Break intermediate measurements on 3+ qubits into single qubit measur…

…ements in `RouteCQC`quantumlib#6293 (quantumlib#6349)

cirq/transformers/routing/route_circuit_cqc.py

@@ -249,26 +249,32 @@ def _get_one_and_two_qubit_ops_as_timesteps(
         output routed circuit, single-qubit operations are inserted before two-qubit operations.
 
         Raises:
-            ValueError: if circuit has intermediate measurement op's that act on 3 or more qubits.
+            ValueError: if circuit has intermediate measurements that act on three or more
+                        qubits with a custom key.
         """
         two_qubit_circuit = circuits.Circuit()
         single_qubit_ops: List[List[cirq.Operation]] = []
 
-        if any(
-            protocols.num_qubits(op) > 2 and protocols.is_measurement(op)
-            for op in itertools.chain(*circuit.moments[:-1])
-        ):
-            # There is at least one non-terminal measurement on 3+ qubits
-            raise ValueError('Non-terminal measurements on three or more qubits are not supported')
-
-        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:
+                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(
+                            'Intermediate measurements on three or more qubits '
+                            'with a custom key 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/transformers/routing/route_circuit_cqc_test.py

@@ -107,35 +107,45 @@ def test_circuit_with_measurement_gates():
     cirq.testing.assert_same_circuits(routed_circuit, circuit)
 
 
-def test_circuit_with_valid_intermediate_multi_qubit_measurement_gates():
-    device = cirq.testing.construct_ring_device(3)
+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)
-    q = cirq.LineQubit.range(2)
-    hard_coded_mapper = cirq.HardCodedInitialMapper({q[i]: q[i] for i in range(2)})
-
-    valid_circuit = cirq.Circuit(cirq.measure_each(*q), cirq.H.on_each(q))
-
-    c_routed = router(
-        valid_circuit, initial_mapper=hard_coded_mapper, context=cirq.TransformerContext(deep=True)
+    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(c_routed)
+    device.validate_circuit(routed_circuit)
 
 
-def test_circuit_with_invalid_intermediate_multi_qubit_measurement_gates():
+def test_circuit_with_multi_qubit_intermediate_measurement_gate_and_with_default_key():
     device = cirq.testing.construct_ring_device(3)
     device_graph = device.metadata.nx_graph
     router = cirq.RouteCQC(device_graph)
-    q = cirq.LineQubit.range(3)
-    hard_coded_mapper = cirq.HardCodedInitialMapper({q[i]: q[i] for i in range(3)})
+    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)
 
-    invalid_circuit = cirq.Circuit(cirq.MeasurementGate(3).on(*q), cirq.H.on_each(*q))
 
+def test_circuit_with_multi_qubit_intermediate_measurement_gate_with_custom_key():
+    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="test")), cirq.Moment(cirq.H.on_each(qs))]
+    )
     with pytest.raises(ValueError):
         _ = router(
-            invalid_circuit,
-            initial_mapper=hard_coded_mapper,
-            context=cirq.TransformerContext(deep=True),
+            circuit, initial_mapper=hard_coded_mapper, context=cirq.TransformerContext(deep=True)
         )