This is the first demonstration of quantum feedback in a solid-state system and reveals its potential in fighting decoherence. I will discuss possible applications in quantum information processing and probing other solid-state systems.
Refreshments: 3:30-4:00 PHY 151