  Complementarity restricts the accuracy with which incompatible quantum
observables can be jointly measured. Despite popular conception, the Heisenberg
uncertainty relation does not quantify this principle. We report the
experimental verification of universally valid complementarity relations,
including an improved relation derived here. We exploit
Einstein-Poldolsky-Rosen correlations between two photonic qubits, to jointly
measure incompatible observables of one. The product of our measurement
inaccuracies is low enough to violate the widely used, but not universally
valid, Arthurs-Kelly relation.
