Summarize the task
Refactor cirq-google to

• Simplify Quantum Engine DeviceSpecification proto and its serialization/deserialization logic.
• Implement Devices 2.0 and GateSets APIs in cirq-core.

Acceptance criteria - when is the task considered done?
When all tasks below are complete:

• DeviceSpecification proto updates
• GateFamily tag support
• CompilationTargetGateset support in GridDeviceMetadata
• GridDevice shell
• Gateset generation
• Gate duration
• Look into refactoring qubit targets
• Remove valid_targets in the new GateSpecification
• Deprecations
  • SerializableDevice
  • SerializableGateset
  • gate_set field in Engine methods. Deprecate gate_set args in various Engine methods #4995
• Handle PhysicalZ in CompilationTargetGatesets used by GridDevice
• Rerouting to GridDevice
  • get_device()
  • Sycamore example devices
  • Virtual engine factory
  • cg.workflow ProcessorRecord
• Update example DeviceSpecifications
• Downstream dependency refactors
  • cirq_google/workflow - use Gateset validation/compilation (cc @mpharrigan )
• Remove deprecations
  • Deprecated create_proto methods in cirq_google.devices.known_devices.py (deprecation PR: Deprecate create_device_proto_for_qubits #5592 and 983875e)
  • SerializableDevice class (deprecation PR: Deprecate SerializableDevice #5522)
  • Global SerializableGateSets (cirq_google/serialization/gate_sets.py) and the SerializableGateSet class (deprecation PR: SerializableGateSet deprecation #5573)
  • Common serializers (cirq_google/serialization/common_serializers.py) (deprecation PR: Deprecate common serializers #5611)
  • Unused serializer classes (deprecation PR: Serializers deprecation #5589)
  • GridDevice.qubits property (deprecation PR: Temporary GridDevice.qubits property #5593)
  • [?] Program.language.gateset proto field (deprecation PR: Deprecate Program.language.gate_set proto field #5591)
  • Optimized_for_sycamore, optimized_for_xmon [blocked on Google target gateset implementation] (deprecation PR: Deprecate optimize_for_sycamore and optimize_for_xmon #5531)

Future improvements:

• Serialize GridDevice back to DeviceSpecification
• Documentation
  • Update cirq_google tutorials
  • Document how to add/remove gates
• SycamoreTargetGateset additional_gates arg
• Deprecate ASYMMETRIC and SUBSET_PERMUTATION target types.
• Make gate_durations optional (requires protobuf 3.15)
  • While the protoc flag --experimental_allow_proto3_optional can be used for protoc 3.12-3.14, protoc-gen-mypy doesn't support it.
• Removing deprecated fields in program.proto and device.proto. Can consider removing few months after Cirq 1.0, or can be left untouched until v3 protos.
• More frequent Device refreshes [unclear if necessary]
  • Update devices documentation

Related
Cirq Device 2.0 design
Cirq Gateset design

Related issues: #abc, #xyz, ...