Physics Department, Princeton University

Physics Colloquium - Michel Devoret, Yale University - “The Road to the Poles: Quantum Measurements that Steer rather than Collapse"
A quantum system subject to the infinitely-strong measurement of textbook physics undergoes a discontinuous, random state collapse. All phase information in a superposition of the eigenstates of the measurement apparatus is then suddenly erased from the system under observation. However, in practice, measurements often involve a finite-strength, continuous process whose iteration leads to a projective evolution only asymptotically. Moreover, if the observation apparatus is fully efficient informationally, the measured system can remain at all times in a pure state. The stochastic evolution of this pure state is trackable from the measurement record. Thus, an initial superposition of states can be usefully transformed by a partial measurement rather than be entirely destroyed. This striking property has been demonstrated in superconducting qubit experiments in which readout is performed by a microwave signal sent through a cavity dispersively coupled to the qubit, and thereafter processed by an amplifier operating at the quantum limit. Accurately monitoring a qubit state is an essential prerequisite for measurement-based feedback control of quantum systems.
Jadwin A10  ·  4:30 p.m.– 5:30 p.m.