GridDevice: Exclude MeasurementGates in validation of qubit pairs

[verult]

I'm comfortable with special-casing MeasurementGate because it's the only gate today with the property that it can be applied to any subset of qubits.

Fixes #5652

@maffoo

[maffoo]

Would be good to add some test cases of measurement on more qubits, for example triplets of qubits or all qubits (cirq.measure(*device_info.grid_qubits)).

[CirqBot]

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[dabacon]

What about identity gate, does it also need to be allowed? (Yes odd to do identity across multiple qubits, but I think it is technically a multiqubit gate. Reset isn't)

[verult]

Ah right. I left identity gate as a TODO which will be addressed soon in a separate PR. Will change the check to something like not any(operation in gf for gf in SUBSET_GATE_FAMILIES) in a separate PR

[dstrain115]

What about wait gate? Does that need to be handled here?

[verult]

@dstrain115 does this mean only single-qubit WaitGates are allowed by the CircuitSerializer since both num_qubits and qid_shape are None in the WaitGate constructor?

Cirq/cirq-google/cirq_google/serialization/circuit_serializer.py

Line 568 in b592ae7

op = cirq.WaitGate(duration=cirq.Duration(nanos=total_nanos or 0.0))(*qubits)

[dstrain115]

It's been a while since I dealt with this, but I think wait gates are similar to measurement gates in that they can be any permutation of qubits. I think they were the PERMUTATION_SET in the old device spec.

[verult]

Added WaitGate support

[maffoo]

I'd suggest using the term "variadic" to describe these gates, otherwise this looks good to me.

[maffoo]

Could we perhaps call this _VARIADIC_GATE_FAMILIES instead? I think the term "variadic" captures the property we want, that these are gates that can be applied to any number of qubits.

[dstrain115]

I am going to remember this vocabulary word for when I take the Quantum SATs!

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