How to create the equivalent of the Qiskit rxx gate in Pennylane?

Question

How can I create the Pennylane equivalent of:

from qiskit import QuantumCircuit

circ = QuantumCircuit(2, 2)
circ.rxx(theta=0.3, qubit1=0, qubit2=1)

Answer 1

Here's a simple example if you're looking for a quick hack:

import pennylane as qml
import numpy as np

def RXX(theta):
    rxx = np.array([
        [np.cos(theta/2), 0, 0, -1j*np.sin(theta/2)],
        [0, np.cos(theta/2), -1j*np.sin(theta/2), 0],
        [0, -1j*np.sin(theta/2), np.cos(theta/2), 0],
        [-1j*np.sin(theta/2), 0, 0, np.cos(theta/2)]
    ])
    return rxx

dev = qml.device('default.qubit', wires=2)
@qml.qnode(dev)
def circuit(theta):
    qml.QubitUnitary(RXX(theta), wires=[0, 1])
    return qml.expval(qml.PauliZ(0))

Alternatively, you can create a new RXX class as they do in this custom gate tutorial:

import pennylane as qml
from pennylane.operation import Operation
from pennylane import numpy as np

class RXX(Operation):
    num_params = 1
    num_wires = 2
    par_domain = "R"

    grad_method = "A"
    grad_recipe = None # This is the default but we write it down explicitly here.

    generator = [(qml.PauliX(0) @ qml.PauliX(1)).matrix, -0.5]

    @staticmethod
    def decomposition(theta, wires):
        return [qml.PauliRot(theta, 'XX', wires=wires)]

    @staticmethod
    def _matrix(*params):
        theta = params[0]
        c = np.cos(0.5 * theta)
        s = np.sin(0.5 * theta)
        return np.array(
            [
                [c, 0, 0, -s],
                [0, c, -s, 0],
                [0, -s, c, 0],
                [-s, 0, 0, c]
            ]
        )

    def adjoint(self):
        return RXX(-self.data[0], wires=self.wires)

Answer 2

In this tutorial in PennyLane, they guide you to create a custom gate (Rxx gate)

https://pennylane.ai/blog/2021/05/how-to-add-custom-gates-and-templates-to-pennylane/

After creating it you can simply use these code to add it:

dev = qml.device('default.qubit', wires=3)
dev.operations.add("RXX")