PauliSumCollector is not compatible with engine

[MichaelBroughton]

In PauliSum Collector when doing the basis transformations with

Cirq/cirq/work/pauli_sum_collector.py

Line 112 in 3469e5c

circuit.append(ops.Moment(pauli_string.to_z_basis_ops()))

The gates that get returned are cirq.SingleQubitClifford(...) which won't play nice with the serializers. A quick fix is to just add a cirq.decompose around the result of pauli_string.to_z_basis_ops(), but I wasn't sure if people might have other opinions ?

[mpharrigan]

xref #2781

[balopat]

Just to clarify this is what you are facing, right?

cirq.google.SYC_GATESET.serialize(cirq.Circuit(cirq.SingleQubitCliffordGate.Y(cirq.NamedQubit("a"))))

Throws an error

ValueError: Cannot serialize op cirq.SingleQubitCliffordGate(X:-X, Y:+Y, Z:-Z).on(cirq.NamedQubit('a')) of type <class 'cirq.ops.clifford_gate.SingleQubitCliffordGate'>

While

 cirq.google.SYC_GATESET.serialize(cirq.Circuit(cirq.decompose(cirq.SingleQubitCliffordGate.Y(cirq.NamedQubit("a")))))

does the trick.

To answer your question: cirq.decompose happens to compile to the gateset that is compatible with the SYC_GATESET serializers, so today that seems to be your best bet.

Thoughts:

1. there is no nicer option than cirq.decompose() to convert a SingleQubitCliffordGate in general to regular Pauli gates that are recognized by our common serializers
2. we could change to_z_basis_ops so it returns a sequence of regular Pauli gate (using cirq.decompose) - the problem with that is that some algorithms might like the fact that it returns the SingleQubitCliffordGate representation.
3. we could add decomposition in the serializers as an automated fallback strategy, guarded by a parameter - eg. SYC_GATESET.serialize(circuit, decompose_on_fail=True) would try to decompose an unrecognized gate.
4. We could make the serialization error message better and suggest decomposition
5. We could add a SingleQubitCliffordGate serializer to our gatesets that would do the decomposition.

I think 3 and 4 would be my preference here.
@viathor @dstrain115 @dabacon ?