# An experimental proposal to study collapse of the wave function in travelling-wave parametric amplifiers

### Submission summary

 As Contributors: Xavier Le Large · Louk Rademaker · Tom van der Reep Arxiv Link: https://arxiv.org/abs/1811.01698v4 Date submitted: 2020-01-13 Submitted by: van der Reep, Tom Submitted to: SciPost Physics Discipline: Physics Subject area: Quantum Physics Approaches: Theoretical, Computational

### Abstract

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 did we really measure' the quantum state? In order to investigate whether the measurement' takes place in the amplification chain, we propose to construct a microwave interferometer that has a parametric amplifier added to each of its arms. Feeding the interferometer with single photons, the visibility depends on the gain of the amplifiers and whether a measurement collapse has taken place during the amplification process. We calculate the interference visibility as given by standard quantum mechanics as a function of gain, insertion loss and temperature and find a magnitude of $1/3$ in the limit of large gain without taking into account losses. This number reduces to $0.26$ in case the insertion loss of the amplifiers is $2.2$ dB at a temperature of $50$ mK. We show that if the wave function collapses within the interferometer, we will measure a reduced visibility compared to the prediction from standard quantum mechanics once this collapse process sets in.

###### Current status:
Editor-in-charge assigned

Dear Editor-in-charge,
We hereby resubmit our paper “An experimental proposal to study collapse of the wave function in travelling-wave parametric ampliﬁers” in response to the referee reports submitted so far.

We have chosen to simplify the message of the paper. To avoid the readers of the paper to think in line of the spontaneous collapse theories available to date, we have chosen a more pragmatic line of reasoning independent from the available spontaneous collapse theories.

If one considers a microwave qubit experiment, we know that the qubit can be in a superposition of states, e.g. resulting in a superposition containing zero and one photon. If this state is measured, we always measure either zero or one photon, implying the state has collapsed at some point in between the qubit and the output of the measurement apparatus. In our proposed experiment we want to investigate at which point this happens.
Typically, the qubit is connected to the measurement apparatus by an amplification chain. As a first stage we therefore want to know whether the superposed state survives amplification, or that the collapse happens during the amplification process.

We hope that this simplification in the motivation for our paper will avoid the reader to run into unintended obstacles and that this satisfies you and the referees so that you can decide to publish our manuscript.

Sincerely,
Tom van der Reep