Implement general tensor_pow via Base.power_by_squaring

Author: akirakyle

src/QuantumInterface.jl

@@ -45,7 +45,7 @@ function tensor end
 const ⊗ = tensor
 tensor() = throw(ArgumentError("Tensor function needs at least one argument."))
 
-function tensor_pow end # TODO should Base.^ be the same as tensor_pow?
+function tensor_pow end
 
 function traceout! end
 

src/bases.jl

@@ -93,12 +93,7 @@ function tensor(b1::Basis, b2::CompositeBasis)
 end
 tensor(bases::Basis...) = reduce(tensor, bases)
 
-function Base.:^(b::Basis, N::Integer)
-    if N < 1
-        throw(ArgumentError("Power of a basis is only defined for positive integers."))
-    end
-    tensor([b for i=1:N]...)
-end
+Base.:^(b::Basis, N::Integer) = tensor_pow(b, N)
 
 """
     equal_shape(a, b)

src/tensor.jl

@@ -8,3 +8,10 @@ tensor(op::AbstractOperator) = op
 tensor(operators::AbstractOperator...) = reduce(tensor, operators)
 tensor(state::StateVector) = state
 tensor(states::Vector{T}) where T<:StateVector = reduce(tensor, states)
+
+"""
+    tensor_pow(a, N)
+
+Gives the tensor product of `a` `N` times.
+"""
+tensor_pow(a, N) = Base.power_by_squaring(a, N; mul=⊗)