fix for wrong SIMD mul_left! #320 (#322)

Author: Krastanov

CHANGELOG.md

@@ -5,6 +5,9 @@
 
 # News
 
+## v0.9.7 - 2024-07-23
+
+- **(fix `#320`)** Fix a serious correctness bug in the SIMD implementation of Pauli string multiplication (affects the correctness of canonicalization and traceout for tableaux bigger than ~500 qubits; does not affect symbolic gates or Pauli frame simulations of any scale)
 ## v0.9.6 - 2024-07-12
 
 - `inv` implementation for single-qubit "symbolic" Clifford operators (subtypes of `AbstractSingleQubitOperator`).

Project.toml

@@ -1,7 +1,7 @@
 name = "QuantumClifford"
 uuid = "0525e862-1e90-11e9-3e4d-1b39d7109de1"
 authors = ["Stefan Krastanov <[email protected]> and QuantumSavory community members"]
-version = "0.9.6"
+version = "0.9.7"
 
 [deps]
 Combinatorics = "861a8166-3701-5b0c-9a16-15d98fcdc6aa"

src/mul_leftright.jl

@@ -89,7 +89,7 @@ function mul_ordered!(r::AbstractVector{T}, l::AbstractVector{T}; phases::Val{B}
             r[i+len+lane] = newz1 = z1 ⊻ z2
             x1z2 = x1 & z2
             anti_comm = (x2 & z1) ⊻ x1z2
-            cnt2 ⊻= (newx1 ⊻ newz1 ⊻ x1z2) & anti_comm
+            cnt2 ⊻= (cnt1 ⊻ newx1 ⊻ newz1 ⊻ x1z2) & anti_comm
             cnt1 ⊻= anti_comm
         end
         for i in 1:length(cnt1)

test/test_mul_leftright.jl

@@ -5,7 +5,7 @@ using Test
 
 test_sizes = [1,2,10,63,64,65,127,128,129] # Including sizes that would test off-by-one errors in the bit encoding.
 
-@testset "Inner product between stabilizer states" begin
+@testset "Pauli string multiplication" begin
     for n in test_sizes
         for _ in 1:20
             p1 = random_pauli(n)
@@ -22,3 +22,18 @@ test_sizes = [1,2,10,63,64,65,127,128,129] # Including sizes that would test off
         end
     end
 end
+
+# test for #320
+@testset "verify SIMD implementation" begin
+    for i in 1:10,
+        n in 1:30,
+        T in [UInt8, UInt16, UInt32, UInt64]
+        a = rand(T, n)
+        b = rand(T, n)
+        c1,c2 = QuantumClifford.mul_ordered!(copy(a),copy(b))
+        n1,n2 = QuantumClifford._mul_ordered_nonvec!(copy(a),copy(b))
+        np = ((n1 ⊻ (n2<<1))&0x3)
+        cp = ((c1 ⊻ (c2<<1))&0x3)
+        @test np==cp
+    end
+end

test/test_stabcanon.jl

@@ -74,3 +74,16 @@ test_sizes = [1,2,10,63,64,65,127,128,129] # Including sizes that would test off
         end
     end
 end
+
+@testset "canonicalization invariants" begin
+    s = random_stabilizer(40,100)
+    ss = tensor_pow(s,20)
+    sa1 = canonicalize!(canonicalize_rref!(copy(ss))[1])
+    sa2 = canonicalize!(copy(ss))
+    @test sa1 == sa2
+    ms = MixedDestabilizer(s)
+    mss = tensor_pow(ms, 20)
+    msa1 = canonicalize!(canonicalize_rref!(copy(mss))[1])
+    msa2 = canonicalize!(copy(mss))
+    @test stabilizerview(msa1) == stabilizerview(msa2) == sa1
+end