Decompose.run discards global_phase

[jlapeyre]

from qiskit.circuit.library import RZGate
from qiskit.quantum_info import Operator
from qiskit import QuantumCircuit

import scipy
import numpy as np

# Illustrate the problem with an RZGate, which lowers to a U1Gate plus a global phase
g = RZGate(1.0)

# Manually create an array containing the correct matrix
zmat = np.array([[1, 0], [0, -1]])
correct_matrix = scipy.linalg.expm(- 0.5j * zmat)

# Operator correctly converts RZGate to a matrix
assert np.allclose(Operator(g).data, correct_matrix)

# g.definition returns a QuantumCircuit with a U1 gate and a global_phase
# Converting this gate + phase to a matrix via Operator gives the correct result
assert g.definition.global_phase == -0.5
assert np.allclose(Operator(g.definition).data, correct_matrix)

# Create a circuit with an RZGate (not lowered to U1)
qc = QuantumCircuit(1)
qc.append(g, [0])
# qc.decompose() also lowers RZGate to a U1 gate, calling "definition" as above,
#  but the global_phase is discarded, resulting in a matrix that is incorrect by a phase.
assert np.allclose(np.exp(g.definition.global_phase * 1.0j) * Operator(qc.decompose()).data, correct_matrix)

A substitution is made using a gate obtained this way
https://github.com/Qiskit/qiskit-terra/blob/a0ee52400cb21139b509cdba5c14455c27150b01/qiskit/transpiler/passes/basis/decompose.py#L50

But, node.op.definition.global_phase is lost.

Information

• Qiskit Terra version:
  08a1f37

[ewinston]

I believe this should be resolved by pr #4915