QASM output for multiple R gates

[christian512]

Environment

• Qiskit Terra version: 0.25.2.1
• Python version: 3.11.5
• Operating system: Fedora

What is happening?

I create a QuantumCircuit from a qasm string that contains multiple R gates. E.g.:

OPENQASM 2.0;
include "qelib1.inc";
gate r(param0,param1) q0 { u3(param0,param1 - pi/2,pi/2 - 1.0*param1) q0; }
qreg q[2];
r(0.1,0.1) q[0];
r(0.5,0.5) q[1];

Generating the qasm string from the previously generated QuantumCircuit does result in the R gates being defined multiple times:

OPENQASM 2.0;
include "qelib1.inc";
gate r(param0,param1) q0 { u3(0.1,-1.4707963267948965,1.4707963267948965) q0; }
gate r_140480215738000(param0,param1) q0 { u3(0.5,-1.0707963267948966,1.0707963267948966) q0; }
qreg q[2];
r(0.1,0.1) q[0];
r_140480215738000(0.5,0.5) q[1];

How can we reproduce the issue?

qc = QuantumCircuit(2)
qc.r(0.1,0.1,0)
qc.r(0.5,0.5,1)

qc2 = QuantumCircuit.from_qasm_str(qc.qasm())
qc2.qasm()
``

### What should happen?

I would expect the `qasm` input and output to a `QuantumCircuit` to be the same. Especially, if there are no changes applied to the object.

### Any suggestions?

_No response_

[jakelishman]

This is in principle a duplicate of #5082, which requires a larger rework of our data model to handle (which we are working on, I promise! - it's just a very very big change), but the reason you only see this after a re-import from the OQ2 string is because Qiskit can't know for certain that it's safe for it to use its built-in RGate to represent the gate r statement.

We're currently reworking how we'll define header files for OpenQASM 2, which will make this easier, but in the meantime, a direct workaround is to replace the legacy QuantumCircuit.from_qasm_str with qiskit.qasm2.loads, which is far more configurable. If you do this:

from qiskit import qasm2
from qiskit.circuit.library import RGate

prog = """
OPENQASM 2.0;
include "qelib1.inc";
gate r(param0,param1) q0 { u3(param0,param1 - pi/2,pi/2 - 1.0*param1) q0; }
qreg q[2];
r(0.1,0.1) q[0];
r(0.5,0.5) q[1];
"""

custom_instructions = list(qasm2.LEGACY_CUSTOM_INSTRUCTIONS)
custom_instructions.append(qasm2.CustomInstruction("r", 2, 1, RGate))

qc = qasm2.loads(prog, custom_instructions=custom_instructions)
print(qasm2.dumps(qc))

I think you'll get what you expect.

edit: just modified the code block - I'd flipped the arguments in CustomInstruction by accident.

[jakelishman]

The gist of #10737 (linked above) is that we're going to start exposing new header files called things like qiskit/v1.qasm, and that will include definitions of all of Qiskit's standard gates, not just the legacy ones that kind of got mashed into Qiskit's not-very-accurate version of qelib1.inc (see #4312). That will let us safely know when we're importing OQ2 that's targetted at our standard gates, without making assumptions about user-defined gates.

[christian512]

Thanks @jakelishman for the detailed response!
Using qasm2.loads() solved my issue. Looking forward to the new implementation.

I close this issue, since it is a duplicate of #5082.