Quantum circuits with a C3SX gate fail to convert back from qasm

[jond01]

Information

• Qiskit Terra version: 0.18.3
• Python version: 3.8.5
• Operating system: macOS-10.16-x86_64-i386-64bit

What is the current behavior?

A circuit that contains a C3SX gate can be translated to qasm, but the generated qasm cannot be translated back to a quantum circuit.
The error stems from a mismatch in the name of this gate between qelib1.inc ("c3sqrtx"):
https://github.com/Qiskit/qiskit-terra/blob/6af71f9d1e249a3747772449f299160df4047b83/qiskit/qasm/libs/qelib1.inc#L242
and qiskit's python code ("c3sx"):
https://github.com/Qiskit/qiskit-terra/blob/cf4005ff9fdc672111a827582e6b7d8ae2683be5/qiskit/circuit/library/standard_gates/x.py#L446
https://github.com/Qiskit/qiskit-terra/blob/6af71f9d1e249a3747772449f299160df4047b83/qiskit/circuit/quantumcircuit.py#L1563

Steps to reproduce the problem

import qiskit

qc = qiskit.QuantumCircuit(4)
gate = qiskit.circuit.library.C3SXGate()
qc.append(gate, qargs=qc.qubits)

qasm = qc.qasm()
qc_from_qasm = qiskit.QuantumCircuit.from_qasm_str(qasm)  # fails:
# QasmError(
# qiskit.qasm.exceptions.QasmError: "Cannot find gate definition for 'c3sx', line 4 file "

What is the expected behavior?

I would like the gate names to concur and the qasm to be convertible to a quantum circuit.

Suggested solutions

• Rename the gate in qelib1.inc to "c3sx".
• Change the python code to use "c3sqrtx".

(This issue probably relates to #4943.)

[Cryoris]

Good catch! I think renaming the qasm definition to c3sx is the way to go so we stay consistent in our naming policy.

[jond01]

Thanks, if this solution is accepted I would like to submit a PR that replaces all the occurrences of c3sqrtx with c3sx - please let me know.

[Cryoris]

Yeah please go ahead!