Return the Choi matrix representation of this super
        operator

def choi(self) -> bk.BKTensor:
        """Return the Choi matrix representation of this super
        operator"""
        # Put superop axes in [ok, ib, ob, ik] and reshape to matrix
        N = self.qubit_nb
        return bk.reshape(self.sharp.tensor, [2**(N*2)] * 2)