Heisenberg's Uncertainty Principle: Three Faces, Two Roles

A review is given of precise formulations of three conceptually distinct but related manifestations of Heisenberg's Uncertainty Principle. This principle appears in the form of trade-off inequalities: for the widths of the position and momentum distributions in any quantum state; for the inaccuracies of any joint measurement of these quantities; and for the inaccuracy of a measurement of one of the quantities and the ensuing disturbance in the distribution of the other quantity. The uncertainty principle is often described as expressing a limitation of operational possibilities imposed by quantum mechanics. Here we demonstrate that apart from this negative role, the full content of the uncertainty principle also includes its positive role as a condition ensuring that mutually exclusive experimental options can indeed be reconciled if an appropriate trade-off is accepted. Finally, we survey models and experimental implementations of joint measurements of position and momentum and comment briefly on the status of experimental tests of the uncertainty principle