optimize_1q_decomposition: 'NoneType' object has no attribute 'global_phase'

[1ucian0]

Environment

• Qiskit Terra version: 0.24.0.dev0+84a0fdd
• Python version: 3.9
• Operating system: MacOS

What is happening?

When unitary_synthesis is called with a particularly restrictive basis_gates, it seems to generate a None output as best_synth_circuit, which makes optimize_1q_decomposition raise.

How can we reproduce the issue?

from qiskit import *
circuit = QuantumCircuit(1)
circuit.append(Operator([[0.96891242+0.j        , 0.        -0.24740396j],
          [0.        -0.24740396j, 0.96891242+0.j        ]]), [0])
transpile(circuit, basis_gates=['h', 'rx']).draw()

...
<reducted>/qiskit/transpiler/passes/synthesis/unitary_synthesis.py in run(self, unitary, **options)
    741         if unitary.shape == (2, 2):
    742             _decomposer1q = Optimize1qGatesDecomposition(basis_gates, target)
--> 743             return _decomposer1q._gate_sequence_to_dag(
    744                 _decomposer1q._resynthesize_run(unitary, qubits[0])
    745             )

<reducted>/qiskit/transpiler/passes/optimization/optimize_1q_decomposition.py in _gate_sequence_to_dag(self, best_synth_circuit)
    144         out_dag = DAGCircuit()
    145         out_dag.add_qubits(qubits)
--> 146         out_dag.global_phase = best_synth_circuit.global_phase
    147 
    148         for gate_name, angles in best_synth_circuit:

AttributeError: 'NoneType' object has no attribute 'global_phase'

What should happen?

work or nicer error

Any suggestions?

No response

[1ucian0]

Notice that transpile(circuit, basis_gates=['rx', 'ry', 'rz']) works, and returns:

   ┌─────────┐
q: ┤ Rx(0.5) ├
   └─────────┘

[jakelishman]

The immediate cause for why it doesn't work is because ['h', 'rx'] isn't a combination that we have a synthesis routine for. There are no syntheses found, and the pass should return an error. Of course, we should catch that case and return a good error rather than this one, though.

[jakelishman]

To be clear, I mean UnitarySynthesis or the totality of the synthesis steps should return the error - it's part of the contract of that internal method of Optimize1qDecomposition to return None if it couldn't find a synthesis.

[jakelishman]

Fixed by #9843.

[1ucian0]

The immediate cause for why it doesn't work is because ['h', 'rx'] isn't a combination that we have a synthesis routine for

even when the synthesis result is a single RxGate?

[jakelishman]

How would you know that the synthesis is a single rx gate without a synthesis routine that can target the general basis?

[1ucian0]

I see.. so UnitarySynthesis needs to validate that you provide a "universal" basis? It is possible to do that?

[jakelishman]

Whether you need a "universal" basis depends on the synthesis plugin you're using. The default one has a variety of possible universal 1q decomposers, but your basis set has to be a superset of at least one of them to work. These are the available decomposers:

ONE_QUBIT_EULER_BASIS_GATES = {
    "U3": ["u3"],
    "U321": ["u3", "u2", "u1"],
    "U": ["u"],
    "PSX": ["p", "sx"],
    "U1X": ["u1", "rx"],
    "RR": ["r"],
    "ZYZ": ["rz", "ry"],
    "ZXZ": ["rz", "rx"],
    "XZX": ["rz", "rx"],
    "XYX": ["rx", "ry"],
    "ZSXX": ["rz", "sx", "x"],
    "ZSX": ["rz", "sx"],
}