SingleQubitCliffordGate does not properly serialize to JSON

Description of the issue

SingleQubitCliffordGates write incomplete information to JSON, and their type cannot be understood by the JSON deserializer.

How to reproduce the issue

>>> import cirq
>>> print(cirq.__version__)
0.10.0
>>> q = cirq.LineQubit(0)
>>> json_text = cirq.to_json(cirq.SingleQubitCliffordGate.X(q))
>>> print(json_text)
{
  "cirq_type": "GateOperation",
  "gate": {
    "cirq_type": "SingleQubitCliffordGate"
  },
  "qubits": [
    {
      "cirq_type": "LineQubit",
      "x": 0
    }
  ]
}
>>> deserialized_gate = cirq.read_json(json_text=json_text)
ValueError: Could not resolve type 'SingleQubitCliffordGate' during deserialization

Cirq version

0.10.0

[ghost]

Yes, please take it. I will be able to contribute to Cirq from the month of September.

[balopat]

Hi @PoPaT1618 - thank you for volunteering!

I saw that you are new to the project, so, just to clarify, the following needs to be done:

• remove SingleQubitCliffordGate from this list: https://github.com/quantumlib/Cirq/blob/a41df7f97a9eddba8fcaa4c19c6366acd67de4de/cirq-core/cirq/protocols/json_test_data/spec.py#L67
• add the class between these two lines: https://github.com/quantumlib/Cirq/blob/a41df7f97a9eddba8fcaa4c19c6366acd67de4de/cirq-core/cirq/json_resolver_cache.py#L125-L126
• then we’ll have to make the cirq-core/cirq/protocols/json_serialization_test.py pass (you can run it with check/pytest cirq-core/cirq/protocols/json_serialization_test.py)
  • you’ll need to implement the _json_dict_ magic method for the json serialization protocol within the SingleQubitCliffordGate class, along the lines of:

    def _json_dict_(self) -> Dict[str, Any]:
        return protocols.obj_to_dict_helper(
            self, ['_rotation_map', '_inverse_map']
        )

At this point you might be ready to commit, and create a pull request, but ping us if you run into issues.

Thanks!