QASM exporter error with CSwapGate and CCXGate: Cannot find gate definition for 'unitary...'

[MattePalte]

Environment

• Qiskit Terra version: 0.19.1
• Python version: 3.8
• Operating system: Ubuntu 18.04.6 LTS

What is happening?

Using the CSwapGate and CCXGate in a subcircuit and converting it to qasm leads to an incorrect qasm file, where a unitary is never defined. Note that this happens only with optimization level 3 and not with others.

How can we reproduce the issue?

Run the following script:

import qiskit
from qiskit import QuantumCircuit, ClassicalRegister, QuantumRegister
from qiskit.circuit.library.standard_gates import *
from qiskit import Aer, transpile, execute

qr = QuantumRegister(3, name='qr')
cr = ClassicalRegister(3, name='cr')
qc = QuantumCircuit(qr, cr, name='qc')
subcircuit = QuantumCircuit(qr, name='subcircuit')
subcircuit.append(CSwapGate(), qargs=[qr[0], qr[1], qr[2]], cargs=[])
subcircuit.append(CCXGate(), qargs=[qr[0], qr[1], qr[2]], cargs=[])
qc.append(subcircuit, qargs=qr)
qc.append(subcircuit.inverse(), qargs=qr)
qc.measure(qr, cr)
qc = transpile(qc, optimization_level=3)
my_qasm = qc.qasm()
print(my_qasm)

Output qasm:

OPENQASM 2.0;
include "qelib1.inc";
gate unitary139839044026176 p0,p1 {
	u3(pi/2,pi/2,-pi/2) p0;
	u3(pi/2,0,-1.569473) p1;
	cx p0,p1;
	u3(pi/2,-pi/2,pi/2) p0;
	u3(3.1402694,-3*pi/4,pi/2) p1;
}
qreg qr[3];
creg cr[3];
unitary139839044026176 qr[1],qr[2];
cx qr[0],qr[2];
t qr[2];
cx qr[1],qr[2];
t qr[1];
tdg qr[2];
cx qr[0],qr[2];
cx qr[0],qr[1];
t qr[0];
tdg qr[1];
cx qr[0],qr[1];
t qr[2];
h qr[2];
unitary139839044101600 qr[1],qr[2];
cx qr[0],qr[2];
t qr[2];
cx qr[1],qr[2];
t qr[1];
tdg qr[2];
cx qr[0],qr[2];
cx qr[0],qr[1];
t qr[0];
tdg qr[1];
cx qr[0],qr[1];
t qr[2];
cx qr[1],qr[2];
tdg qr[2];
cx qr[0],qr[2];
t qr[2];
cx qr[1],qr[2];
t qr[1];
tdg qr[2];
cx qr[0],qr[2];
cx qr[0],qr[1];
t qr[0];
tdg qr[1];
cx qr[0],qr[1];
t qr[2];
h qr[2];
unitary139839044129696 qr[1],qr[2];
cx qr[0],qr[2];
t qr[2];
cx qr[1],qr[2];
t qr[1];
tdg qr[2];
cx qr[0],qr[2];
cx qr[0],qr[1];
t qr[0];
tdg qr[1];
cx qr[0],qr[1];
t qr[2];
h qr[2];
cx qr[2],qr[1];
measure qr[0] -> cr[0];
measure qr[1] -> cr[1];
measure qr[2] -> cr[2];

Read the qasm:

qc = QuantumCircuit.from_qasm_str(my_qasm)

output:

QasmError: "Cannot find gate definition for 'unitary139839047598864', line 25 file "

What should happen?

The QASM exporter should create the definition for unitary139839047598864 as well.

Any suggestions?

This is similar to another issue (#7772) but in that case the problem was with CSwap and DCX, thus the error might be due to the interaction between the CSwap and other CX gates.

[jakelishman]

This is likely because UnitaryGate has some pretty questionable special handling to turn itself into OQ 2, including statefully producing a custom definition (the only gate in the circuit library to do so). Probably this false definition is getting copied into an object that it should be in, or some other confusion like this, but neither the OQ 2 exporter nor UnitaryGate behave well and predictably, so it's pretty hard to know.