MCMT generalized gate accepts a label kwarg that seems ignored

Information

• Qiskit Terra version: main @ c3792f9d
• Python version: 3.6
• Operating system: osx 10.15.7

What is the current behavior?

mcmt from library allows a label kwarg, but it doens’t go anywhere?

Steps to reproduce the problem

The generalized gate MCMT accepts a label kwarg, which it will set to be self.label, but as a subclass of QuantumCircuit, it does not look like this attribute will be read anywhere (or at least, I couldn’t find anywhere where it was used.)

>>> from qiskit.circuit.library import *
>>> MCMT(XGate(), 3, 1, label='foo').draw()
          
q_0: ──■──
       │  
q_1: ──■──
       │  
q_2: ──■──
     ┌─┴─┐
q_3: ┤ X ├
     └───┘
>>> MCMT(XGate(), 3, 1, label='foo').name
'mcmt'
>>> MCMT(XGate(), 3, 1, label='foo').to_instruction()
<qiskit.circuit.instruction.Instruction at 0x14b031898>
>>> MCMT(XGate(), 3, 1, label='foo').to_instruction().label
>>> MCMT(XGate(), 3, 1, label='foo').to_instruction().name
'mcmt'

What is the expected behavior?

Suggested solutions

[kevinhartman]

After some investigation, it looks like label is actually not ignored, though I imagine this is a bug.

When multiple target qubits are used, the name of the QuantumCircuit created for broadcast is set to the value of label. When control() is called on the resulting gate, that name is actually eventually used in some special-casing in add_control.py::control.

if gate.name == "x":
   controlled_circ.mct(q_control, q_target[bit_indices[qargs[0]]], q_ancillae)
elif gate.name == "rx":
  ...

I was able to get different circuits by specifying a special-cased label.

Default (label=“2-X”)

mcmt1 = MCMT(XGate(), num_ctrl_qubits=1, num_target_qubits=2)
print(mcmt1.qasm())

OPENQASM 2.0;
include "qelib1.inc";
gate c2-X q0,q1,q2 { cx q0,q1; cx q0,q2; }
qreg q[3];
c2-X q[0],q[1],q[2];

label=“x”

mcmt2 = MCMT(XGate(), num_ctrl_qubits=1, num_target_qubits=2, label="x")
print(mcmt2.qasm())

OPENQASM 2.0;
include "qelib1.inc";
qreg q[3];
cx q[0],q[1],q[2];

@kdk

[Cryoris]

I think a label doesn’t make much sense for the MCMT since it’s not drawn as a box but as multicontrolled gate. We could make label be the label of the controlled gate, or (probably the better, cleaner solution) just remove it.