utils: (qssa-builder) introduce builder for qssa

[alexarice]

Lets a qssa program be built in a style similar to qref programs.

Still needs some unit tests. I also don't think the name hints are propagating correctly yet.

[superlopuh]

I'm curious about this, what's the use-case and intended API? Do you have a circuit that you'd like to have some sugar for?

[alexarice]

I have a circuit which was a real pain to write without this. Essentially it's much easier to write a circuit when thinking of a gate as an operation which mutates qubits rather than as an SSA operation, and this builder lets you write in the first style and converts to the SSA style

Edit: circuit appears in future PR which has not been pushed yet

[alexarice]

@superlopuh https://github.com/xdslproject/inconspiquous/blob/temp/inconspiquous/transforms/qec/inline.py here is some code that uses this builder. Would be keen to know if you can think of a better way to achieve this, or if you think subclassing the builder is a bad idea

[superlopuh]

Nice! It's not a bad idea, although I wonder if there's already some quantum circuit DSL/library that you could IR gen from instead? This already looks rather pretty so it's your call.

[alexarice]

I don't think there's a dsl that would let you mix in the classical mlir operations in this way.

[alexarice]

An alternative could be to have a direct generation method on GateAttr, instead of using a builder. I might investigate what that would look like next week

[alexarice]

I had a play around and don't particularly like the method on GateAttr as it doesn't extend to dynamic gates. I'll merge this for now, can always iterate on the design.

[alexarice]

I also played around with having a contextmanager for ancilla qubits, which would allocate them and then measure them an the end, but I couldn't find a satisfying interface for returning the measured value