
on which we will construct the su(3) generators. The basis vectors |1⟩, |2⟩, |3⟩ are the three skew-
pair states of the trapped node; the inner product is the standard Hermitian inner product on C
3
.
The states are by construction orthonormal: ⟨i|j⟩ = δ
ij
.
4 The Weyl group from anchor selection
4.1 The action of S
4
on the skew pairs
The full permutation group of {A, B, C, D} is the symmetric group S
4
of order 4! = 24. Each
element σ ∈ S
4
acts on edges by permuting endpoints, σ({X, Y }) = {σ(X), σ(Y )}, and on skew
pairs by acting on each edge:
σ(P ) = σ({e
1
, e
2
}) = {σ(e
1
), σ(e
2
)}. (4)
Since the property of being a skew pair is preserved under permutations (vertex-disjointness is
permutation-invariant), σ(P
i
) is again a skew pair. We obtain a homomorphism
π : S
4
→ Sym({P
1
, P
2
, P
3
})
∼
=
S
3
, σ 7→
P
i
7→ σ(P
i
)
. (5)
Proposition 1. The kernel of π is the Klein four-group
V
4
= {e, (AB)(CD), (AC)(BD), (AD)(BC)} ⊂ S
4
. (6)
Proof. We compute π(g) for each g ∈ S
4
by direct enumeration, focusing on the four elements of
(6).
• π(e) fixes each pair: e(P
i
) = P
i
. Trivially in the kernel.
• π((AB)(CD)): applying (AB)(CD) to P
1
= {AB, CD} gives {(AB)(CD)({A, B}), (AB)(CD)({C, D})} =
{{B, A}, {D, C}} = {AB, CD} = P
1
. Applying it to P
2
= {AC, BD}gives {{B, D}, {A, C}} =
P
2
. Similarly P
3
7→ P
3
. In the kernel.
• π((AC)(BD)) and π((AD)(BC)): by the same computation, each fixes P
1
, P
2
, P
3
as sets. In
the kernel.
For any other element of S
4
, π(g) = e. For example, π((AB)): applying (AB) to P
2
= {AC, BD}
gives {BC, AD} = P
3
, so (AB) sends P
2
to P
3
, and is not in the kernel. The four elements of V
4
are therefore the complete kernel.
By the first isomorphism theorem,
S
4
/V
4
∼
=
Im(π) = S
3
. (7)
The image is all of S
3
because every transposition of skew pairs (P
i
P
j
) is realized by some element
of S
4
(e.g., the swap (P
1
P
2
) is induced by (AC)).
Anchor selection in the matter paper picks one of the four vertices — without loss of generality
A — as the channel coupling the defect to the bulk. The stabilizer of A in S
4
is the symmetric group
on {B, C, D}, isomorphic to S
3
. Composing the inclusion S
3
→ S
4
with π gives an isomorphism
S
3
∼
−→ S
4
/V
4
, (8)
since no non-trivial permutation of {B, C, D} lies in V
4
. The residual symmetry of the defect after
anchor selection is therefore precisely the quotient S
4
/V
4
∼
=
S
3
.
3