adding iscss for issue #212 (#241)

Fixes #212

---------

Co-authored-by: Stefan Krastanov <[email protected]>

CHANGELOG.md

@@ -5,6 +5,10 @@
 
 # News
 
+## v0.9.1 - 2024-03-20
+
+- Implemented `iscss` function to identify whether a given code is known to be a CSS (Calderbank-Shor-Steane) code.
+
 ## v0.9.0 - 2024-03-19
 
 - **(breaking)** The defaults in `random_pauli` are now `realphase=true` and `nophase=true`.
@@ -13,7 +17,6 @@
 - Implement an inplace `random_pauli!`, a non-allocating alternative to `random_pauli`.
 - Significant improvement in the performance of the ECC decoder pipeline (but many low-hanging fruits still remain).
 
-
 ## v0.8.21 - 2024-03-17
 
 - Implemented the Gottesman code family, also known as [[2^j, 2^j - j - 2, 3]] quantum Hamming codes.

src/ecc/ECC.jl

@@ -12,7 +12,7 @@ using Nemo: ZZ, residue_ring, matrix
 
 abstract type AbstractECC end
 
-export parity_checks, parity_checks_x, parity_checks_z,
+export parity_checks, parity_checks_x, parity_checks_z, iscss,
     code_n, code_s, code_k, rate, distance,
     isdegenerate, faults_matrix,
     naive_syndrome_circuit, shor_syndrome_circuit, naive_encoding_circuit,
@@ -49,6 +49,14 @@ function parity_checks_z(code::AbstractECC)
     throw(lazy"Codes of type $(typeof(code)) do not have separate X and Z parity checks, either because they are not a CSS code and thus inherently do not have separate checks, or because its separate checks are not yet implemented in this library.")
 end
 
+function iscss(::Type{T}) where T<:AbstractECC
+    return false
+end
+
+function iscss(c::AbstractECC)
+    return iscss(typeof(c))
+end
+
 parity_checks(s::Stabilizer) = s
 Stabilizer(c::AbstractECC) = parity_checks(c)
 MixedDestabilizer(c::AbstractECC; kwarg...) = MixedDestabilizer(Stabilizer(c); kwarg...)

src/ecc/codes/bitflipcode.jl

@@ -2,5 +2,9 @@ struct Bitflip3 <: AbstractECC end
 
 code_n(c::Bitflip3) = 3
 
+function iscss(::Type{Bitflip3})
+    return true
+end
+
 parity_checks(c::Bitflip3) = S"_ZZ
                                Z_Z"

src/ecc/codes/css.jl

@@ -17,6 +17,10 @@ struct CSS <: AbstractECC
     end
 end
 
+function iscss(::Type{CSS})
+    return true
+end
+
 function parity_checks(c::CSS)
     extended_Hx = Matrix{Bool}(vcat(c.Hx, zeros(size(c.Hz))))
     extended_Hz = Matrix{Bool}(vcat(zeros(size(c.Hx)), c.Hz))

src/ecc/codes/shorcode.jl

@@ -2,6 +2,10 @@ struct Shor9 <: AbstractECC end
 
 code_n(c::Shor9) = 9
 
+function iscss(::Type{Shor9})
+    return true
+end
+
 parity_checks(c::Shor9) = S"ZZ_______
                             _ZZ______
                             ___ZZ____

src/ecc/codes/steanecode.jl

@@ -2,6 +2,10 @@
 
 struct Steane7 <: AbstractECC end
 
+function iscss(::Type{Steane7})
+    return true
+end
+
 parity_checks(c::Steane7) = S"___XXXX
                               _XX__XX
                               X_X_X_X

src/ecc/codes/toric.jl

@@ -4,6 +4,10 @@ struct Toric <: AbstractECC
     dz::Int
 end
 
+function iscss(::Type{Toric})
+    return true
+end
+
 code_n(c::Toric) = 2*c.dx*c.dz
 
 function parity_checks_xz(c::Toric)

test/runtests.jl

@@ -63,6 +63,7 @@ end
 @doset "ecc_encoding"
 @doset "ecc_syndromes"
 @doset "ecc_gottesman"
+@doset "ecc_iscss"
 @doset "precompile"
 @doset "pauliframe"
 @doset "allocations"

test/test_ecc_iscss.jl

@@ -0,0 +1,27 @@
+using Test
+using QuantumClifford
+using QuantumClifford.ECC
+using QuantumClifford.ECC: AbstractECC
+
+function is_css_matrix(H)
+    nrows, ncols = size(H)
+    for i in 1:nrows
+        has_x = false
+        has_z = false
+        for j in 1:ncols
+            has_x |= H[i,j][1]
+            has_z |= H[i,j][2]
+            has_x && has_z && return false
+        end
+    end
+    return true
+end
+
+known_all_codes = [Shor9(), Steane7(), Gottesman(3), Cleve8(), Perfect5(), Toric(8,8), CSS([0 1 1 0; 1 1 0 0], [1 1 1 1]), Bitflip3()]
+
+@testset "is CSS" begin
+    for code in known_all_codes
+        H = parity_checks(code)
+        @test iscss(code) == is_css_matrix(H)
+    end
+end