  We investigate the measurement-induced dephasing of a qubit coupled with a
single-mode cavity in the vacuum limit. Dephasing of the qubit state takes
place through the ntanglement of the qubit and the single probe photon sent to
the cavity, while the cavity mode never occupies a photon. We find that the
qubit state is dephased even if the cavity is always in the vacuum state. This
dephasing is caused purely by the interaction between the qubit and the vacuum
field. We also show that this vacuum-fluctuation-induced dephasing takes place
much faster than the spontaneous decay of the qubit excited state, and
therefore our prediction is observable in a real experiment.
