# To\_openqasm() support for multi-controlled operations **URL:** \ **Category:** PennyLane Help\ **Created:** [August 6, 2025, 10:35pm UTC](https://discuss.pennylane.ai/t/to-openqasm-support-for-multi-controlled-operations/8880 "2025-08-06T22:35:32Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![vpn](https://yyz2.discourse-cdn.com/flex012/user_avatar/discuss.pennylane.ai/vpn/32/1753_2.png) [@vpn](https://discuss.pennylane.ai/u/vpn)\ **Post date:** [August 6, 2025, 10:35pm UTC](https://discuss.pennylane.ai/t/to-openqasm-support-for-multi-controlled-operations/8880/1 "2025-08-06T22:35:32Z")
Hi all, I’m wondering if there has been support for the `qml.to_openqasm()` function for multi-controlled operations, including inverse QFT and PauliZ. It seems that for \>=3-controlled inverse QFT operation, or \>=4-controlled PauliZ cannot be converted. ```python import pennylane as qml dev = qml.device("default.qubit", wires=6) @qml.qnode(dev) def circuit(): for wire in range(3): qml.Hadamard(wires=wire) qml.ctrl(qml.QFT, control=[3, 4, 5])(wires=range(3)) qml.ctrl(qml.PhaseAdder, control=[3, 4, 5])(k=1, x_wires=range(3)) # qml.ctrl(qml.adjoint(qml.QFT), control=[3, 4, 5])(wires=range(3)) # qml.ctrl(qml.PauliZ, control=[1, 2, 3, 4, 5])(wires=0) return qml.probs(wires=range(3)) qml.draw_mpl(circuit, style='pennylane', wire_order=[0, 1, 2, 3, 4, 5])() t = qml.to_openqasm(circuit, measure_all=False)() ``` The code above runs fine, until you uncomment _any_ of two `qml.ctrl()` lines. Both yield the following error ```auto ValueError: Operation GlobalPhase not supported by the QASM serializer ``` As I dig around, it seems [QFT](https://docs.pennylane.ai/en/stable/_modules/pennylane/templates/subroutines/qft.html#QFT) utilizes the [ControlledPhaseShift() operation](https://docs.pennylane.ai/en/stable/_modules/pennylane/ops/op_math/controlled_ops.html#ControlledPhaseShift), which is decomposed to some `qml.GlobalPhase()` gates that cannot be parsed to openqasm. Is there a way we can use the [qml.transforms.combine\_global\_phase()](https://docs.pennylane.ai/en/stable/code/api/pennylane.transforms.combine_global_phases.html) and then remove the combined global phase, or would there be another workaround this? Here’s my `qml.about()`: ```plaintext Version: 0.42.0 Summary: PennyLane is a cross-platform Python library for quantum computing, quantum machine learning, and quantum chemistry. Train a quantum computer the same way as a neural network. Home-page: Author: Author-email: License-Expression: Apache-2.0 Location: (omitted) Requires: (omitted) Required-by: PennyLane-Cirq, PennyLane-qiskit, pennylane_lightning Platform info: Windows-10 (omitted) Python version: 3.11.9 Numpy version: 2.3.0 Scipy version: 1.15.3 Installed devices: - default.clifford (pennylane-0.42.0) - default.gaussian (pennylane-0.42.0) - default.mixed (pennylane-0.42.0) - default.qubit (pennylane-0.42.0) - default.qutrit (pennylane-0.42.0) - default.qutrit.mixed (pennylane-0.42.0) - default.tensor (pennylane-0.42.0) - null.qubit (pennylane-0.42.0) - reference.qubit (pennylane-0.42.0) - cirq.mixedsimulator (PennyLane-Cirq-0.41.0) - cirq.pasqal (PennyLane-Cirq-0.41.0) - cirq.qsim (PennyLane-Cirq-0.41.0) - cirq.qsimh (PennyLane-Cirq-0.41.0) - cirq.simulator (PennyLane-Cirq-0.41.0) - lightning.qubit (pennylane_lightning-0.42.0) - qiskit.aer (PennyLane-qiskit-0.42.0) - qiskit.basicaer (PennyLane-qiskit-0.42.0) - qiskit.basicsim (PennyLane-qiskit-0.42.0) - qiskit.remote (PennyLane-qiskit-0.42.0) ``` Thanks for your help! ---
**Author:** ![CatalinaAlbornoz](https://yyz2.discourse-cdn.com/flex012/user_avatar/discuss.pennylane.ai/catalinaalbornoz/32/1196_2.png) [@CatalinaAlbornoz](https://discuss.pennylane.ai/u/CatalinaAlbornoz)\ **Post date:** [August 8, 2025, 12:59am UTC](https://discuss.pennylane.ai/t/to-openqasm-support-for-multi-controlled-operations/8880/2 "2025-08-08T00:59:08Z")
Hi @vpn , thank you for reporting this. It does look like there’s an issue when you use more than 4 control qubits, as seen in the simplified code below: ```auto import pennylane as qml dev = qml.device("default.qubit", wires=6) @qml.qnode(dev) def circuit(): qml.ctrl(qml.PauliZ, control=[2, 3, 4, 5])(wires=0) # works qml.ctrl(qml.PauliZ, control=[1, 2, 3, 4, 5])(wires=0) # causes an error return qml.probs(wires=range(3)) qml.draw_mpl(circuit, style='pennylane', wire_order=[0, 1, 2, 3, 4, 5])() t = qml.to_openqasm(circuit, measure_all=False)() ``` Here’s the full error traceback ```auto --------------------------------------------------------------------------- KeyError Traceback (most recent call last) /usr/local/lib/python3.11/dist-packages/pennylane/tape/qscript.py in to_openqasm(self, wires, rotations, measure_all, precision) 1255 try: -> 1256 gate = OPENQASM_GATES[op.name] 1257 except KeyError as e: KeyError: 'GlobalPhase' The above exception was the direct cause of the following exception: ValueError Traceback (most recent call last) 2 frames /tmp/ipython-input-330596400.py in () 8 return qml.probs(wires=range(3)) 9 qml.draw_mpl(circuit, style='pennylane', wire_order=[0, 1, 2, 3, 4, 5])() ---> 10 t = qml.to_openqasm(circuit, measure_all=False)() /usr/local/lib/python3.11/dist-packages/pennylane/io/io.py in wrapper(*args, **kwargs) 746 def wrapper(*args, **kwargs) -> str: 747 tape = construct_tape(qnode)(*args, **kwargs) --> 748 return tape.to_openqasm( 749 wires=wires, rotations=rotations, measure_all=measure_all, precision=precision 750 ) /usr/local/lib/python3.11/dist-packages/pennylane/tape/qscript.py in to_openqasm(self, wires, rotations, measure_all, precision) 1256 gate = OPENQASM_GATES[op.name] 1257 except KeyError as e: -> 1258 raise ValueError(f"Operation {op.name} not supported by the QASM serializer") from e 1259 1260 wire_labels = ",".join([f"q[{wires.index(w)}]" for w in op.wires.tolist()]) ValueError: Operation GlobalPhase not supported by the QASM serializer ``` The implementation of `to_openqasm` comes from the QuantumScript class ([see the source code here](https://docs.pennylane.ai/en/stable/_modules/pennylane/tape/qscript.html)). Unfortunately there’s something hardcoded that states that for `to_openqasm` the gates will be decomposed up to `depth=10`. What I think is happening is that for gates with more than 4 control qubits this depth is not enough to decompose everything into gates supported by OpenQASM. As evidence, below is a code example that does work, even though it includes a `GlobalPhase` gate. ```auto dev = qml.device("default.qubit", wires=2) @qml.qnode(dev) def circuit(phi): qml.X(0) qml.GlobalPhase(phi) return qml.expval(qml.Z(0)), qml.expval(qml.Z(1)) qml.draw_mpl(circuit, style='pennylane')(0.123) t = qml.to_openqasm(circuit, measure_all=False)(0.123) ``` So what can you do? The fastest and easiest option is to create a fork of PennyLane, increase the depth for the `to_openqasm` decompositions in the QuantumScript class, and test if this solves your problem. In addition to this we would appreciate it if you can open an issue on our [GitHub](https://github.com/pennylaneai/pennylane) so that we can keep track of this issue and tackle it when we have capacity, or prioritize it if it’s affecting many people. I know this is a lot of information so please let us know if you still have any questions or issues. Thanks for bringing this to our attention!