Error "too many subscripts in einsum", UnitaryGate()

Question

I'd like to build a multi qubit controlled gate in the following way.
The circuit has $n+1$ qubits, the first qubit is the control qubit, and the operation on the rest $n$ qubits is the tensor product of $n$ phase rotations $R^{\otimes n}$.

from qiskit import *
from qiskit.extensions import *
import qiskit.extensions.unitary
import cmath as cm
import numpy as np
import math as m
from qiskit.aqua.utils import tensorproduct


n=10
c = QuantumRegister(1, "c")
q = QuantumRegister(n, "q")
cl = ClassicalRegister(2, "cl")
circ = QuantumCircuit( c, q, cl)


phase=cm.exp(cm.pi*complex(0,1)*(1/4))
matrixR=np.array([[phase,0],[0,1]])
matrixTe= matrixR
for _ in range(n-1):
    matrixTe=tensorproduct(matrixTe,matrixR)
matrixOp=matrixTe

gateCR=UnitaryGate(matrixOp).control(num_ctrl_qubits=1)

qubits=[m for m in reversed(q)]
circ.append(gateCR, qubits) 

For $n\geq10$, I get the following error message:

I understand the problem lies in the size of the matrix when it has phase in it. I tried to build a gate for $n=10$ when the operation has no phase, and it worked without any problem!

My question is, is there any workaround to fix this problem? What does the comment hack to correct global phase; should fix to prevent need for correction here in the source code mean?

Answer 1

You can use Qiskit's MatrixOp, and TensoredOp classes:

_list = []
for _ in range(n):
    _list.append(MatrixOp(matrixR))

tensored_op = TensoredOp(_list)
gateCR=tensored_op.to_circuit().to_gate().control(num_ctrl_qubits = 1)