  Quantum operations describe any state change allowed in quantum mechanics,
including the evolution of an open system or the state change due to a
measurement. In this letter we present a general method based on quantum
tomography for measuring experimentally the matrix elements of an arbitrary
quantum operation. As input the method needs only a single entangled state. The
feasibility of the technique for the electromagnetic field is shown, and the
experimental setup is illustrated based on homodyne tomography of a twin-beam.
