embed for single-qubit Clifford Operator

src/dense_cliffords.jl

@@ -104,6 +104,30 @@ function apply!(r::CliffordOperator, l::AbstractCliffordOperator; phases=false)
     r
 end
 
+function embed(n::Int, i::Int, t::CliffordOperator)
+    t = tab(t)
+    if nqubits(t) == 1
+        tout = zero(typeof(t), 2*n, n)
+        for j in 1:n
+            tout[j, i] = t[nqubits(t)][1]
+            tout.phases[j] = t.phases[1]
+        end
+        for j in 1:n
+            tout[n + j, i] = t[nqubits(t)+1][1]
+            tout.phases[n + j] = t.phases[nqubits(t)+1]
+        end
+        return CliffordOperator(tout)
+    else
+        throw(ArgumentError("""
+        You are trying to embed a small CliffordOperator into a larger CliffordOperator.
+        However, you have not given all the positions at which the CliffordOperator needs to be embedded.
+        If you are directly calling `embed`, use the form `embed(nqubits, indices::Tuple, t::CliffordOperator)`.
+        If you are not using `embed` directly, then `embed` must have been incorrectly called
+        by one of the functions you have called.
+        """))
+    end
+end
+
 # TODO create Base.permute! and getindex(..., permutation_array)
 function permute(c::CliffordOperator,p) # TODO this is a slow stupid implementation
     CliffordOperator(Tableau([tab(c)[i][p] for i in 1:2*nqubits(c)][vcat(p,p.+nqubits(c))]))

test/test_embed.jl

@@ -1,7 +1,30 @@
-@testitem "embed PauliOperator" begin
+@testitem "embed PauliOperator, CliffordOperator" begin
     @test embed(5,3,P"-Y") == P"-__Y__"
     @test embed(5,(3,5),P"-YZ") == P"-__Y_Z"
     @test embed(5,[3,5],P"-YZ") == P"-__Y_Z"
     @test_throws ArgumentError embed(5,[3,5],P"-YZX")
     @test_throws ArgumentError embed(5,3,P"-ZX")
+
+    # embed for single-qubit Clifford gates
+    @test embed(5, 5, tHadamard) == C"____Z
+                                      ____Z
+                                      ____Z
+                                      ____Z
+                                      ____Z
+                                      ____X
+                                      ____X
+                                      ____X
+                                      ____X
+                                      ____X"
+
+    @test embed(5, 3, CliffordOperator(sHadamard)) == C"__Z__
+                                                        __Z__
+                                                        __Z__
+                                                        __Z__
+                                                        __Z__
+                                                        __X__
+                                                        __X__
+                                                        __X__
+                                                        __X__
+                                                        __X__"
 end