I am using Qiskit to measure the Hamiltonian H, which isbasically sum of Pauli strings, say something like 2*X^X+0.5*Z^Y. For one variable, I need to get the analytically form of the inverse of H. It seems that there is no easy way to directly get the inverse observable. So, I am wondering that how can I get this done by using qiskit?


1 Answer 1


It seems that Qiskit does not have this feature. However, you can get the matrix, inverse it using numpy.linalg.inv(), then convert it to operator object again:

from qiskit.opflow import X, Y, Z
from qiskit.opflow.primitive_ops import MatrixOp
import numpy as np

op = 2*(X^X)+0.5*(Z^Y)
inv_matrix = np.linalg.inv(op.to_matrix())
operator = MatrixOp(inv_matrix)


And to get a PauliSumOp:

pauli_list = [(p.primitive.to_label(), p.coeff) for p in operator.to_pauli_op().oplist]

pauli_sum_op = PauliSumOp.from_list(pauli_list)
  • $\begingroup$ Thanks! That helps a lot. $\endgroup$ Jul 6, 2021 at 13:54
  • $\begingroup$ The resulting output is a SummedOp object. Is there a way to transform it t a PauliSumOp object? $\endgroup$ Jul 6, 2021 at 15:51
  • $\begingroup$ I updated the answer to show how to get PauliSumOp. $\endgroup$ Jul 6, 2021 at 16:46
  • $\begingroup$ Thanks for the update. $\endgroup$ Jul 7, 2021 at 3:25

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