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In the Rev. Mod. Phys. 80, 1083 (2008) Non-Abelian Anyons and Topological Quantum Computation, they make an aside in Section II.1.a that

as an aside, we mention that in 1 + 1D, quantum statistics is not well-defined since particle interchange is impossible without one particle going through another, and bosons with hard-core repulsion are equivalent to fermions.)

However, I have a naive understanding that the Kitaev wire is a 1D model that realizes Majorana Zero Modes. This is often thought of as a model to create anyons for topological quantum computation. How then do anyons appear in 1D?

In general, we have 1D models (1D Hubbard or Heisenberg model for example) where people do talk about fermion exchange statistics. In what sense are these not well defined? I generally thought of quantum statistics not as a `literal' moving of particles.

PS: I have some naive idea that anyons in the Kitaev wire model are actually MZM paired with SC vortices so this might be 2D. If someone could explain that concept that might clear up my confusion? Thanks in advance.

Edit: I am revising my question to be more precise. How are anyons braided in 1D? nanowire proposals? In particular, how do the proposals in Figure 3 of this review coincide with the 2D restriction? I do not see how one can move an anyon off the localized 0D end point into these T-junctions, how T-junctions count as exchange or interaction, or how projective measurements count as the 2D braid group.

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  • $\begingroup$ unusable link. can you provide the full reference? $\endgroup$ Commented Jul 9, 2021 at 22:46
  • $\begingroup$ Why is the link unusable? It takes you straight to the pdf Unpaired Majorana fermions in quantum wires, by Alexei Yu. Kitaev. $\endgroup$
    – joseph h
    Commented Jul 9, 2021 at 23:17
  • $\begingroup$ @josephh first link gives me an unreachable site…. You can have a look at the link by starting an edit to the post… $\endgroup$ Commented Jul 10, 2021 at 0:22
  • $\begingroup$ @ZeroTheHero Yes, you're right. The first link does not even point to a website. It's just "https:// " {then some words}. $\endgroup$
    – joseph h
    Commented Jul 10, 2021 at 0:32
  • $\begingroup$ This link seems to work : arxiv.org/abs/0707.1889 $\endgroup$
    – Kurt G.
    Commented Jul 10, 2021 at 7:03

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I'll try to answer the questions posed in the edit. To start with, I'll talk through the various methods for performing braids using T-junctions. Then I'll discuss the last question, about projective measurements.

Braiding with T-Junctions

To start with the proposal of Figure 3 in the review paper you cited, it might be worth going to the original paper (https://www.nature.com/articles/nphys1915) for a more in-depth explanation, particularly the supplemental information.

Basically, unpaired Majoranas in nanowires are pinned to the boundary between topological and non-topological regions of the superconducting wires. Whether or not a region is in the topological phase is controlled by electrical gates which can change the chemical potential of the region. So by tuning these gates, you can extend or shrink the length of a topological region in the wire, thus moving the Majoranas at its boundary.

In terms of how to move Majoranas past a T-junction, have a look at Figure 2 in the supplementary info of the paper I linked. We can think of a T-junction as being the spot where three different Majorana-carrying nanowires meet. Each of these wires can host Majoranas at its ends. However, there are tunnel-couplings set up between the endpoints of the three wires located at the junction, meaning that only one zero-energy Majorana can sit at that junction, while the other two Majoranas pair up into a finite energy fermion. This basically means that if an even number nanowires around the junction are in the topological phase, there is no unpaired Majorana located at the junction (both Majoranas located at the junction are paired up into a finite energy fermion), while if there are an odd number of topological wires around the junction, there is an unpaired Majorana located there. This effectively means that you can move Majoranas through the junction onto any of its branches by tuning the electrical gates.

So to braid two Majoranas, you move them around the T-junction according to one of the patterns shown in Figure 3 of the paper.

There are more-complex "flux-controlled" braiding proposals, like that of this paper (https://arxiv.org/abs/1111.6001). The idea with these proposals is that you have three topological nanowires meeting at a T-junction, which results in a single zero-energy Majorana located at the junction $\gamma_0$, along with three other Majoranas located at the other ends of the three wires, $\gamma_1$, $\gamma_2$, $\gamma_3$. The effective low-energy Hamiltonian of this system includes three terms, $\Delta_k i \gamma_0\gamma_k$ for $k=1,2,3$, with strengths $\Delta_k$ that can be tuned.

By tuning these three $\Delta_k$'s adiabatically you can braid any two of the Majoranas $\gamma_1$, $\gamma_2$, $\gamma_3$. This procedure is effectively an adiabatic version of a measurement-only protocol which I'll explain below.

Measurement-Only Braiding

These protocols involve a series of bilinear measurements - measurements of operators of the form $i\gamma_j\gamma_k$. For example see this paper (https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.101.010501) on measurement-only topological computation with arbitrary anyons, and this paper (https://arxiv.org/abs/1909.03002) for a more recent Majorana-specific discussion.

The idea is that you need two ancillary Majoranas, which we will call $\gamma_a$ and $\gamma_b$, in a known parity state $i\gamma_a\gamma_b = 1$, on top of the two Majoranas you wish to braid, $\gamma_1$ and $\gamma_2$. Suppose these Majoranas form a topological qubit with two others, $\gamma_3$ and $\gamma_4$, which is in the state $i\gamma_1\gamma_3 = +1$.

If we make a measurement of the operator $i\gamma_1\gamma_a$ and determine the result to be $+1$, we have effectively transferred the "ancilla state" to now lie between anyons $a$ and $1$. But the original logical state of the qubit stored between Majoranas 1-4 is not destroyed by this measurement since you cannot retrieve any information about the original state from the measurement. Hence, the logical state now must be stored in the anyons $\gamma_b, \gamma_2, \gamma_3,\gamma_4$.

You then similarly measure the operator $i\gamma_2\gamma_a$, and if the result is $+1$, the logical state is stored in Majoranas $\gamma_b,\gamma_1,\gamma_3,\gamma_4$ while the ancilla state is in $\gamma_2$ and $\gamma_a$. Finally we measure $i\gamma_a\gamma_b$ again. And if the result is once again $+1$ the logical state is now stored between $\gamma_2,\gamma_1,\gamma_3,\gamma_4$ and the ancilla state is back to where it started.

So how is the logical qubit affected? It started in the state $i\gamma_1\gamma_3 = +1$, then after the first measurement the logical Majoranas are in an eigenstate of $i\gamma_b\gamma_3$. This operator commutes with the second measurement but after the third measurement, the logical Majoranas are in an eigenstate of $i\gamma_2\gamma_3$. Hence the series of measurements have resulted in the transformation $\gamma_1 \rightarrow \pm \gamma_2$. Similarly you can show that $\gamma_2\rightarrow \mp \gamma_1$ (I haven't worked out the signs but one can do so).

These measurements have to produce the result $+1$ for the procedure to work as specified, so you more or less have to repeat the protocol until you get the desired measurement outcomes.

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  • $\begingroup$ This looks like a very thorough answer. Unfortunately it may not collect the number of upvotes it deserves as it will take time (for me at least) to digest it in full. $\endgroup$ Commented Aug 1, 2021 at 14:30
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    $\begingroup$ Well I hope it is useful! I don't mind taking the time to write it, even if it doesn't get too many upvotes, as it's the focus of my research so I'm happy to discuss it. $\endgroup$
    – Campbell
    Commented Aug 2, 2021 at 0:24
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As far as I know quasi particles in 2D allow braiding thus making topological quantum computers fault tolerant. Please see Kitaev's other paper. A working link to the paper that led to your question is in arXiv.

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  • $\begingroup$ @Andrew, regarding your more precise revised question: the guys write about Figure 3: " There are in fact many theoretical proposals on how to braid the end-localised MZMs using electrical gates..." and "..., it turns out not to be necessary to move quasiparticles to braid them." How that's done ? They refer to yet some other references. $\endgroup$
    – Kurt G.
    Commented Jul 10, 2021 at 19:16

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