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I am giving this bare list of references its own entry, so that it may be !include
-ed into related entries (such as topological quantum computation, anyon and Chern-Simons theory but maybe also elsewhere) for ease of updating and synchronizing
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I was looking for discussion whether pure braid gates are sufficient to model all topological quantum operations. These authors show that all “weaves” suffice (braids with a single “mobile” strand). All the example weaves in this article and the followup arXiv:2008.03542 are in fact pure, but the authors never comment on this.
My understanding is that their method shows that every braid is approximated by a pure weave up to, possibly, appending a single elementary braiding of a fixed pair of neighbouring strands. Which in practice should be good enough. But I am wondering if authors ever comment on this aspect.
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This shows many more examples of “weave” gates. Again, all of them are in fact pure braids (except for “injection weaves” and “F weaves”, but these are all meant to be intermediate).
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Jiannis K. Pachos, Quantum computation with abelian anyons on the honeycomb lattice, International Journal of Quantum Information 4 6 (2006) 947-954 (arXiv:quant-ph/0511273)
James Robin Wootton, Dissecting Topological Quantum Computation, 2010 (pdf)
“non-Abelian anyons are usually assumed to be better suited to the task. Here we challenge this view, demonstrating that Abelian anyon models have as much potential as some simple non-Abelian models.”
Seth Lloyd, Quantum computation with abelian anyons, Quantum Information Processing 1 1/2 (2002) (arXiv:quant-ph/0004010, doi:10.1023/A:1019649101654)
In
the authors write (first page):
all concrete known examples of non-abelian anyon theories with integer global dimension are constructed from a gauging of an abelian anyon theory.
I’d like to understand this statement in detail (preferably I’d like to understand if it can be expressed as a statement about something like “equivariant braid representations”?), but I have only a vague notion so far.
The authors don’t seem to come back to this statement in their main text, but from their preceding sentence it seems that it is meant to be taken from:
The discussion there is about taking a “unitary” modular tensor category equipped with a group action to something like the corresponding crossed product MTC. While I haven’t gone through it in much detail, that seems quite natural/plausible. But I don’t yet see an argument that “all concrete known examples” of MTCs (with integer global dimensions) arise this way.
I have a vague memory of statements like this from back when I was surrounded more by MTC theorists in Hamburg, but will need to remind myself.
On a different point:
I suspect that the relevant group of phases for physically realizable abelian anyons is not the full circle group $\mathbb{R}/\mathbb{Z}$, but just the “rational circle” $\mathbb{Q}/\mathbb{Z}$. Is there an authorative reference that would admit this?
Asking Google, I find a single decent hit:
Here the statement appears in the second paragraph, but just in passing and parenthetically, and then again on p. 2, above (11), in somewhat weaker form.
Is there a better reference?
Regarding #12:
I see from Kohno 2014 (pdf) that the flat line bundle on config space of interest in KZ-theory has phases given by a rational function of two real parameters (his (6.6)) and that the parameter values of interest form an open subset (his Thm. 7.1). So at least there is a large supply of “abelian anyons” of interest that have rational phases.
regarding #7:
Just to highlight a fun observation: What those physicist call “weave quantum gates” corresponds to the mondromy braid representations induced by the configuration space that is called “$Y_{\mathbf{z},1}$” in Etingof, Frenkel & Kirillov 1998, around Cor. 7.4.2.
It’s evident once one sees it. But I wonder if this has been made explicit anywhere before?
In fact it’s even better: it’s exactly the case of rational mondromy that corresponds to CFT-realizations: by the discussion in Etingof, Frenkel & Kirillov 1998, Sec. 13.4
For when the edit functionality is back, to add pointer to this preprint from today:
Here is a question on the (potential) experimental realization of anyon defects in crystalline materials:
For pure crystals, we may and usually do model them, mathematically, as the quotient space of Euclidean space by the given crystallographic group. For instance, this is done when classifying their topological phases by computing the twisted equivariant K-theory of these quotients.
This makes it appear natural to model punctures by puncturing this quotient space. While mathematically natural, in the real crystal this means, of course, to add an impurity not just in one single location – which might be most natural from an experimental perspective – but periodically, i.e. including for any one impurity also all its images under the crystallographic group.
So my question is: Do existing experimental realizations consider such periodic impurities?
It’s not easy to search for the answer to this question, but eventually I found this article:
Ville Lahtinen, Andreas W. W. Ludwig, Simon Trebst,
Perturbed vortex lattices and the stability of nucleated topological phases
On p. 1 this has the following parenthetical remark:
… anyons, usually arranged in a regular array to enable systematic control…
This seems to mean that the answer to the above question is Yes. But is there a reference that would say this a little more explicitly?
I see that theorists, at least, are happy with anyons on tori (the special case of the above where the point group is trivial):
Roberto Iengo, Kurt Lechner, Quantum mechanics of anyons on a torus, Nuclear Physics B 346 2–3 (1990) 551-575 (doi:10.1016/0550-3213(90)90292-L)
Yutaka Hosotani, Choon-Lin Ho, Anyons on a Torus, AIP Conference Proceedings 272 (1992) 1466 ; (arXiv:hep-th/9210112, doi:10.1063/1.43444)
Songyang Pu, J. K. Jain, Composite anyons on a torus, Phys. Rev. B 104 (2021) 115135 (arXiv:2106.15705, doi:10.1103/PhysRevB.104.115135)
Songyang Pu, Study of Fractional Quantum Hall Effect in Periodic Geometries (etda:21203sjp5650)
Ah, Guo Chuan Thiang kindly points out to me that punctures in the momentum-space torus are being considered and known to make good sense; these are the “Weyl points” in:
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Carlos Mochon, Anyons from non-solvable finite groups are sufficient for universal quantum computation, Phys. Rev. A 67 022315 (2003) $[$arXiv:quant-ph/0206128, doi:10.1103/PhysRevA.67.022315$]$
Carlos Mochon, Anyon computers with smaller groups, Phys. Rev. A 69 032306 (2004) $[$arXiv:quant-ph/0306063, doi:10.1103/PhysRevA.69.032306$]$
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finally hyperlinked these author names, hoping that I identified the initials correctly:
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These and similar claims are curious, in that they start with a setup that is manifestly not a topological quantum system, in this case
Quantinuum’s H2 trapped-ion quantum processor
and then let that system do something which is then claimed to be a topological quantum system.
I am not necessarily doubting that there is a way for this to be true, but since at face value it can’t be true just by definition of the terms, it can only by true in a more subtle sense, and therefore I’d like to see a careful explanation of what that more subtle sense really is.
The evident guess that we are really looking at a non-topological quantum system just simulating aspects of a truly topological quantum system is biefly addressed by the authors in the paragraph starting with
These experiments go beyond merely simulating non-Abelian order and statistics.
The reason given in the following sentence
The ions are entangled in precisely the same way as…
maybe leaves room to be expanded on.
We remember people tending to make an ontological leap of faith in these situations, for instance when they have their quantum computer exhibit a kind of entanglement expected of some kind of wormhole, and then claim it to be that wormhole. Here we have a quantum computer exhibit entanglement as expected of some kind of topologically ordered phase and next the claim for it to be in that phase.
I guess there is some sense in which these claims are correct, but it seems subtle and I’d like to see that subtlety discussed in detail.
Thanks for checking it out, since you’ve been immersing yourself in this literature, it’s a better take than mine. I agree that one has to be careful with what amounts to a press release from a QC company, hence the link to the paper for more details. But that needs to go through the usual publication scrutiny…
the usual publication scrutiny…
For a sobering reminder, recall the (de-)publication story of anyons in the form of Majorana zero modes.
On this desaster, Das Sarma & Pan (2021) concluded (p. 1):
serious problem of potential confirmation bias in the putative topological experimental discoveries often claimed in the literature since the theoretical prediction is precise, and condensed matter physics imposes no community standards on the definition of an experimental discovery as is common in high-energy physics.
This is particularly problematic for topological discoveries since…
But luckily Quantinuum is a commercial company and (or so is my understanding) will proceed with the proof of the pudding by building recognizably robust quantum gates in the forthfoming future using their latest results. Seeing these in action will change the face of the field.
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