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I have stumbled across these papers 1 and 2. I can't seem to work out how they are defining a 2 way quantum computer other than an analogy and then they jump to something that seems vastly more powerful than a regular quantum computer (in that it can run 'Grovers' In a single query). Is this a different version of quantum computers that we don't know how to realise in the real world?

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    $\begingroup$ From a cursory look at the abstracts and introductions of those references, the papers seem extremely hard to parse, and at some points borderline meaningless. I doubt you'll be able to understand much of what's in there (if you want to be optimistic and say that there is something there). $\endgroup$
    – glS
    Commented Jun 17 at 12:05
  • $\begingroup$ Sentences like "As e.g. pull/push, negative/positive pressure, stimulated emission/absorption causing deexcitation/excitation are CPT analogs, and one can be used for state preparation, the second should allow for its CPT analog, referred here as CPT(state preparation) - allowing for additional chosen enforcement of the final state, its more active treatment than measurement." don't mean much of anything. Also weirdly enough, both papers contain essentially the same above "puzzling" wording, with very minor changes, which is a bit sus (it might just be laziness, but still, not a good sign). $\endgroup$
    – glS
    Commented Jun 17 at 12:06
  • $\begingroup$ I also just realised one of the paper was previously discussed here, see quantumcomputing.stackexchange.com/q/33411/55 $\endgroup$
    – glS
    Commented Jun 17 at 12:07
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    $\begingroup$ @glS ... by the author. $\endgroup$ Commented Jun 17 at 19:56
  • $\begingroup$ Please see some introduction to 2WQC with code: community.wolfram.com/web/community/groups/-/m/t/3157512 , or recent talk: youtube.com/watch?v=OEywgfbqUas $\endgroup$
    – Jarek Duda
    Commented Jun 18 at 6:29

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In my view, the whole concept of 2WQC is built on a misconception of the CPT symmetry of physics.

Yes, the laws of nature as we know them are invariant under the combined transformation of charge inversion (C), parity (P) and time inversion (T).

But this does not mean that we can simply invert time in the laboratory or in any other practical context. The authors seem to suggest that we could do so and hence force a specific measurement outcome (or: "CPT state preparation" as they call it).

If their claims turned out to be solid, we would actually have to rethink quantum mechanics and physics as a whole. To put it mildly: I am highly skeptical.

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  • $\begingroup$ It is not about inverting time, but reversing process used for state preparation - e.g. sequence of EM impulses for silicon quantum dots. Generally perform process which in CPT symmetry perspective is the process used for state preparation |0>, and you get conjugated <0|. $\endgroup$
    – Jarek Duda
    Commented Jun 18 at 6:27
  • $\begingroup$ This "process inversion" means you can force measurement outcomes on superpositions. If this were possible you could effectively (by some transformations) do it also on one part of a spacelike seperated Bell-pair and thus, send information with over-light speed (which would mean that in some reference frame you send the information back in time). $\endgroup$
    – qubitzer
    Commented Jun 18 at 7:29
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2WQC combines assumption that there is available process for state preparation, with requirement of CPT symmetry of physics:

If in CPT perspective we perform the process used for state preparation $|0\rangle$, in standard perspective it would prepare conjugated $\langle 0 |$.

While some state preparations require irreversible processes like thermalization, spontaneous emission, there are also state preparations using reversible processes e.g. stimulated emission-absorption equations being switched in CPT perspective - with absorption of laser light we can prepare excited $| 1\rangle$, hence stimulated emission equation would allow to prepare $\langle 1|$, and it is confirmed to act on external targets (e.g. https://onlinelibrary.wiley.com/doi/full/10.1002/anie.202305817 ).

Or silicon quantum dots performing all operations with EM impulses - just use reversed state preparation impulse sequence at the end, and from CPT perspective you have performed state preparation.

Introduction with Wolfram Quantum Framework code: https://community.wolfram.com/web/community/groups/-/m/t/3157512

enter image description here

Update: Turned out there is ~experimental confirmation: STED microscopy using stimulated emission on external target (donut below from https://onlinelibrary.wiley.com/doi/10.1111/joim.12278 ) - as required for 2WQC.

enter image description here

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  • $\begingroup$ Can you describe the rules of the 2WQC without the physics and just the rules that this form of QC obeys? My impression is that its unitary transformations combined with deterministic projections, which is where all the speed ups is coming from? $\endgroup$ Commented Jun 18 at 8:11
  • $\begingroup$ @Steve_smith, Sure, mathematically it is just multiplication of unitary operator from both sides, supported e.g. by Wolfram Quantum Framework: community.wolfram.com/web/community/groups/-/m/t/3157512 , generally it acts as postselection, replacing selection with physical constraints. $\endgroup$
    – Jarek Duda
    Commented Jun 18 at 8:56
  • $\begingroup$ A ket is a state in my Hilbert space and a unitary is a mapping from the Hilbert state to itself. Multiplication is only defined on the left hand side between a unitary and a ket? Would there even be a description for this in the density matrix representation? $\endgroup$ Commented Jun 18 at 9:28
  • $\begingroup$ @Steve_smith, to multiply from both directions you need to use psi, psi^dagger - such conjugation is performed by CPT symmetry - using process which in CPT perspective is the original state preparation process, should give conjugated state preparation. $\endgroup$
    – Jarek Duda
    Commented Jun 18 at 9:37
  • $\begingroup$ A normal density matrix \rho would evolve as U\rho U^\dagger, are you suggesting instead to do U \rho U? If not I dont see how this description is different to regular QM? $\endgroup$ Commented Jun 18 at 9:41

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