Break intermediate measurements on 3+ qubits into single qubit measurements in RouteCQC#6293

cirq-core/cirq/transformers/routing/route_circuit_cqc.py

@@ -245,17 +245,32 @@ def _get_one_and_two_qubit_ops_as_timesteps(
         The i'th entry in the nested two-qubit and single-qubit ops correspond to the two-qubit
         gates and single-qubit gates of the i'th timesteps respectively. When constructing the
         output routed circuit, single-qubit operations are inserted before two-qubit operations.
+
+        Raises:
+            ValueError: if circuit has intermediate measurement op's that act on three or more qubits and result is stored.
         """
         two_qubit_circuit = circuits.Circuit()
         single_qubit_ops: List[List[cirq.Operation]] = []
-        for moment in circuit:
+
+        for i, moment in enumerate(circuit):
             for op in moment:
                 timestep = two_qubit_circuit.earliest_available_moment(op)
                 single_qubit_ops.extend([] for _ in range(timestep + 1 - len(single_qubit_ops)))
                 two_qubit_circuit.append(
                     circuits.Moment() for _ in range(timestep + 1 - len(two_qubit_circuit))
                 )
-                if protocols.num_qubits(op) == 2 and not protocols.is_measurement(op):
+                if protocols.num_qubits(op) > 2 and protocols.is_measurement(op):
+                    key = op.gate.key  # type: ignore
+                    default_key = ops.measure(op.qubits).gate.key  # type: ignore
+                    if len(circuit.moments) == i + 1:
+                        single_qubit_ops[timestep].append(op)
+                    elif key in ('', default_key):
+                        single_qubit_ops[timestep].extend(ops.measure(qubit) for qubit in op.qubits)
+                    else:
+                        raise ValueError(
+                            'Non-terminal measurements on three or more qubits when result is stored are not supported'
+                        )
+                elif protocols.num_qubits(op) == 2:
                     two_qubit_circuit[timestep] = two_qubit_circuit[timestep].with_operation(op)
                 else:
                     single_qubit_ops[timestep].append(op)

cirq-core/cirq/transformers/routing/route_circuit_cqc_test.py

@@ -107,6 +107,48 @@ def test_circuit_with_measurement_gates():
     cirq.testing.assert_same_circuits(routed_circuit, circuit)
 
 
+def test_circuit_with_two_qubit_intermediate_measurement_gate():
+    device = cirq.testing.construct_ring_device(2)
+    device_graph = device.metadata.nx_graph
+    router = cirq.RouteCQC(device_graph)
+    qs = cirq.LineQubit.range(2)
+    hard_coded_mapper = cirq.HardCodedInitialMapper({qs[i]: qs[i] for i in range(2)})
+    circuit = cirq.Circuit([cirq.Moment(cirq.measure(qs)), cirq.Moment(cirq.H.on_each(qs))])
+    routed_circuit = router(
+        circuit, initial_mapper=hard_coded_mapper, context=cirq.TransformerContext(deep=True)
+    )
+    device.validate_circuit(routed_circuit)
+
+
+def test_circuit_with_multi_qubit_intermediate_measurement_gate_and_result_not_stored():
+    device = cirq.testing.construct_ring_device(3)
+    device_graph = device.metadata.nx_graph
+    router = cirq.RouteCQC(device_graph)
+    qs = cirq.LineQubit.range(3)
+    hard_coded_mapper = cirq.HardCodedInitialMapper({qs[i]: qs[i] for i in range(3)})
+    circuit = cirq.Circuit([cirq.Moment(cirq.measure(qs)), cirq.Moment(cirq.H.on_each(qs))])
+    routed_circuit = router(
+        circuit, initial_mapper=hard_coded_mapper, context=cirq.TransformerContext(deep=True)
+    )
+    expected = cirq.Circuit([cirq.Moment(cirq.measure_each(qs)), cirq.Moment(cirq.H.on_each(qs))])
+    cirq.testing.assert_same_circuits(routed_circuit, expected)
+
+
+def test_circuit_with_multi_qubit_intermediate_measurement_gate_and_result_stored():
+    device = cirq.testing.construct_ring_device(3)
+    device_graph = device.metadata.nx_graph
+    router = cirq.RouteCQC(device_graph)
+    qs = cirq.LineQubit.range(3)
+    hard_coded_mapper = cirq.HardCodedInitialMapper({qs[i]: qs[i] for i in range(3)})
+    circuit = cirq.Circuit(
+        [cirq.Moment(cirq.measure(qs, key="key_name")), cirq.Moment(cirq.H.on_each(qs))]
+    )
+    with pytest.raises(ValueError):
+        _ = router(
+            circuit, initial_mapper=hard_coded_mapper, context=cirq.TransformerContext(deep=True)
+        )
+
+
 def test_circuit_with_non_unitary_and_global_phase():
     device = cirq.testing.construct_ring_device(4)
     device_graph = device.metadata.nx_graph