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Reject multi-qubit noise channels with clear error message #1020

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Reject multi-qubit noise channels with clear error message #1020

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27 changes: 19 additions & 8 deletions tensorflow_quantum/core/serialize/serializer.py
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Original file line number Diff line number Diff line change
Expand Up @@ -30,6 +30,17 @@
_CONSTANT_TRUE = lambda x: True


def _single_qubit_channel_check(x):
"""Check that a noise channel operates on exactly one qubit."""
if len(x.qubits) != 1:
raise ValueError(
"Multi-qubit noise channels are not supported in TFQ. "
f"Got {x} acting on {len(x.qubits)} qubits. "
"Consider decomposing into single-qubit channels applied "
"via .on_each().")
return True


def _round(x):
return np.round(x, 6) if isinstance(x, float) else x

Expand Down Expand Up @@ -196,7 +207,7 @@ def _asymmetric_depolarize_serializer():
gate_type=cirq.AsymmetricDepolarizingChannel,
serialized_gate_id="ADP",
args=args,
can_serialize_predicate=_CONSTANT_TRUE)
can_serialize_predicate=_single_qubit_channel_check)


def _asymmetric_depolarize_deserializer():
Expand Down Expand Up @@ -234,7 +245,7 @@ def _depolarize_channel_serializer():
gate_type=cirq.DepolarizingChannel,
serialized_gate_id="DP",
args=args,
can_serialize_predicate=_CONSTANT_TRUE)
can_serialize_predicate=_single_qubit_channel_check)


def _depolarize_channel_deserializer():
Expand Down Expand Up @@ -272,7 +283,7 @@ def _gad_channel_serializer():
gate_type=cirq.GeneralizedAmplitudeDampingChannel,
serialized_gate_id="GAD",
args=args,
can_serialize_predicate=_CONSTANT_TRUE)
can_serialize_predicate=_single_qubit_channel_check)


def _gad_channel_deserializer():
Expand Down Expand Up @@ -309,7 +320,7 @@ def _amplitude_damp_channel_serializer():
gate_type=cirq.AmplitudeDampingChannel,
serialized_gate_id="AD",
args=args,
can_serialize_predicate=_CONSTANT_TRUE)
can_serialize_predicate=_single_qubit_channel_check)


def _amplitude_damp_channel_deserializer():
Expand Down Expand Up @@ -341,7 +352,7 @@ def _reset_channel_serializer():
gate_type=cirq.ResetChannel,
serialized_gate_id="RST",
args=args,
can_serialize_predicate=_CONSTANT_TRUE)
can_serialize_predicate=_single_qubit_channel_check)


def _reset_channel_deserializer():
Expand Down Expand Up @@ -370,7 +381,7 @@ def _phase_damp_channel_serializer():
gate_type=cirq.PhaseDampingChannel,
serialized_gate_id="PD",
args=args,
can_serialize_predicate=_CONSTANT_TRUE)
can_serialize_predicate=_single_qubit_channel_check)


def _phase_damp_channel_deserializer():
Expand Down Expand Up @@ -403,7 +414,7 @@ def _phase_flip_channel_serializer():
gate_type=cirq.PhaseFlipChannel,
serialized_gate_id="PF",
args=args,
can_serialize_predicate=_CONSTANT_TRUE)
can_serialize_predicate=_single_qubit_channel_check)


def _phase_flip_channel_deserializer():
Expand Down Expand Up @@ -437,7 +448,7 @@ def _bit_flip_channel_serializer():
gate_type=cirq.BitFlipChannel,
serialized_gate_id="BF",
args=args,
can_serialize_predicate=_CONSTANT_TRUE)
can_serialize_predicate=_single_qubit_channel_check)


def _bit_flip_channel_deserializer():
Expand Down
10 changes: 10 additions & 0 deletions tensorflow_quantum/core/serialize/serializer_test.py
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Original file line number Diff line number Diff line change
Expand Up @@ -720,6 +720,16 @@ def test_serialize_noise_channel_unsupported_value(self):
with self.assertRaises(ValueError):
serializer.serialize_circuit(simple_circuit)

def test_serialize_multi_qubit_noise_channel(self):
"""Ensure multi-qubit noise channels are rejected with clear error."""
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@mhucka mhucka May 5, 2026

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Suggested change
"""Ensure multi-qubit noise channels are rejected with clear error."""
"""Ensure multi-qubit noise channels are rejected with a clear error."""

q0 = cirq.GridQubit(0, 0)
q1 = cirq.GridQubit(0, 1)
multi_qubit_circuit = cirq.Circuit(
cirq.DepolarizingChannel(p=0.1, n_qubits=2)(q0, q1))
with self.assertRaisesRegex(ValueError,
"Multi-qubit noise channels"):
serializer.serialize_circuit(multi_qubit_circuit)
Comment on lines +723 to +731
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@gemini-code-assist gemini-code-assist Bot Mar 19, 2026

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medium

Great job on adding a test for this new validation! To make it more robust, I suggest expanding it to also test cirq.asymmetric_depolarize, which was the motivating example from the issue description. This will ensure that channels defined by error_probabilities with multi-qubit Pauli strings are also correctly rejected.

Additionally, the n_qubits argument in cirq.DepolarizingChannel is deprecated. It's better to omit it and let cirq infer the number of qubits from the on() call.

Suggested change
def test_serialize_multi_qubit_noise_channel(self):
"""Ensure multi-qubit noise channels are rejected with clear error."""
q0 = cirq.GridQubit(0, 0)
q1 = cirq.GridQubit(0, 1)
multi_qubit_circuit = cirq.Circuit(
cirq.DepolarizingChannel(p=0.1, n_qubits=2)(q0, q1))
with self.assertRaisesRegex(ValueError,
"Multi-qubit noise channels"):
serializer.serialize_circuit(multi_qubit_circuit)
def test_serialize_multi_qubit_noise_channel(self):
"""Ensure multi-qubit noise channels are rejected with clear error."""
q0 = cirq.GridQubit(0, 0)
q1 = cirq.GridQubit(0, 1)
depol_circuit = cirq.Circuit(
cirq.DepolarizingChannel(p=0.1)(q0, q1))
with self.assertRaisesRegex(ValueError, "Multi-qubit noise channels"):
serializer.serialize_circuit(depol_circuit)
asym_depol_circuit = cirq.Circuit(
cirq.asymmetric_depolarize(error_probabilities={'XX': 0.1})(q0, q1))
with self.assertRaisesRegex(ValueError, "Multi-qubit noise channels"):
serializer.serialize_circuit(asym_depol_circuit)


@parameterized.parameters([{'inp': v} for v in ['wrong', 1.0, None, []]])
def test_serialize_circuit_wrong_type(self, inp):
"""Attempt to serialize invalid objects types."""
Expand Down
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