Given a unitary matrix, output the gate name using Qiskit

Question

Given a unitary matrix of a unknown gate, can we write a program in Qiskit to output the name of the corresponding gate if that is a standard gate? So suppose I have $U = \begin{pmatrix} 0 & 1\\ 1 & 0 \end{pmatrix}$, I want Qiskit to output the Pauli $X$ gate name.

Answer 1

I don't think Qiskit itself has some functionality to directly get what you want but you could implement it by yourself using something like the following:

from inspect import getmembers, isclass
import numpy as np
from qiskit.circuit.library import standard_gates


def get_qiskit_gate(u):
    for name, gate in getmembers(standard_gates, isclass):
        try:
            unitary = gate().__array__()
        except:
            continue
        if unitary.shape == u.shape and np.allclose(unitary, u):
            return name

However, this will work only for standard gates (defined in the qiskit.circuit.library.standard_gates module) that implement the __array__ attribute. If the given unitary u does not correspond to any of these gates, the function will simply returns None.

Here is an example for the Pauli $X$ gate:

unitary = np.array([[0, 1],
                    [1, 0]])

print(get_qiskit_gate(u=unitary))

XGate