Information and Measurements

The topic of measurements, information and measurement ‘back-action’ in quantum mechanics was endlessly confusing to the founders of the field who were forced to argue with each other solely in terms of ‘gedanken’ experiments.   The fantastic progress that has occurred in recent years means that we must face this confusion head on.  Yesterday’s gedanken experiments are today’s real experiments and these exquisitely precise results are putting our understanding of all the twists and turns of quantum measurements to the test.  As just one example, it is now possible to use optical techniques to measure the position of a macroscopic mirror with an uncertainty less than the diameter of a single proton!  However if we try to measure with even greater precision, the results actually become less certain rather than more certain. 

The last two decades have witnessed a revolution in our understanding of the role of information in quantum mechanics and have led to the notion of the quantum computer.  Similarly we now better understand the flow of information that occurs when measurements are made and we understand in a precise sense that ideal quantum measurements are ones that do not waste any information.  Finally we now understand that it is not just the experimentalist who makes intentional measurements but any ‘observer’ including the environment surrounding a quantum system.     The quantum coherence that is essential for the operation of any quantum computer is maintained only as long as no information is lost to the environment.

This talk will present a simple introductory tutorial on how real measurements work by entangling the state of the quantum system with the state of the measurement apparatus.  This will allow us to understand and how the choice of what is measured affects the state of the quantum system being observed.