ABSTRACT
In Chapter 3, we have discussed the direct measurement scheme in
which a measured field is directly absorbed by photodetectors. In
the direct detection process, the field is destroyed as the detector
absorbs all the field interacting with it. From the point of view of
quantum physics, the measurement destroys the state vector of the
field such that the state collapses to the appropriate eigenstate of the
measured field. For example, if prior to the measurement the state
vector of the field is |α〉 + |β〉, at the measurement the state vector collapses to |α〉 if the value α is obtained, or |β〉 if β is obtained.
