An Experimental Proposal to Study Collapse of the Wave Function in Traveling‐Wave Parametric Amplifiers

The readout 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”? 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. A visibility of 1/3 is found in the limit of large gain without considering 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 with its magnitude predicted by standard quantum mechanics once this collapse process sets in. A novel experiment is proposed to test the limits of quantum mechanics by building a single‐photon microwave interferometer with a parametric amplifier in both interferometer arms. The experiment's expected visibility is calculated if transmission through the interferometer is governed by quantum mechanics and points out what differences may be observed in the outcome if a spontaneous collapse occurs.