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Error transpiling circuit containing MCMT gate #13563

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cqc-alec opened this issue Dec 13, 2024 · 2 comments · May be fixed by #13596
Open

Error transpiling circuit containing MCMT gate #13563

cqc-alec opened this issue Dec 13, 2024 · 2 comments · May be fixed by #13596
Labels
bug Something isn't working mod: circuit Related to the core of the `QuantumCircuit` class or the circuit library
Milestone
1.3.2

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@cqc-alec
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Environment

  • Qiskit version: 1.3.1
  • Qiskit_aer version: 0.15.1
  • Python version: 3.12.3
  • Operating system: Linux

What is happening?

Transpiling a circuit containing an MCMT gate with the aer_simulator_unitary fails. A very long exception trace ends with: AttributeError: 'Instruction' object has no attribute 'num_ctrl_qubits'.

This is a regression from qiskit 1.2.4.

How can we reproduce the issue?

from qiskit import QuantumCircuit, QuantumRegister, transpile
from qiskit.circuit.library import MCMT, RYGate
from qiskit_aer import Aer


qr = QuantumRegister(2, "q")
qcirc = QuantumCircuit(qr)
ry_gate = RYGate(0.1)
cry_gate = MCMT(gate=ry_gate, num_ctrl_qubits=1, num_target_qubits=1)
qcirc.append(cry_gate, [qr[0], qr[1]])

backend = Aer.get_backend("aer_simulator_unitary")

transpile(qcirc, backend)

What should happen?

Transpilation should succeed (as it does with qiskit 1.2.4).

Any suggestions?

No response
@cqc-alec cqc-alec added the bug Something isn't working label Dec 13, 2024
@cqc-alec cqc-alec mentioned this issue Dec 13, 2024
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@cqc-alec
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However if we just replace MCMT with MCMTGate there is no error.

@Cryoris
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Cryoris commented Dec 13, 2024

That's a good catch! This seems to happen because the high-level synthesis in the transpiler mistakes the MCMT circuit (called "mcmt") with the MCMTGate (also called "mcmt"!) and tries to synthesize the circuit, when it only supports the gate. (@alexanderivrii so you were right adding the instance checks, I forgot them here 😉).

Using the MCMTGate as you suggested is the better solution anyways, since the MCMT is merely a circuit containing the MCMTGate. Using the gate directly is more efficient as it requires less instruction conversions and unwrapping. Nevertheless we should fix this for 1.3.2.

@Cryoris Cryoris added this to the 1.3.2 milestone Dec 13, 2024
@ShellyGarion ShellyGarion added the mod: circuit Related to the core of the `QuantumCircuit` class or the circuit library label Dec 15, 2024
@Cryoris Cryoris linked a pull request Dec 23, 2024 that will close this issue
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bug Something isn't working mod: circuit Related to the core of the `QuantumCircuit` class or the circuit library
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1.3.2
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Successfully merging a pull request may close this issue.

Only apply MCMT plugin on MCMTGate Cryoris/qiskit-terra
3 participants
@Cryoris @ShellyGarion @cqc-alec