  The interference pattern of coherent electrons is effected by coupling to the
quantized electromagnetic field. The amplitudes of the interference maxima are
changed by a factor which depends upon a double line integral of the photon
two-point function around the closed path of the electrons. The interference
pattern is sensitive to shifts in the vacuum fluctuations in regions from which
the electrons are excluded. Thus this effect combines aspects of both the
Casimir and the Aharonov-Bohm effects. The coupling to the quantized
electromagnetic field tends to decrease the amplitude of the interference
oscillations, and hence is a form of decoherence. The contributions due to
photon emission and to vacuum fluctuations may be separately identified. It is
to be expected that photon emission leads to decoherence, as it can reveal
which path an electron takes. It is less obvious that vacuum fluctuations also
can cause decoherence. What is directly observable is a shift in the
fluctuations due, for example, to the presence of a conducting plate. In the
case of electrons moving parallel to conducting boundaries, the dominant
decohering influence is that of the vacuum fluctuations. The shift in the
interference amplitudes can be of the order of a few percent, so experimental
verification of this effect may be possible. The possibility of using this
effect to probe the interior of matter, e.g., to determine the electrical
conductivity of a rod by means of electrons encircling it is discussed.
(Presented at the Conference on Fundamental Problems in Quantum Theory,
University of Maryland, Baltimore County, June 18-22, 1994.)
