  In a beautiful experiment performed about a decade ago, Goulielmakis et al.
made a direct measurement of the electric field of light waves [E. Goulielmakis
et al., Science 305, 1267-1269 (2004)]. However, they used a laser source to
produce the light field, whose quantum state has a null expectation value for
the electric field operator, so how was it possible to measure this electric
field? Here we present a quantum treatment for the f:2f interferometer used to
calibrate the carrier-envelope phase of the light pulses in the experiment. We
show how the special nonlinear features of the f:2f interferometer can change
the quantum state of the electromagnetic field inside the laser cavity to a
state with a definite oscillating electric field, explaining how the
"classical" electromagnetic field emerges in the experiment. We discuss that
this experiment was, to our knowledge, the first demonstration of an absolute
coherent superposition of different photon number states in the optical regime.
