Support prepending noise (#5410)

Fixes #2570.

This is essentially a more friendly version of #5327 which maintains existing behavior but gives users the option to prepend noise (or append, in the case of readout error) on various noise models.

Author: 95-martin-orion

cirq-core/cirq/contrib/noise_models/noise_models.py

@@ -29,26 +29,29 @@ class DepolarizingNoiseModel(devices.NoiseModel):
     also contain gates.
     """
 
-    def __init__(self, depol_prob: float):
+    def __init__(self, depol_prob: float, prepend: bool = False):
         """A depolarizing noise model
 
         Args:
             depol_prob: Depolarizing probability.
+            prepend: If True, put noise before affected gates. Default: False.
         """
         value.validate_probability(depol_prob, 'depol prob')
         self.qubit_noise_gate = ops.DepolarizingChannel(depol_prob)
+        self._prepend = prepend
 
     def noisy_moment(self, moment: 'cirq.Moment', system_qubits: Sequence['cirq.Qid']):
         if validate_all_measurements(moment) or self.is_virtual_moment(moment):
             # coverage: ignore
             return moment
 
-        return [
+        output = [
             moment,
             circuits.Moment(
                 self.qubit_noise_gate(q).with_tags(ops.VirtualTag()) for q in system_qubits
             ),
         ]
+        return output[::-1] if self._prepend else output
 
 
 class ReadoutNoiseModel(devices.NoiseModel):
@@ -63,25 +66,28 @@ class ReadoutNoiseModel(devices.NoiseModel):
     also contain gates.
     """
 
-    def __init__(self, bitflip_prob: float):
+    def __init__(self, bitflip_prob: float, prepend: bool = True):
         """A noise model with readout error.
 
         Args:
             bitflip_prob: Probability of a bit-flip during measurement.
+            prepend: If True, put noise before affected gates. Default: True.
         """
         value.validate_probability(bitflip_prob, 'bitflip prob')
         self.readout_noise_gate = ops.BitFlipChannel(bitflip_prob)
+        self._prepend = prepend
 
     def noisy_moment(self, moment: 'cirq.Moment', system_qubits: Sequence['cirq.Qid']):
         if self.is_virtual_moment(moment):
             return moment
         if validate_all_measurements(moment):
-            return [
+            output = [
                 circuits.Moment(
                     self.readout_noise_gate(q).with_tags(ops.VirtualTag()) for q in system_qubits
                 ),
                 moment,
             ]
+            return output if self._prepend else output[::-1]
         return moment
 
 
@@ -97,25 +103,28 @@ class DampedReadoutNoiseModel(devices.NoiseModel):
     also contain gates.
     """
 
-    def __init__(self, decay_prob: float):
+    def __init__(self, decay_prob: float, prepend: bool = True):
         """A depolarizing noise model with damped readout error.
 
         Args:
             decay_prob: Probability of T1 decay during measurement.
+            prepend: If True, put noise before affected gates. Default: True.
         """
         value.validate_probability(decay_prob, 'decay_prob')
         self.readout_decay_gate = ops.AmplitudeDampingChannel(decay_prob)
+        self._prepend = prepend
 
     def noisy_moment(self, moment: 'cirq.Moment', system_qubits: Sequence['cirq.Qid']):
         if self.is_virtual_moment(moment):
             return moment
         if validate_all_measurements(moment):
-            return [
+            output = [
                 circuits.Moment(
                     self.readout_decay_gate(q).with_tags(ops.VirtualTag()) for q in system_qubits
                 ),
                 moment,
             ]
+            return output if self._prepend else output[::-1]
         return moment
 
 

cirq-core/cirq/contrib/noise_models/noise_models_test.py

@@ -29,6 +29,17 @@ def test_depol_noise():
         assert isinstance(g.gate, cirq.DepolarizingChannel)
 
 
+def test_depol_noise_prepend():
+    noise_model = ccn.DepolarizingNoiseModel(depol_prob=0.005, prepend=True)
+    qubits = cirq.LineQubit.range(2)
+    moment = cirq.Moment([cirq.X(qubits[0]), cirq.Y(qubits[1])])
+    noisy_mom = noise_model.noisy_moment(moment, system_qubits=qubits)
+    assert len(noisy_mom) == 2
+    assert noisy_mom[1] == moment
+    for g in noisy_mom[0]:
+        assert isinstance(g.gate, cirq.DepolarizingChannel)
+
+
 # Composes depolarization noise with readout noise.
 def test_readout_noise_after_moment():
     program = cirq.Circuit()
@@ -81,6 +92,17 @@ def test_readout_noise_after_moment():
     assert_equivalent_op_tree(true_noisy_program, noisy_circuit)
 
 
+def test_readout_noise_no_prepend():
+    noise_model = ccn.ReadoutNoiseModel(bitflip_prob=0.005, prepend=False)
+    qubits = cirq.LineQubit.range(2)
+    moment = cirq.Moment([cirq.measure(*qubits, key="meas")])
+    noisy_mom = noise_model.noisy_moment(moment, system_qubits=qubits)
+    assert len(noisy_mom) == 2
+    assert noisy_mom[0] == moment
+    for g in noisy_mom[1]:
+        assert isinstance(g.gate, cirq.BitFlipChannel)
+
+
 # Composes depolarization, damping, and readout noise (in that order).
 def test_decay_noise_after_moment():
     program = cirq.Circuit()
@@ -138,6 +160,17 @@ def test_decay_noise_after_moment():
     assert_equivalent_op_tree(true_noisy_program, noisy_circuit)
 
 
+def test_damped_readout_noise_no_prepend():
+    noise_model = ccn.DampedReadoutNoiseModel(decay_prob=0.005, prepend=False)
+    qubits = cirq.LineQubit.range(2)
+    moment = cirq.Moment([cirq.measure(*qubits, key="meas")])
+    noisy_mom = noise_model.noisy_moment(moment, system_qubits=qubits)
+    assert len(noisy_mom) == 2
+    assert noisy_mom[0] == moment
+    for g in noisy_mom[1]:
+        assert isinstance(g.gate, cirq.AmplitudeDampingChannel)
+
+
 # Test the aggregate noise models.
 def test_aggregate_readout_noise_after_moment():
     program = cirq.Circuit()

cirq-core/cirq/devices/insertion_noise_model.py

@@ -37,7 +37,7 @@ class InsertionNoiseModel(devices.NoiseModel):
             type is more specific (e.g. A is a subtype of B, but B defines
             qubits and A does not) then the first one appering in this dict
             will match.
-        prepend: whether to add the new moment before the current one.
+        prepend: If True, put noise before affected gates. Default: False.
         require_physical_tag: whether to only apply noise to operations tagged
             with PHYSICAL_GATE_TAG.
     """

cirq-core/cirq/devices/noise_model.py

@@ -212,10 +212,20 @@ class ConstantQubitNoiseModel(NoiseModel):
     operation is given as "the noise to use" for a `NOISE_MODEL_LIKE` parameter.
     """
 
-    def __init__(self, qubit_noise_gate: 'cirq.Gate'):
+    def __init__(self, qubit_noise_gate: 'cirq.Gate', prepend: bool = False):
+        """Noise model which applies a specific gate as noise to all gates.
+
+        Args:
+            qubit_noise_gate: The "noise" gate to use.
+            prepend: If True, put noise before affected gates. Default: False.
+
+        Raises:
+            ValueError: if qubit_noise_gate is not a single-qubit gate.
+        """
         if qubit_noise_gate.num_qubits() != 1:
             raise ValueError('noise.num_qubits() != 1')
         self.qubit_noise_gate = qubit_noise_gate
+        self._prepend = prepend
 
     def _value_equality_values_(self) -> Any:
         return self.qubit_noise_gate
@@ -227,12 +237,13 @@ def noisy_moment(self, moment: 'cirq.Moment', system_qubits: Sequence['cirq.Qid'
         # Noise should not be appended to previously-added noise.
         if self.is_virtual_moment(moment):
             return moment
-        return [
+        output = [
             moment,
             moment_module.Moment(
                 [self.qubit_noise_gate(q).with_tags(ops.VirtualTag()) for q in system_qubits]
             ),
         ]
+        return output[::-1] if self._prepend else output
 
     def _json_dict_(self):
         return protocols.obj_to_dict_helper(self, ['qubit_noise_gate'])
@@ -246,6 +257,11 @@ def _has_mixture_(self):
 
 class GateSubstitutionNoiseModel(NoiseModel):
     def __init__(self, substitution_func: Callable[['cirq.Operation'], 'cirq.Operation']):
+        """Noise model which replaces operations using a substitution function.
+
+        Args:
+            substitution_func: a function for replacing operations.
+        """
         self.substitution_func = substitution_func
 
     def noisy_moment(

cirq-core/cirq/devices/noise_model_test.py

@@ -124,6 +124,25 @@ def test_constant_qubit_noise():
         _ = cirq.ConstantQubitNoiseModel(cirq.CNOT**0.01)
 
 
+def test_constant_qubit_noise_prepend():
+    a, b, c = cirq.LineQubit.range(3)
+    damp = cirq.amplitude_damp(0.5)
+    damp_all = cirq.ConstantQubitNoiseModel(damp, prepend=True)
+    actual = damp_all.noisy_moments([cirq.Moment([cirq.X(a)]), cirq.Moment()], [a, b, c])
+    expected = [
+        [
+            cirq.Moment(d.with_tags(ops.VirtualTag()) for d in [damp(a), damp(b), damp(c)]),
+            cirq.Moment([cirq.X(a)]),
+        ],
+        [
+            cirq.Moment(d.with_tags(ops.VirtualTag()) for d in [damp(a), damp(b), damp(c)]),
+            cirq.Moment(),
+        ],
+    ]
+    assert actual == expected
+    cirq.testing.assert_equivalent_repr(damp_all)
+
+
 def test_noise_composition():
     # Verify that noise models can be composed without regard to ordering, as
     # long as the noise operators commute with one another.

cirq-core/cirq/devices/thermal_noise_model.py

@@ -175,6 +175,7 @@ def __init__(
         dephase_rate_GHz: Union[float, Dict['cirq.Qid', float], None] = None,
         require_physical_tag: bool = True,
         skip_measurements: bool = True,
+        prepend: bool = False,
     ):
         """Construct a ThermalNoiseModel data object.
 
@@ -203,6 +204,7 @@ def __init__(
             require_physical_tag: whether to only apply noise to operations
                 tagged with PHYSICAL_GATE_TAG.
             skip_measurements: whether to skip applying noise to measurements.
+            prepend: If True, put noise before affected gates. Default: False.
 
         Returns:
             The ThermalNoiseModel with specified parameters.
@@ -225,6 +227,7 @@ def __init__(
         self.rate_matrix_GHz: Dict['cirq.Qid', np.ndarray] = rate_dict
         self.require_physical_tag: bool = require_physical_tag
         self.skip_measurements: bool = skip_measurements
+        self._prepend = prepend
 
     def noisy_moment(
         self, moment: 'cirq.Moment', system_qubits: Sequence['cirq.Qid']
@@ -277,4 +280,5 @@ def noisy_moment(
             noise_ops.append(ops.KrausChannel(kraus_ops).on(qubit))
         if not noise_ops:
             return [moment]
-        return [moment, moment_module.Moment(noise_ops)]
+        output = [moment, moment_module.Moment(noise_ops)]
+        return output[::-1] if self._prepend else output

cirq-core/cirq/devices/thermal_noise_model_test.py

@@ -225,6 +225,36 @@ def test_noisy_moment_one_qubit():
     )
 
 
+def test_noisy_moment_one_qubit_prepend():
+    q0, q1 = cirq.LineQubit.range(2)
+    model = ThermalNoiseModel(
+        qubits={q0, q1},
+        gate_durations_ns={cirq.PhasedXZGate: 25.0, cirq.CZPowGate: 25.0},
+        heat_rate_GHz={q0: 1e-5, q1: 2e-5},
+        cool_rate_GHz={q0: 1e-4, q1: 2e-4},
+        dephase_rate_GHz={q0: 3e-4, q1: 4e-4},
+        require_physical_tag=False,
+        prepend=True,
+    )
+    gate = cirq.PhasedXZGate(x_exponent=1, z_exponent=0.5, axis_phase_exponent=0.25)
+    moment = cirq.Moment(gate.on(q0))
+    noisy_moment = model.noisy_moment(moment, system_qubits=[q0, q1])
+    # Noise applies to both qubits, even if only one is acted upon.
+    assert len(noisy_moment[0]) == 2
+    noisy_choi = cirq.kraus_to_choi(cirq.kraus(noisy_moment[0].operations[0]))
+    assert np.allclose(
+        noisy_choi,
+        [
+            [9.99750343e-01, 0, 0, 9.91164267e-01],
+            [0, 2.49656565e-03, 0, 0],
+            [0, 0, 2.49656565e-04, 0],
+            [9.91164267e-01, 0, 0, 9.97503434e-01],
+        ],
+    )
+    # Prepend puts noise before the original moment.
+    assert noisy_moment[1] == moment
+
+
 def test_noise_from_wait():
     # Verify that wait-gate noise is duration-dependent.
     q0 = cirq.LineQubit(0)