Quantum computers and Annular Josephson junctions

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Discussion Overview

The discussion centers on the relationship between annular Josephson junctions and their role as qubits in quantum computing, as well as the definitions and distinctions between vortices and fluxons in this context. Participants explore both theoretical and practical aspects of these concepts.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant inquires whether annular Josephson junctions are currently used as qubits in quantum computers and seeks updates on progress since a 2003 article.
  • Another participant suggests that early superconducting qubit technologies, including annular Josephson junctions, have largely been replaced by more modern designs, particularly transmons.
  • A distinction is made between vortices, described as quantized magnetic flux rings that can be imaged, and fluxons, which are considered a more abstract concept related to quantized flux in circuits.
  • A question is raised about the possibility of vortices and fluxons being synonymous in the context of annular Josephson junctions.
  • One participant explains that the annular Josephson junction operates by using vortices as qubit states, with the quantized variable being flux, thus relating fluxons to the tunneling process, but emphasizes that they are not the same entity.

Areas of Agreement / Disagreement

Participants express differing views on the current relevance of annular Josephson junctions in quantum computing, with some suggesting they are outdated while others explore their theoretical implications. The relationship between vortices and fluxons remains contested, with no consensus reached on their definitions or equivalence.

Contextual Notes

Participants reference various types of qubits and their characteristics, indicating a spectrum from charge qubits to flux qubits, but do not resolve the complexities of these classifications or the implications for annular Josephson junctions.

LagrangeEuler
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Are annular Josephson junctions qubits in some quantum computer right now?
https://www.nature.com/articles/425133aI found this article from 2003. What is the progress right now?

Also are vortices and fluxons same thing?
 
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No, I believe it is fair to say that it was a bit of a dead end. None of the early superconducting qubit technologies are in much use anymore; they haven been superseded by more modern topologies (usually based on transmons)

A vortex is "physical thing", it is a "ring" (with a core) of quantised magnetic flux (google "image of Abricosov vortices") which can be imaged if you use the right type of microscope.

"Fluxons" is a more abstract term which typically refers to some form of quantised flux in a circuit.
There is a range of different types of qubits with charge qubits (tunnelling Cooper pairs) at one end and flux qubits at the other (tunnelling flux). Modern qubits sit somewhere closer to the middle of this range.
The "Fluxonium" is is an example of a modern flux-based design which looks promising, there are also more complicated versions such as the "heavy fluxonium" which is quite recent.
 
Is there any chance that in annular Josephson junctions fluxons and vortex are the same thing? Or are in any case those two are synonyms?
 
The basic idea of the annular JJ was -as far as I remember- to use vortices in one of two states as the qubit. Because of the design the quantised variable in this type of qubit is flux, which means that the "abstract things" which tunnels are fluxons.
So, at least using the terminology I am used to, the two are related but they are not the the same; you can have fluxons in qubits without vortices.
 

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