Universal quantum observables

It is shown how one can estimate the ensemble average of all operators of a quantum system by measuring only one fixed universal observable on an extended Hilbert space. This is equivalent to run a tomographic reconstruction in a kind of quantum parallelism, measuring all the quorum observables with a single universal observable. An experimental implementation in quantum optics is given, based on Kerr cross-phase modulated homodyning. (C) 2002 Elsevier Science B.V. All rights reserved.