Add Google-specific variant for noise properties. (quantumlib#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 quantumlib#4666; part 2 was quantumlib#4964. Remaining steps include:

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

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/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/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.