Squinting at quantum systems

摘要

For the purpose of controlling a system, two facts appear self-evident. First, the more information one can obtain about the system, the better one can control it. Second, one needs to do more than just obtain information in order to control the system. In the quantum world, however, self-evidence cannot be trusted. Writing in Physical Review A, Ashhab and Nori refute the two 'facts' just given and show that a quantum system can be quickly driven to any desired state using a fixed type of measurement. Although various schemes have been proposed2'3 for driving a quantum system from one state to another using only quantum measurements, this is the first time it has been shown to be achievable using repetitions of a given measurement. Crucially, the authors' proposal requires the measurement to be unsharp. That is, one must avoid obtaining too much information about the system.