Fix phase of cu3 gate to be compatible with u3 gate definition

[chriseclectic]

Summary

Fixes #2703

Currently the cu3 gate is not a controlled u3 gate under the current definition. It differs by a local phase on the u3 gate (which would be an ignorable global phase on normal u3, but not when it is controlled). This adds an additional u1 gate the the definition of the cu3 gate so that it has the correct matrix definition.

Details and comments

Before this PR the following would return falls because the output unitary for cu3 was not equal (up to global phase) to I ⊗ |0X0| + U3(a, b, c) ⊗ |1X1|

import numpy as np
from qiskit import QuantumCircuit, execute, BasicAer
from qiskit.quantum_info.operators.predicates import matrix_equal

# U3 gate params
a, b, c = 0.2, 0.3, 0.4

# U3 gate
circ1 = QuantumCircuit(1)
circ1.u3(a, b, c, 0)

# CU3 gate
circ2= QuantumCircuit(2)
circ2.cu3(a, b, c, 0, 1)

job = execute([circ1, circ2], BasicAer.get_backend('unitary_simulator'))
result = job.result()

# Circuit unitaries
mat_u3 = result.get_unitary(0)
mat_cu3 = result.get_unitary(1)

# Target Controlled-U3 unitary
target = np.kron(mat_u3, np.diag([0, 1])) + np.kron(np.eye(2), np.diag([1, 0]))

matrix_equal(target, mat_cu3, ignore_phase=True) 

[1ucian0]

Is this a fix for #546 also?

[chriseclectic]

@1ucian0 yep