  We discuss the influence of a zero-temperature environment on a coherent
quantum system. First, we calculate the reduced density operator of the system
in the framework of the well-known, exactly solvable model of an oscillator
coupled to a bath of harmonic oscillators. Then, we propose the sketch of an
Aharonov--Bohm-like interferometer showing, through interference measurements,
the decrease of the coherence length of the system due to the interaction with
the environment, even in the zero temperature limit.
