In this work we are interested in dephasing or decoherence in the zero-temperature limit. The only source of decoherence are then provided by vacuum (zero-point) fluctuations. Concern with vacuum fluctuations has a long history starting with theories of black-body radiation and the Planck spectrum and important effects like the Lamb shift, the Casimir effect and the Debye-Waller factor. More recently, the role of vacuum fluctuations was discussed in theories of macroscopic quantum tunneling and even more closely related to our subject in theories of macroscopic quantum coherence. In mesoscopic physics, we deal with systems that are so small and are cooled to such low temperatures that the wave nature of electrons becomes important and interference effects become measurable. Dephasing processes are therefore also of central importance in mesoscopic physics. Ultimately in the zero-temperature limit only vacuum fluctuations remain and it is clearly very interesting and fascinating to inquire about a possible role of such fluctuations.
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