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Only apply MCMT plugin on MCMTGate #13596

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Only apply MCMT plugin on MCMTGate #13596

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10 changes: 10 additions & 0 deletions qiskit/transpiler/passes/synthesis/hls_plugins.py
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Original file line number Diff line number Diff line change
Expand Up @@ -420,6 +420,7 @@
C3XGate,
C4XGate,
PauliEvolutionGate,
MCMTGate,
ModularAdderGate,
HalfAdderGate,
FullAdderGate,
Expand Down Expand Up @@ -1156,6 +1157,9 @@ class MCMTSynthesisDefault(HighLevelSynthesisPlugin):

def run(self, high_level_object, coupling_map=None, target=None, qubits=None, **options):
# first try to use the V-chain synthesis if enough auxiliary qubits are available
if not isinstance(high_level_object, MCMTGate):
return None

if (
decomposition := MCMTSynthesisVChain().run(
high_level_object, coupling_map, target, qubits, **options
Expand All @@ -1170,6 +1174,9 @@ class MCMTSynthesisNoAux(HighLevelSynthesisPlugin):
"""A V-chain based synthesis for ``MCMTGate``."""

def run(self, high_level_object, coupling_map=None, target=None, qubits=None, **options):
if not isinstance(high_level_object, MCMTGate):
return None

base_gate = high_level_object.base_gate
ctrl_state = options.get("ctrl_state", None)

Expand All @@ -1195,6 +1202,9 @@ class MCMTSynthesisVChain(HighLevelSynthesisPlugin):
"""A V-chain based synthesis for ``MCMTGate``."""

def run(self, high_level_object, coupling_map=None, target=None, qubits=None, **options):
if not isinstance(high_level_object, MCMTGate):
return None

if options.get("num_clean_ancillas", 0) < high_level_object.num_ctrl_qubits - 1:
return None # insufficient number of auxiliary qubits

Expand Down
9 changes: 9 additions & 0 deletions releasenotes/notes/fix-mcmt-to-gate-ec84e1c625312444.yaml
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Original file line number Diff line number Diff line change
@@ -0,0 +1,9 @@
---
fixes:
- |
Fixed a bug where any instruction called ``"mcmt"`` would be passed into the high-level
synthesis routine for a :class:`.MCMTGate`, which causes a failure or invalid result.
In particular, this could happen accidentally when handling the :class:`.MCMT` _circuit_,
named ``"mcmt"``, and implicitly converting it into an instruction e.g. when appending
it to a circuit.
Fixed `#13563 <https://github.com/Qiskit/qiskit/issues/13563>`__.
13 changes: 13 additions & 0 deletions test/python/circuit/library/test_mcmt.py
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Original file line number Diff line number Diff line change
Expand Up @@ -285,6 +285,19 @@ def test_gate_with_parameters_vchain(self):
self.assertEqual(circuit.num_parameters, 1)
self.assertIs(circuit.parameters[0], theta)

def test_mcmt_circuit_as_gate(self):
"""Test the MCMT plugin is only triggered for the gate, not the same-named circuit.

Regression test of #13563.
"""
circuit = QuantumCircuit(2)
gate = RYGate(0.1)
mcmt = MCMT(gate=gate, num_ctrl_qubits=1, num_target_qubits=1)
circuit.append(mcmt, circuit.qubits) # append the MCMT circuit as gate called "MCMT"

transpiled = transpile(circuit, basis_gates=["u", "cx"])
self.assertTrue(Operator(transpiled).equiv(gate.control(1)))


if __name__ == "__main__":
unittest.main()
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