QASM3 exporter with parameterized circuit support

[doichanj]

What should we add?

Qiskit C++ supports building parameterized circuits, but can not run them. One reason it does not support is QASM3 exporter implemented in QuantumCircuit class
(

qiskit-cpp/src/circuit/quantumcircuit_def.hpp

Line 1542 in 91f4b5d

std::string to_qasm3(void)

)
does not support parameterized circuit.

• Move to_qasm3 function to outside of QuantumCircuit class (to other source file)
• implement QASM3 exporter with parameter support

[1ucian0]

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[Mayank447]

@doichanj Hi, I am working on this issue. Does qiskit have a C-API to iterate over the parameters of a Quantum Circuit? The only implemented C-API I found was for number of parameters.

Also as a side note some operations currently supported in parameters like a**b are not valid QASM3 operations (should be pow(a,b)). There might be more such operations, is there a doc of all supported operations on parameters by qiskit or is it just Python operations?

[1ucian0]

Also as a side note some operations currently supported in parameters like a**b are not valid QASM3 operations (should be pow(a,b)). There might be more such operations, is there a doc of all supported operations on parameters by qiskit or is it just Python operations?

If you think there is an unrelated issue, please report it as a bug with a reproduction example.

[1ucian0]

Thank you to everyone who has submitted a PR for this issue! We currently have several promising fixes under review, so please hold off on submitting new PRs until the Qiskit team posts a follow-up comment here.

We will review the existing submissions in due course. Thank you for your patience!

[doichanj]

I reviewed all the PRs related to this issue, and I noticed we do not have C-API to get a list of parameters used in the circuit (A list of the parameters is stored on rust side, so it is not good to have duplicate list on C++ side). So I would like to propose a stab function in QuantumCircuit class to get a list of parameters and replace the implementation of the function to C-API when it will be available.

std::vector<std::string> QuantumCircuit::parameter_symbols(void);  // please implement this one

// std::vector<Parameter> QuantumCircuit::parameters(void); // this is not required currently, it is compatible to Qiskit

Then pass the list of parameter symbols to QASM3 exporter.

Also I would like to add a limitation, we do not accept the same symbols in a circuit, because we do not have C-API to identify by UUID of Parameters, for example the following should be different parameter, but we do not accept this in Qiskit C++ at this moment.

Parameter a0("a");
Parameter a1("a");

circ.ry(a0, 0);
circ.ry(a1, 1);

So please collect a list of parameter symbols using the original symbol name for a stab function. (The original symbol name will be also used in a list of parameter set for SamplerPub/EstimatorPub)