Why does `ControlledGate` have a notion of qubits

ControlledGate.control_qubits is a tuple of Qid (or Nones). I thought “gates” don’t have qubits and that that is reserved for Operations.

[mpharrigan]

If you can have partially specified qubits, I’m led to ask why bother having a difference between Gate and Operation at all? There could be one object that has any of its qubits specified or not. If all of them are there, it would be similar to an Operation. If none of them are there, it would be similar to (most) Gates.

Earlier versions of pyquil provided first-class support for partial specification of qubits for each gate/operation. I ripped that out since people typically didn’t use it (especially for NISQ algorithms) and in special cases you could use closures (at the python language level) for the same effect.

[smitsanghavi]

The original reason for going this route was to have the flexibility of calling control(GateX, my_control_qubits) to generate a ControlledGate where you can specify the qubits GateX uses (and also some more controls) later. The assumption was that control qubits might be used differently than “regular” qubits and having an option to pre-specify some controls without being asked to resolve the underlying Gate to an Operation might come in handy.

But if in the subsequent months no such use case has shown up, cleaning up the additional complexity is probably a good idea.