Fix ignore_measurement_results=True for subcircuits (quantumlib#4760)

Density matrix simulator has a bug that if `ignore_measurement_results` is set, that setting is overlooked within subcircuits.

The reason is that this setting check is done in `SimulatorBase.core_iterator`, but that only iterates the outermost circuit.

The fix is to move the check into `ActOnArgs.measure`, to ensure that any measurement is replaced with a dephase operation.

Surprisingly we did not even have a test for the non-subcircuit case. This PR adds a test for that and the subcircuit case.

cirq/sim/act_on_args.py

@@ -29,7 +29,7 @@
 
 import numpy as np
 
-from cirq import protocols
+from cirq import protocols, ops
 from cirq.protocols.decompose_protocol import _try_decompose_into_operations_and_qubits
 from cirq.sim.operation_target import OperationTarget
 
@@ -47,6 +47,7 @@ def __init__(
         prng: np.random.RandomState = None,
         qubits: Sequence['cirq.Qid'] = None,
         log_of_measurement_results: Dict[str, List[int]] = None,
+        ignore_measurement_results: bool = False,
     ):
         """Inits ActOnArgs.
 
@@ -58,6 +59,10 @@ def __init__(
                 ordering of the computational basis states.
             log_of_measurement_results: A mutable object that measurements are
                 being recorded into.
+            ignore_measurement_results: If True, then the simulation
+                will treat measurement as dephasing instead of collapsing
+                process, and not log the result. This is only applicable to
+                simulators that can represent mixed states.
         """
         if prng is None:
             prng = cast(np.random.RandomState, np.random)
@@ -68,13 +73,18 @@ def __init__(
         self._set_qubits(qubits)
         self.prng = prng
         self._log_of_measurement_results = log_of_measurement_results
+        self._ignore_measurement_results = ignore_measurement_results
 
     def _set_qubits(self, qubits: Sequence['cirq.Qid']):
         self._qubits = tuple(qubits)
         self.qubit_map = {q: i for i, q in enumerate(self.qubits)}
 
     def measure(self, qubits: Sequence['cirq.Qid'], key: str, invert_mask: Sequence[bool]):
-        """Adds a measurement result to the log.
+        """Measures the qubits and records to `log_of_measurement_results`.
+
+        Any bitmasks will be applied to the measurement record. If
+        `self._ignore_measurement_results` is set, it dephases instead of
+        measuring, and no measurement result will be logged.
 
         Args:
             qubits: The qubits to measure.
@@ -86,6 +96,9 @@ def measure(self, qubits: Sequence['cirq.Qid'], key: str, invert_mask: Sequence[
         Raises:
             ValueError: If a measurement key has already been logged to a key.
         """
+        if self.ignore_measurement_results:
+            self._act_on_fallback_(ops.phase_damp(1), qubits)
+            return
         bits = self._perform_measurement(qubits)
         corrected = [bit ^ (bit < 2 and mask) for bit, mask in zip(bits, invert_mask)]
         if key in self._log_of_measurement_results:
@@ -184,6 +197,10 @@ def _on_transpose_to_qubit_order(self: TSelf, qubits: Sequence['cirq.Qid'], targ
     def log_of_measurement_results(self) -> Dict[str, List[int]]:
         return self._log_of_measurement_results
 
+    @property
+    def ignore_measurement_results(self) -> bool:
+        return self._ignore_measurement_results
+
     @property
     def qubits(self) -> Tuple['cirq.Qid', ...]:
         return self._qubits

cirq/sim/act_on_density_matrix_args.py

@@ -41,6 +41,7 @@ def __init__(
         prng: np.random.RandomState = None,
         log_of_measurement_results: Dict[str, Any] = None,
         qubits: Sequence['cirq.Qid'] = None,
+        ignore_measurement_results: bool = False,
     ):
         """Inits ActOnDensityMatrixArgs.
 
@@ -60,8 +61,12 @@ def __init__(
                 effects.
             log_of_measurement_results: A mutable object that measurements are
                 being recorded into.
+            ignore_measurement_results: If True, then the simulation
+                will treat measurement as dephasing instead of collapsing
+                process. This is only applicable to simulators that can
+                model dephasing.
         """
-        super().__init__(prng, qubits, log_of_measurement_results)
+        super().__init__(prng, qubits, log_of_measurement_results, ignore_measurement_results)
         self.target_tensor = target_tensor
         self.available_buffer = available_buffer
         self.qid_shape = qid_shape

cirq/sim/density_matrix_simulator.py

@@ -209,6 +209,7 @@ def _create_partial_act_on_args(
             qid_shape=qid_shape,
             prng=self._prng,
             log_of_measurement_results=logs,
+            ignore_measurement_results=self._ignore_measurement_results,
         )
 
     def _can_be_in_run_prefix(self, val: Any):

cirq/sim/density_matrix_simulator_test.py

@@ -514,6 +514,31 @@ def test_simulate(dtype: Type[np.number], split: bool):
     assert len(result.measurements) == 0
 
 
+@pytest.mark.parametrize('split', [True, False])
+def test_simulate_ignore_measurements(split: bool):
+    q0 = cirq.LineQubit(0)
+    simulator = cirq.DensityMatrixSimulator(
+        split_untangled_states=split, ignore_measurement_results=True
+    )
+    circuit = cirq.Circuit(cirq.H(q0), cirq.measure(q0))
+    result = simulator.simulate(circuit)
+    np.testing.assert_almost_equal(result.final_density_matrix, np.eye(2) * 0.5)
+    assert len(result.measurements) == 0
+
+
+@pytest.mark.parametrize('split', [True, False])
+def test_simulate_ignore_measurements_subcircuits(split: bool):
+    q0 = cirq.LineQubit(0)
+    simulator = cirq.DensityMatrixSimulator(
+        split_untangled_states=split, ignore_measurement_results=True
+    )
+    circuit = cirq.Circuit(cirq.H(q0), cirq.measure(q0))
+    circuit = cirq.Circuit(cirq.CircuitOperation(circuit.freeze()))
+    result = simulator.simulate(circuit)
+    np.testing.assert_almost_equal(result.final_density_matrix, np.eye(2) * 0.5)
+    assert len(result.measurements) == 0
+
+
 @pytest.mark.parametrize('dtype', [np.complex64, np.complex128])
 @pytest.mark.parametrize('split', [True, False])
 def test_simulate_qudits(dtype: Type[np.number], split: bool):

cirq/sim/simulator_base.py

@@ -207,18 +207,13 @@ def _core_iterator(
         for moment in noisy_moments:
             for op in ops.flatten_to_ops(moment):
                 try:
-                    # TODO: support more general measurements.
-                    # Github issue: https://github.com/quantumlib/Cirq/issues/3566
-
                     # Preprocess measurements
                     if all_measurements_are_terminal and measured[op.qubits]:
                         continue
                     if isinstance(op.gate, ops.MeasurementGate):
                         measured[op.qubits] = True
                         if all_measurements_are_terminal:
                             continue
-                        if self._ignore_measurement_results:
-                            op = ops.phase_damp(1).on(*op.qubits)
 
                     # Simulate the operation
                     protocols.act_on(op, sim_state)