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

by T. H. A. van der Reep, L. Rademaker, X. G. A. Le Large, R. H. Guis, T. H. Oosterkamp

Submission summary

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


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

Author comments upon resubmission

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.

Tom van der Reep
Louk Rademaker
Tjerk Oosterkamp

List of changes

We rewrote the introduction and abstract along the lines of the letter above. Throughout the paper we removed references to spontaneous collapse theories, as these do not contribute to our pragmatic line of reasoning and they distract from the point that we want to make. Furthermore, we added a comment on the stochasticity of the collapse as brought up by referee report 3. Without performing the actual calculation, we gave a sketch of what such a calculation and its expected result would look like at the end of section 6.

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