Apply the action of this channel upon a density

def evolve(self, rho: Density) -> Density:
        """Apply the action of this channel upon a density"""
        N = rho.qubit_nb
        qubits = rho.qubits

        indices = list([qubits.index(q) for q in self.qubits]) + \
            list([qubits.index(q) + N for q in self.qubits])

        tensor = bk.tensormul(self.tensor, rho.tensor, indices)
        return Density(tensor, qubits, rho.memory)