Gate.control method throwing error

[sharipr]

Environment

• Qiskit Terra version: 0.24.0
• Python version: Python 3.9.6

What is happening?

Given a quantum circuit object, we execute the to_gate() method and attempt to call the Gate.control() method with appropriate parameters and sometimes observe the following error: *** UnboundLocalError: local variable 'mat2' referenced before assignment

How can we reproduce the issue?

from qiskit.extensions import UnitaryGate
from qiskit import QuantumCircuit, QuantumRegister

# initialize a unitary matrix

A = np.array([[ 0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  1.+0.j,  0.+0.j,  0.+0.j,
         0.+0.j],
       [ 0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  1.+0.j,  0.+0.j,
         0.+0.j],
       [ 0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j, -1.+0.j,
         0.+0.j],
       [ 0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,
        -1.+0.j],
       [ 1.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,
         0.+0.j],
       [ 0.+0.j,  1.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,
         0.+0.j],
       [ 0.+0.j,  0.+0.j, -1.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,
         0.+0.j],
       [ 0.+0.j,  0.+0.j,  0.+0.j, -1.+0.j,  0.+0.j,  0.+0.j,  0.+0.j,
         0.+0.j]])

gate = UnitaryGate(A)
qc = QuantumCircuit(3)
qc.append(gate,[0,1,2])

# This happens in a a different function than the operations above

CU = qc.to_gate().control(6,28)

What should happen?

I would assume that the error is not thrown

Any suggestions?

The error appears to come from the file qsd.py, and in particular because the list ind2q (line 234) has only one element. The for loop that starts on line 242 is thus never executed and the the variable mat2 is never initialized.

[jakelishman]

I believe this is a duplicate of #10036, so the fix should have been released in Terra 0.24.1. Could you try upgrading and see if the problem persists?

[jakelishman]

Oh no, this still seems to be present on main despite #10126. @ewinston, please can you look at this?

[sharipr]

@jakelishman @ewinston I see the lines of code that are meant to fix this (from 2 months ago). Lines 246 and 247 are:

if not ind2q:
        return ccirc

I could be wrong but it also seems like if the list ind2q has just one element, the for loop that comes next will not be executed and the variable mat2 will never be initialized, thus throwing the same error.