  The read-out of a microwave qubit state occurs using an amplification chain
that enlarges the quantum state to a signal detectable with a classical
measurement apparatus. However, at what point in this process is the quantum
state really 'measured'? In order to investigate whether the `measurement'
takes place in the amplification chain, in which a parametric amplifier is
often chosen as the first amplifier, it is proposed to construct a microwave
interferometer that has such an amplifier added to each of its arms. Feeding
the interferometer with single photons, the interference visibility depends on
the gain of the amplifiers and whether a measurement collapse has taken place
during the amplification process. The visibility as given by standard quantum
mechanics is calculated as a function of gain, insertion loss and temperature.
We find a visibility of 1/3 in the limit of large gain without taking into
account losses, which is reduced to 0.26 in case the insertion loss of the
amplifiers is 2.2 dB at a temperature of 50 mK. It is shown that if the wave
function collapses within the interferometer, the measured visibility is
reduced compared to its magnitude predicted by standard quantum mechanics once
this collapse process sets in.
