What object should we use for DAG manipulation?

[jordandsullivan]

What are we doing now?

For DAG optimization, currently we are making use of the qiskit.DAGCircuit object, which relies on being initialized by a QiskitCircuit object. As of Sep 13, 2024, if a user inputs a QiskitCircuit, ucc.compile is doing to wholly unnecessary steps of translating from QiskitCircuit --> OpenQasm --> QiskitCircuit --> DAGCircuit:

ucc/ucc/compile.py

Lines 20 to 23 in eb83869

	translator = QuantumTranslator(circuit, return_format)
	qasm_code = translator.to_qasm(circuit, version=qasm_version)
	compiled_qasm = UCCTranspiler.transpile(qasm_code)

ucc/ucc/transpilers/ucc_transpiler.py

Lines 20 to 24 in eb83869

	circuit = QuantumCircuit.from_qasm_str(qasm_code)
	# Transpile (transpile) the circuit
	transpiled_circuit = transpile(circuit)
	transpiled_qasm = QuantumTranslator.to_qasm(circuit=transpiled_circuit)

But if we got rid of these unnecessary steps for Qiskit inputs, using a QiskitCircuit object as the fundamenal unit of of our DAG object makes UCC lack parity between frontends (i.e. if I say ucc.compile(cirq_circuit), we are on the back end translating CirqCircuit --> OpenQASM --> QiskitCircuit --> DAGCircuit, whereas a QiskitCircuit requires no translation to initialize a DAGCircuit.

Alternatives

For parity across frontends and to avoid relying on the QiskitCircuit object as such a fundamental part of our compilation, another option would be to use for instance BQSKit, which already accepts QASM code and has their own IR. It seems to have a lot of advanced features that we could choose a subset of for our default compiler pass.

However, Qiskit plans to move most of their code to Rust, and already their DAG object is calling Rust on the backend, so maybe this is fine?

[jordandsullivan]

We have decided to stick with the qiskit.DAGCircuit object for v0.1.0. Since we are using the qbraid.transpile function, which makes it easy to translate any circuit into Qiskit and back, and given the DAGCircuit object is already compatible with the qiskit.PassManager we are using.