Engine creator helper function (quantumlib#5658)

Add a helper function to create a virtual engine with a noisy simulator.

Author: augustehirth

cirq-google/cirq_google/engine/__init__.py

@@ -72,6 +72,7 @@
 from cirq_google.engine.validating_sampler import ValidatingSampler
 
 from cirq_google.engine.virtual_engine_factory import (
+    create_default_noisy_quantum_virtual_machine,
     create_device_from_processor_id,
     create_noiseless_virtual_engine_from_device,
     create_noiseless_virtual_engine_from_proto,

cirq-google/cirq_google/engine/virtual_engine_factory.py

@@ -14,16 +14,19 @@
 
 """Functions to instantiate SimulatedLocalEngines to simulate various Google Devices."""
 import json
-from typing import cast, Iterable, List, Optional, Union
+from typing import cast, Iterable, List, Optional, Union, Type
 import pathlib
 import time
 
 import google.protobuf.text_format as text_format
 import cirq
+from cirq.sim.simulator import SimulatesSamples
 from cirq_google.api import v2
 from cirq_google.engine import calibration, engine_validator, simulated_local_processor, util
 from cirq_google.devices import grid_device
+from cirq_google.devices.google_noise_properties import NoiseModelFromGoogleNoiseProperties
 from cirq_google.serialization import serializable_gate_set
+from cirq_google.engine.calibration_to_noise_properties import noise_properties_from_calibration
 from cirq_google.engine.simulated_local_engine import SimulatedLocalEngine
 from cirq_google.engine.simulated_local_processor import SimulatedLocalProcessor
 
@@ -367,3 +370,44 @@ def create_noiseless_virtual_engine_from_latest_templates() -> SimulatedLocalEng
     processor_ids = list(MOST_RECENT_TEMPLATES.keys())
     template_names = [MOST_RECENT_TEMPLATES[k] for k in processor_ids]
     return create_noiseless_virtual_engine_from_templates(processor_ids, template_names)
+
+
+def create_default_noisy_quantum_virtual_machine(
+    processor_id: str, simulator_class: Type[SimulatesSamples] = None, **kwargs
+) -> SimulatedLocalEngine:
+    """Creates a virtual engine with a noisy simulator based on a processor id.
+
+    Args:
+        processor_id: The string name of a processor that has available noise data.
+        simulator_class: The class of the type of simulator to be initialized. The
+            simulator class initializer needs to support the `noise` parameter.
+        kwargs: Other arguments which are passed through to the simulator initializer.
+            The 'noise' argument will be overwritten with a new noise model.
+
+    Returns:
+        A SimulatedLocalEngine that uses a simulator of type simulator_class with a
+            noise model based on available noise data for the processor processor_id.
+    """
+
+    if simulator_class is None:
+        try:  # coverage: ignore
+            import qsimcirq  # type: ignore
+
+            simulator_class = qsimcirq.Simulator  # coverage: ignore
+        except ImportError:
+            simulator_class = cirq.Simulator  # coverage: ignore
+
+    calibration = load_median_device_calibration(processor_id)
+    noise_properties = noise_properties_from_calibration(calibration)
+    noise_model = NoiseModelFromGoogleNoiseProperties(noise_properties)
+    simulator = simulator_class(noise=noise_model, **kwargs)  # type: ignore
+
+    device = create_device_from_processor_id(processor_id)
+    simulated_processor = SimulatedLocalProcessor(
+        processor_id=processor_id,
+        sampler=simulator,
+        device=device,
+        calibrations={calibration.timestamp // 1000: calibration},
+    )
+
+    return SimulatedLocalEngine([simulated_processor])

cirq-google/cirq_google/engine/virtual_engine_factory_test.py

@@ -175,3 +175,19 @@ def test_create_from_proto_no_qubits():
         _ = factory.create_noiseless_virtual_engine_from_device(
             'sycamore', cirq.UNCONSTRAINED_DEVICE
         )
+
+
+def test_create_default_noisy_quantum_virtual_machine():
+    for processor_id in ["rainbow", "weber"]:
+        engine = factory.create_default_noisy_quantum_virtual_machine(
+            processor_id=processor_id, simulator_class=cirq.Simulator
+        )
+        processor = engine.get_processor(processor_id)
+        bad_qubit = cirq.GridQubit(10, 10)
+        circuit = cirq.Circuit(cirq.X(bad_qubit), cirq.measure(bad_qubit))
+        with pytest.raises(ValueError, match='Qubit not on device'):
+            _ = processor.run(circuit, repetitions=100)
+        good_qubit = cirq.GridQubit(5, 4)
+        circuit = cirq.Circuit(cirq.H(good_qubit), cirq.measure(good_qubit))
+        with pytest.raises(ValueError, match='.* contains a gate which is not supported.'):
+            _ = processor.run(circuit, repetitions=100)