The quantum radiation field inside a double cavity is examined. The double
cavity consists of a cavity containing a reflecting mirror. Two independent
radiation fields are present inside the cavity. It is then assumed that th
e mirror becomes suddenly transparent. The double cavity transforms thus in
to a single cavity. The relationship between the radiation fields before an
d after the change is investigated. It is shown that the sudden change exci
tes the radiation modes of the single cavity. Only modes having a node at t
he position of the mirror are not excited. The sudden change of the mirror'
s reflectivity can thus generate photons from the vacuum. It is also shown
that the sudden change produces squeezing. These features of the double cav
ity are then employed in a gedankenexperiment for the generation of entangl
ement between noninteracting atoms with the quantum eraser technique. [S105
0-2947(99)07007-9].