Add Google-specific variant for noise properties. (#5082)

This PR provides an implementation of `SuperconductingQubitsNoiseProperties` for Google hardware. Now that SQNP is an abstract type, this is a prerequisite for re-enabling calibration-to-noise tools.

This is part 3 of #4666; part 2 was #4964. Remaining steps include:

- Reinstatement of calibration_to_noise_properties using new types
- Reinstatement of compare_generated_noise_to_metrics using new types

cirq-core/cirq/devices/noise_properties.py

@@ -22,7 +22,7 @@
 import abc
 from typing import Iterable, Sequence, TYPE_CHECKING, List
 
-from cirq import _import, ops, protocols, devices
+from cirq import _compat, _import, ops, protocols, devices
 from cirq.devices.noise_utils import (
     PHYSICAL_GATE_TAG,
 )
@@ -54,7 +54,7 @@ def __init__(self, noise_properties: NoiseProperties) -> None:
         self._noise_properties = noise_properties
         self.noise_models = self._noise_properties.build_noise_models()
 
-    def virtual_predicate(self, op: 'cirq.Operation') -> bool:
+    def is_virtual(self, op: 'cirq.Operation') -> bool:
         """Returns True if an operation is virtual.
 
         Device-specific subclasses should implement this method to mark any
@@ -68,6 +68,10 @@ def virtual_predicate(self, op: 'cirq.Operation') -> bool:
         """
         return False
 
+    @_compat.deprecated(deadline='v0.16', fix='Use is_virtual instead.')
+    def virtual_predicate(self, op: 'cirq.Operation') -> bool:
+        return self.is_virtual(op)
+
     def noisy_moments(
         self, moments: Iterable['cirq.Moment'], system_qubits: Sequence['cirq.Qid']
     ) -> Sequence['cirq.OP_TREE']:
@@ -91,7 +95,7 @@ def noisy_moments(
         # using `self.virtual_predicate` to determine virtuality.
         new_moments = []
         for moment in split_measure_moments:
-            virtual_ops = {op for op in moment if self.virtual_predicate(op)}
+            virtual_ops = {op for op in moment if self.is_virtual(op)}
             physical_ops = [
                 op.with_tags(PHYSICAL_GATE_TAG) for op in moment if op not in virtual_ops
             ]

cirq-core/cirq/devices/noise_properties_test.py

@@ -60,3 +60,11 @@ def test_sample_model():
         cirq.Moment(cirq.H(q0), cirq.H(q1)),
     )
     assert noisy_circuit == expected_circuit
+
+
+def test_deprecated_virtual_predicate():
+    q0, q1 = cirq.LineQubit.range(2)
+    props = SampleNoiseProperties([q0, q1], [(q0, q1), (q1, q0)])
+    model = NoiseModelFromNoiseProperties(props)
+    with cirq.testing.assert_deprecated("Use is_virtual", deadline="v0.16"):
+        _ = model.virtual_predicate(cirq.X(q0))

cirq-core/cirq/devices/superconducting_qubits_noise_properties.py

@@ -35,14 +35,15 @@ class SuperconductingQubitsNoiseProperties(devices.NoiseProperties, abc.ABC):
 
     Args:
         gate_times_ns: Dict[type, float] of gate types to their duration on
-            quantum hardware.
+            quantum hardware. Used with t(1|phi)_ns to specify thermal noise.
         t1_ns: Dict[cirq.Qid, float] of qubits to their T_1 time, in ns.
         tphi_ns: Dict[cirq.Qid, float] of qubits to their T_phi time, in ns.
         readout_errors: Dict[cirq.Qid, np.ndarray] of qubits to their readout
             errors in matrix form: [P(read |1> from |0>), P(read |0> from |1>)].
+            Used to prepend amplitude damping errors to measurements.
         gate_pauli_errors: dict of noise_utils.OpIdentifiers (a gate and the qubits it
-            targets) to the Pauli error for that operation. Keys in this dict
-            must have defined qubits.
+            targets) to the Pauli error for that operation. Used to construct
+            depolarizing error. Keys in this dict must have defined qubits.
         validate: If True, verifies that t1 and tphi qubits sets match, and
             that all symmetric two-qubit gates have errors which are
             symmetric over the qubits they affect. Defaults to True.

cirq-google/cirq_google/devices/google_noise_properties.py

@@ -0,0 +1,139 @@
+# Copyright 2022 The Cirq Developers
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+
+"""Class for representing noise on a Google device."""
+
+from dataclasses import dataclass, field
+from typing import Dict, List, Set, Type
+import numpy as np
+
+import cirq, cirq_google
+from cirq import _compat, devices
+from cirq.devices import noise_utils
+from cirq.transformers.heuristic_decompositions import gate_tabulation_math_utils
+
+
+SINGLE_QUBIT_GATES: Set[Type['cirq.Gate']] = {
+    cirq.ZPowGate,
+    cirq.PhasedXZGate,
+    cirq.MeasurementGate,
+    cirq.ResetChannel,
+}
+SYMMETRIC_TWO_QUBIT_GATES: Set[Type['cirq.Gate']] = {
+    cirq_google.SycamoreGate,
+    cirq.FSimGate,
+    cirq.PhasedFSimGate,
+    cirq.ISwapPowGate,
+    cirq.CZPowGate,
+}
+ASYMMETRIC_TWO_QUBIT_GATES: Set[Type['cirq.Gate']] = set()
+
+
+@dataclass
+class GoogleNoiseProperties(devices.SuperconductingQubitsNoiseProperties):
+    """Noise-defining properties for a Google device.
+
+    Inherited args:
+        gate_times_ns: Dict[type, float] of gate types to their duration on
+            quantum hardware. Used with t(1|phi)_ns to specify thermal noise.
+        t1_ns: Dict[cirq.Qid, float] of qubits to their T_1 time, in ns.
+        tphi_ns: Dict[cirq.Qid, float] of qubits to their T_phi time, in ns.
+        readout_errors: Dict[cirq.Qid, np.ndarray] of qubits to their readout
+            errors in matrix form: [P(read |1> from |0>), P(read |0> from |1>)].
+            Used to prepend amplitude damping errors to measurements.
+        gate_pauli_errors: dict of noise_utils.OpIdentifiers (a gate and the qubits it
+            targets) to the Pauli error for that operation. Used to construct
+            depolarizing error. Keys in this dict must have defined qubits.
+        validate: If True, verifies that t1 and tphi qubits sets match, and
+            that all symmetric two-qubit gates have errors which are
+            symmetric over the qubits they affect. Defaults to True.
+
+    Additional args:
+        fsim_errors: Dict[noise_utils.OpIdentifier, cirq.PhasedFSimGate] of gate types
+            (potentially on specific qubits) to the PhasedFSim fix-up operation
+            for that gate. Defaults to no-op for all gates.
+    """
+
+    fsim_errors: Dict[noise_utils.OpIdentifier, cirq.PhasedFSimGate] = field(default_factory=dict)
+
+    def __post_init__(self):
+        super().__post_init__()
+
+        # validate two qubit gate errors.
+        self._validate_symmetric_errors('fsim_errors')
+
+    @classmethod
+    def single_qubit_gates(cls) -> Set[type]:
+        return SINGLE_QUBIT_GATES
+
+    @classmethod
+    def symmetric_two_qubit_gates(cls) -> Set[type]:
+        return SYMMETRIC_TWO_QUBIT_GATES
+
+    @classmethod
+    def asymmetric_two_qubit_gates(cls) -> Set[type]:
+        return ASYMMETRIC_TWO_QUBIT_GATES
+
+    @_compat.cached_property
+    def _depolarizing_error(self) -> Dict[noise_utils.OpIdentifier, float]:
+        depol_errors = super()._depolarizing_error
+
+        def extract_entangling_error(match_id: noise_utils.OpIdentifier):
+            """Gets the entangling error component of depol_errors[match_id]."""
+            unitary_err = cirq.unitary(self.fsim_errors[match_id])
+            fid = gate_tabulation_math_utils.unitary_entanglement_fidelity(unitary_err, np.eye(4))
+            return 1 - fid
+
+        # Subtract entangling angle error.
+        for op_id in depol_errors:
+            if op_id.gate_type not in self.two_qubit_gates():
+                continue
+            if op_id in self.fsim_errors:
+                depol_errors[op_id] -= extract_entangling_error(op_id)
+                continue
+            # Find the closest matching supertype, if one is provided.
+            # Gateset has similar behavior, but cannot be used here
+            # because depol_errors is a dict, not a set.
+            match_id = None
+            candidate_parents = [
+                parent_id for parent_id in self.fsim_errors if op_id.is_proper_subtype_of(parent_id)
+            ]
+            for parent_id in candidate_parents:
+                if match_id is None or parent_id.is_proper_subtype_of(match_id):
+                    match_id = parent_id
+            if match_id is not None:
+                depol_errors[op_id] -= extract_entangling_error(match_id)
+
+        return depol_errors
+
+    def build_noise_models(self) -> List['cirq.NoiseModel']:
+        """Construct all NoiseModels associated with NoiseProperties."""
+        noise_models = super().build_noise_models()
+
+        # Insert entangling gate coherent errors after thermal error.
+        if self.fsim_errors:
+            fsim_ops = {op_id: gate.on(*op_id.qubits) for op_id, gate in self.fsim_errors.items()}
+            noise_models.insert(1, devices.InsertionNoiseModel(ops_added=fsim_ops))
+
+        return noise_models
+
+
+class NoiseModelFromGoogleNoiseProperties(devices.NoiseModelFromNoiseProperties):
+    """A noise model defined from noise properties of a Google device."""
+
+    def is_virtual(self, op: cirq.Operation) -> bool:
+        return isinstance(op.gate, cirq.ZPowGate) and cirq_google.PhysicalZTag not in op.tags
+
+    # noisy_moments is implemented by the superclass.