add check_repr_commutation_relation to test the CSS orthogonality c…

…ondition for 2BGA's Group Algebra with a General Group `G` (#403) Co-authored-by: Stefan Krastanov <github.acc@krastanov.org>
QuantumSavory · Nov 5, 2024 · 5531336 · 5531336
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diff --git a/ext/QuantumCliffordHeckeExt/QuantumCliffordHeckeExt.jl b/ext/QuantumCliffordHeckeExt/QuantumCliffordHeckeExt.jl
@@ -5,14 +5,15 @@ using DocStringExtensions
 import QuantumClifford, LinearAlgebra
 import Hecke: Group, GroupElem, AdditiveGroup, AdditiveGroupElem,
     GroupAlgebra, GroupAlgebraElem, FqFieldElem, representation_matrix, dim, base_ring,
-    multiplication_table, coefficients, abelian_group, group_algebra
+    multiplication_table, coefficients, abelian_group, group_algebra, rand
 import Nemo
 import Nemo: characteristic, matrix_repr, GF, ZZ, lift
 
 import QuantumClifford.ECC: AbstractECC, CSS, ClassicalCode,
     hgp, code_k, code_n, code_s, iscss, parity_checks, parity_checks_x, parity_checks_z, parity_checks_xz,
-    two_block_group_algebra_codes, generalized_bicycle_codes, bicycle_codes
+    two_block_group_algebra_codes, generalized_bicycle_codes, bicycle_codes, check_repr_commutation_relation
 
+include("util.jl")
 include("types.jl")
 include("lifted.jl")
 include("lifted_product.jl")

diff --git a/ext/QuantumCliffordHeckeExt/util.jl b/ext/QuantumCliffordHeckeExt/util.jl
@@ -0,0 +1,15 @@
+"""
+Checks the commutation relation between the left and right representation matrices
+for two randomly-sampled elements `a` and `b` in the group algebra `ℱ[G]` with a general group `G`.
+It verifies the commutation relation that states, `L(a)·R(b) = R(b)·L(a)`. This
+property shows that matrices from the left and right representation sets commute
+with each other, which is an important property related to the CSS orthogonality
+condition.
+"""
+function check_repr_commutation_relation(GA::GroupAlgebra)
+    a, b = rand(GA), rand(GA)
+    # Check commutation relation: L(a)R(b) = R(b)L(a)
+    L_a = representation_matrix(a)
+    R_b = representation_matrix(b, :right)
+    return L_a * R_b == R_b * L_a
+end
diff --git a/src/ecc/codes/util.jl b/src/ecc/codes/util.jl
@@ -6,3 +6,6 @@ function hgp(h₁,h₂)
     hz = hcat(kron(LinearAlgebra.I(n₁), h₂), kron(h₁', LinearAlgebra.I(r₂)))
     hx, hz
 end
+
+"""Implemented in a package extension with Hecke."""
+function check_repr_commutation_relation end
diff --git a/test/test_ecc_base.jl b/test/test_ecc_base.jl
@@ -1,6 +1,7 @@
 using Test
 using QuantumClifford
 using QuantumClifford.ECC
+using QuantumClifford.ECC: check_repr_commutation_relation
 using InteractiveUtils
 
 import Nemo: GF
@@ -46,6 +47,7 @@ other_lifted_product_codes = []
 # [[882, 24, d≤24]] code from (B1) in Appendix B of [panteleev2021degenerate](@cite)
 l = 63
 GA = group_algebra(GF(2), abelian_group(l))
+@test check_repr_commutation_relation(GA) # TODO use this check more pervasively throughout the test suite
 A = zeros(GA, 7, 7)
 x = gens(GA)[]
 A[LinearAlgebra.diagind(A)] .= x^27