  The problem of reduction of the wave packet of a relativistic charged
particle by emission of a photon is studied with help of the path integral
approach. A general expression for arbitrary order correlation function of the
electromagnetic field is obtained. As a specific example an ultrarelativistic
electron circulating in a storage ring is considered. It is shown that the
longitudinal width of the electron wave packet defined by characteristic
difference of time between registrations of two photons emitted by the electron
is of the order of the wave length of the photons.
