UnitarySynthesis ignores basis_gates argument for >= 3 qubit unitaries

[kevinsung]

Environment

• Qiskit version: e8714782376
• Python version: 3.10.12
• Operating system: Arch Linux

What is happening?

I create a circuit with a 3-qubit UnitaryGate. I use UnitarySynthesis and pass a CZ gate basis. It gets synthesized down to CX instead of CZ.

How can we reproduce the issue?

from qiskit import QuantumCircuit, QuantumRegister
from qiskit.circuit.library import UnitaryGate
from qiskit.quantum_info import random_unitary
from qiskit.transpiler import PassManager
from qiskit.transpiler.passes import UnitarySynthesis

qubits = QuantumRegister(3, name="q")
circuit = QuantumCircuit(qubits)
a, b, c = qubits
circuit.append(UnitaryGate(random_unitary(8)), [a, b, c])

pass_manager = PassManager(
    [
        UnitarySynthesis(basis_gates=["cz", "id", "rz", "sx", "x"]),
    ]
)
transpiled = pass_manager.run(circuit)
transpiled.draw("mpl")

What should happen?

Should be synthesized down to CZ as requested.

Any suggestions?

No response

[mtreinish]

The default synthesis plugin used by UnitarySynthesis relies on the Quantum Shannon Decomposition module in qiskit.synthesis (https://github.com/Qiskit/qiskit/blob/main/qiskit/synthesis/unitary/qsd.py) for unitaries of > 2 qubits. It currently doesn't look at any basis gates or target constraints.

At the pass level I'm not sure we can always guarantee the output synthesis is going to respect the basis_gates or target arguments as different synthesis plugins can explicitly ignore the arguments. But I think we should look at fixing this for the default plugin though because it does advertise it does respect the basis gates and target, and this issue would potentially break the synthesis translation stage plugin too.