extended methods for tensor and kron

Author: ytdHuang

src/qobj/functions.jl

@@ -187,11 +187,15 @@ Quantum Object:   type=Operator   dims=[20, 20]   size=(400, 400)   ishermitian=
 ⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠙⠦
 ```
 """
-function LinearAlgebra.kron(
+LinearAlgebra.kron(
     A::QuantumObject{<:AbstractArray{T1},OpType},
     B::QuantumObject{<:AbstractArray{T2},OpType},
-) where {T1,T2,OpType<:Union{KetQuantumObject,BraQuantumObject,OperatorQuantumObject}}
-    return QuantumObject(kron(A.data, B.data), A.type, vcat(A.dims, B.dims))
+) where {T1,T2,OpType<:Union{KetQuantumObject,BraQuantumObject,OperatorQuantumObject}} =
+    QuantumObject(kron(A.data, B.data), A.type, vcat(A.dims, B.dims))
+LinearAlgebra.kron(A::QuantumObject) = A
+function LinearAlgebra.kron(A::Vector{<:QuantumObject})
+    @warn "`tensor(A)` or `kron(A)` with `A` is a `Vector` can hurt performance. Try to use `tensor(A...)` or `kron(A...)` instead."
+    return kron(A...)
 end
 
 @doc raw"""

src/qobj/synonyms.jl

@@ -184,7 +184,7 @@ Quantum Object:   type=Operator   dims=[2, 2, 2]   size=(8, 8)   ishermitian=tru
  1.0+0.0im      ⋅          ⋅             ⋅          ⋅          ⋅
 ```
 """
-tensor(A::QuantumObject...) = kron(A...)
+tensor(A...) = kron(A...)
 
 @doc raw"""
     ⊗(A::QuantumObject, B::QuantumObject)

test/quantum_objects.jl

@@ -309,11 +309,24 @@
             @inferred a .^ 2
             @inferred a * a
             @inferred a * a'
+            @inferred kron(a)
             @inferred kron(a, σx)
             @inferred kron(a, eye(2))
         end
     end
 
+    @testset "tensor" begin
+        σx = sigmax()
+        X3 = kron(σx, σx, σx)
+        @test tensor(σx) == kron(σx)
+        @test tensor(fill(σx, 3)...) == X3
+        X_warn = @test_logs (
+            :warn,
+            "`tensor(A)` or `kron(A)` with `A` is a `Vector` can hurt performance. Try to use `tensor(A...)` or `kron(A...)` instead.",
+        ) tensor(fill(σx, 3))
+        @test X_warn == X3
+    end
+
     @testset "projection" begin
         N = 10
         ψ = fock(N, 3)