  Even if the electromagnetic field does not create pairs, virtual pairs lead
to the appearance of a phase in vacuum-vacuum amplitude. This makes it
necessary to distinguish the in- and out-solutions even when it is commonly
assumed that there is only one complete set of solutions as, for example, in
the case of a constant magnetic field. Then in- and out-solutions differ only
by a phase factor which is in essence the Bogoliubov coefficient. The
propagator in terms of in- and out-states takes the same form as the one for
pair creating fields. The transition amplitude for an electron to go from an
initial in-state to out-state is equal to unity (in diagonal representation).
This is in agreement with Pauli principal: if in the field there is an electron
with given (conserved) set of quantum numbers, virtual pair cannot appear in
this state. So even the phase of transition amplitude remains unaffected by the
field. We show how one may redefine the phases of Bogoliubov coefficients in
order to express the vacuum-vacuum amplitude through them.
