
Spin(2N); four edges give Spin(8), whose spinors are 8
s
⊕8
c
. The minimal D4 defect therefore carries
exactly the Spin(8) spinor content that the bulk lattice supports—no larger group is forced, and none
needs to be broken. This is the representation the triality program of Refs. [4, 5, 6, 8] uses for a fermion
generation: the lattice organically populates the so(8) spinors rather than requiring a grand-unified group
above them.
We note two honest limits. First, the correspondence gives 8
s
⊕ 8
c
, both spinor chiralities on equal
footing; which is the left-handed weakly-coupled fermion, and why only that one couples, is the chiral
selection question left open in Section 7, and it is not resolved by the branching. Second, the D4 code
is high-rate: the logical structure does not saturate at the minimal qubit but grows with region size, so
“Spin(8)” is the group of the minimal logical qubit, not a unique feature of the geometry.
7 Fermionic versus chiral order: the obstruction and the open question
The result of Section 6 is that the defect is a fermion. This is weaker than, and must be distinguished
from, the claim that the construction realizes chiral electroweak structure, and the distinction is where
the relevant no-go theorems act.
A commuting-projector (stabilizer) code in two dimensions cannot realize chiral topological order
with nonzero chiral central charge: the c
−
extracted from the anyon data via the Gauss sum e
2πic
−
/8
=
|A|
−1/2
P
a
θ(a) is obstructed for such models [17, 23]. Fermionic topological order—an excitation
with θ = −1—is a distinct and weaker property, and it is realized by stabilizer codes; recent three-
dimensional stabilizer constructions explicitly realize fermionic and chiral topological orders [22]. Our
θ = −1 establishes the fermionic property. It does not by itself establish c
−
= 0, and the identification
with chiral electroweak structure requires that additional step.
Three features place the construction outside the hypotheses of the two-dimensional obstruction,
and we state them as the reasons the question is open rather than closed. First, the code is three/four-
dimensional, not two-dimensional, and chiral fermionic stabilizer codes are known to exist in three
dimensions [22]; the known examples show the obstruction is not automatically fatal outside the two-
dimensional commuting-projector hypotheses, but they do not prove that the present code is chiral.
Second, the matter defect’s logical operator is nonlocal—the minimal logical operator of Section 6 (five
edges on FCC, four on D4)—and no-go results for chiral states from local circuits explicitly exempt
constructions that are nonlocal in a spatial direction [24]. Third, the object we exhibit is a defect carrying
fermionic self-statistics, not a claimed chiral phase with a computed edge central charge. Whether the
construction realizes genuinely chiral (c
−
= 0) order remains open, and settling it—by computing the
anyon data and the Gauss sum for the FCC/D4 code, or by exhibiting the chiral edge mode—is the
natural next problem. We note that the framework must ultimately confront the Nielsen–Ninomiya
theorem [25] in its appropriate code-theoretic form; the present work does not claim to have done so.
8 Discussion
The defensible content of this paper is contained and checkable. The FCC no-go for a local inter-
nal SU (2)
L
is a representation-invariant exclusion. The main result is that the FCC/D4 matter defect
is a fermion, forced by the homology H
1
= 3, H
2
= 1 of its bond complex on the verified CSS
code, with the striking feature that the electric homology H
1
= 3 is the color multiplicity—so within
this framework color and fermionic statistics are one topological datum. The supporting representation
theory—so(8) triality, the 8
s
chiral fermion, the Spin(10) generation—is not original to this work; it
is the program of Refs. [4, 5, 6, 7, 8, 9], to which we add a stabilizer-code realization of the fermionic
statistics.
We have been explicit about the boundary of the claim. Fermionic statistics (θ = −1) is established;
7