Bug fix to StackOverflowError for md ⊗ SX where typeof(md) is MixedDe…

…stabilizer{...}

src/linalg.jl

@@ -161,8 +161,10 @@ julia> tensor(s, s)
 See also [`tensor_pow`](@ref)."""
 function tensor end
 
-function tensor(ops::AbstractStabilizer...) # TODO optimize this by doing conversion to common type to enable preallocation
-    foldl(⊗, ops[2:end], init=ops[1])
+function tensor(ops::AbstractStabilizer...)
+    ct = promote_type(map(typeof, ops)...)
+    conv_ops = map(x -> convert(ct, x), ops)
+    return foldl(⊗, conv_ops)
 end
 
 """Repeated tensor product of an operators or a tableau.
@@ -264,3 +266,67 @@ function tensor(ops::CliffordOperator...) # TODO implement \otimes for Destabili
     end
     CliffordOperator(tab)
 end
+
+"""
+Tensor product between [`MixedDestabilizer`](@ref) and [`Stabilizer`](@ref).
+
+```jldoctest
+julia> md = MixedDestabilizer(T"Z//X", 1)
+𝒟ℯ𝓈𝓉𝒶𝒷
++ Z
+𝒮𝓉𝒶𝒷
++ X
+
+julia> md ⊗ S"X"
+𝒟ℯ𝓈𝓉𝒶𝒷
++ Z_
++ _Z
+𝒮𝓉𝒶𝒷
++ X_
++ _X
+
+julia> S"X" ⊗ md
+𝒟ℯ𝓈𝓉𝒶𝒷
++ Z_
++ _Z
+𝒮𝓉𝒶𝒷
++ X_
++ _X
+
+julia> S"X" ⊗ md ⊗ S"X" ⊗ md
+𝒟ℯ𝓈𝓉𝒶𝒷
++ Z___
++ _Z__
++ __Z_
++ ___Z
+𝒮𝓉𝒶𝒷━━
++ X___
++ _X__
++ __X_
++ ___X
+```
+"""
+function tensor(md::MixedDestabilizer, s::Stabilizer)
+    ntot_md = nqubits(md)
+    ntot_s = nqubits(s)
+    ntot = ntot_md + ntot_s
+    rtot_md = LinearAlgebra.rank(md)
+    rtot_s = length(s)
+    rtot = rtot_md + rtot_s
+    tab = zero(Tableau, 2*ntot, ntot)
+    last_dvrow = 0
+    last_svrow = ntot
+    last_col = 0
+    md1 = MixedDestabilizer(s)
+    last_lxrow = rtot
+    last_lzrow = ntot+rtot
+    for op in [md, md1]
+        _, last_svrow, _        = puttableau!(tab,   stabilizerview(op), last_svrow, last_col)
+        _, last_dvrow, _        = puttableau!(tab, destabilizerview(op), last_dvrow, last_col)
+        _, last_lxrow, _        = puttableau!(tab,     logicalxview(op), last_lxrow, last_col)
+        _, last_lzrow, last_col = puttableau!(tab,     logicalzview(op), last_lzrow, last_col)
+    end
+    return MixedDestabilizer(tab, rtot)
+end
+
+tensor(s::Stabilizer, md::MixedDestabilizer) = tensor(md, s)

test/test_stabs.jl

@@ -55,6 +55,12 @@
             stabs = [s[1:i] for s in [random_stabilizer(n) for n in [32,16,16,64,63,65,129,128,127]] for i in rand(1:10)];
             mdstabs = MixedDestabilizer.(stabs);
             @test canonicalize!(⊗(stabs...)) == canonicalize!(stabilizerview(⊗(mdstabs...)))
+            md = MixedDestabilizer(random_destabilizer(n))
+            s = random_stabilizer(n)
+            mds = md⊗s
+            @test mixed_destab_looks_good(mds)
+            estab = stabilizerview(md)⊗s
+            @test canonicalize!(copy(stabilizerview(mds))) == canonicalize!(estab)
         end
     end