QASM instruction not parameterized for two-qubit Pauli rotation gates

[epelaaez]

Information

• Qiskit Terra version: 0.19.0.dev0+0ee0824
• Python version: 3.9.6
• Operating system: Windows 11

What is the current behavior?

The following snippet

from qiskit import QuantumCircuit
from qiskit.circuit.library.standard_gates import RZXGate
import numpy as np

qc = QuantumCircuit(2)
qc.append(RZXGate(np.pi), [0, 1])
qc.append(RZXGate(np.pi / 2), [0, 1])

qc.qasm()

Produces

OPENQASM 2.0;
include "qelib1.inc";
gate rzx(param0) q0,q1 { h q1; cx q0,q1; rz(pi) q1; cx q0,q1; h q1; }
gate rzx_1491288775456(param0) q0,q1 { h q1; cx q0,q1; rz(pi/2) q1; cx q0,q1; h q1; }
qreg q[2];
rzx(pi) q[0],q[1];
rzx_1491288775456(pi/2) q[0],q[1];

As you can see, instead of creating a single rzx instruction that uses param0, it creates a single rzx for each value present in the circuit.

Steps to reproduce the problem

This happens with at least all other RPPGates, where PP is placeholder for some Pauli matrices. I haven't tested it with other parameterized gates.

What is the expected behavior?

I think the output QASM string should look like:

OPENQASM 2.0;
include "qelib1.inc";
gate rzx(param0) q0,q1 { h q1; cx q0,q1; rz(param0) q1; cx q0,q1; h q1; }
qreg q[2];
rzx(pi) q[0],q[1];
rzx(pi/2) q[0],q[1];

I tested it and using that string to build a QuantumCircuit gives the intended circuit.

string = """OPENQASM 2.0;
include "qelib1.inc";
gate rzx(param0) q0,q1 { h q1; cx q0,q1; rz(param0) q1; cx q0,q1; h q1; }
qreg q[2];
rzx(pi) q[0],q[1];
rzx(pi/2) q[0],q[1];"""

qc = QuantumCircuit.from_qasm_str(string)

qc.decompose().draw()

Output:

q_0: ───────■─────────────■──────────────■───────────────■───────
     ┌───┐┌─┴─┐┌───────┐┌─┴─┐┌───┐┌───┐┌─┴─┐┌─────────┐┌─┴─┐┌───┐
q_1: ┤ H ├┤ X ├┤ Rz(π) ├┤ X ├┤ H ├┤ H ├┤ X ├┤ Rz(π/2) ├┤ X ├┤ H ├
     └───┘└───┘└───────┘└───┘└───┘└───┘└───┘└─────────┘└───┘└───┘

[yjt98765]

I have tried to export this program into OpenQASM3. The result is:

OPENQASM 3;
include "stdgates.inc";
gate rzx(param_0) q_0, q_1 {
  h q_1;
  cx q_0, q_1;
  rz(pi) q_1;
  cx q_0, q_1;
  h q_1;
}
qubit[2] _q;
let q = _q[0] || _q[1];
rzx(pi) q[0], q[1];
rzx(pi/2) q[0], q[1];

There is only one rzx definition. However, the parameter is not used in the body. Therefore, the program is not correct.

[yjt98765]

The problem I mentioned above, i.e., not using parameters in a gate's body, is the same as this example:

                "gate u3(param_0, param_1, param_2) q_0 {",
                "  U(0, 0, pi/2) q_0;",
                "}",
                "gate u1(param_0) q_0 {",
                "  u3(0, 0, pi/2) q_0;",
                "}",
                "gate rz(param_0) q_0 {",
                "  u1(pi/2) q_0;",
                "}",
                "gate sdg q_0 {",
                "  u1(-pi/2) q_0;",
                "}",
                "gate u2(param_0, param_1) q_0 {",
                "  u3(pi/2, 0, pi) q_0;",
                "}",

Therefore, the authors should be aware of this problem. There are probably some documents, issues, pull requests, or code comments that explain the design choices or the difficulties leading to the current status. I have not found them, though.