An “almost diagonal” reduced density matrix (in coordinate representation) is usually a result of environment induced decherence and is considered the sign of classical behavior. We show that the proton of a ground state hydrogen atom can indeed possess such a density matrix. This example demonstrates that the “almost diagonal” structure may be derived from an interaction with a low number of degrees of freedom which play the role of the environment. We also show that decoherence effects in our example can only be observed if the interaction with the measuring device is significantly faster than the interaction with the environment (the electron). In the opposite case, when the interaction with the environment is significant during the measurement process, coherence is maintained. Finally, we propose a neutron scattering experiment on cold He atoms to observe decoherence which shows up as an additional positive contribution to the differential scattering cross section. This contribution is inversely proportional to the bombarding energy.
 D. Giuliani, E. Joos, C. Kiefer, J. Kupsch, I.-O. Stamatescu, H.D. Zeh: Decoherence and the Appearence of a Classical World, Springer, New-York, Heidelberg, Berlin, 1995 and references therein.
 J.P. Paz and W.H. Zurek: “Environment-induced Decoherence and the Transition from Quantum to Classical”, Course lectures at the 72nd Les Houches Summer School on “Coherent Matter Waves”, July–August, 1999, http://xxx.lanl.gov/quantph/0010011.
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