Calling AerState::probabilities() crashes for the MPS simulator

[aromanro]

Informations

• Qiskit Aer version:

Latest from the main branch.

• Python version:

Irrelevant as the call is from c++ code.

• Operating system:

Windows 11

What is the current behavior?

Calling AerState::probabilities() for a MPS simulator crashes. Might crash for other simulators, too, but I didn't check.

The reason is that op.qubits is empty.
The call goes through (skipping here some intermediate calls) MPS::get_probabilities_vector_internal -> MPS::diagonal_of_density_matrix -> MPS::centralize_qubits -> MPS::find_centralized_indices with no qubits and it crashes because the for (uint_t index = 0; index < num_qubits - 1; index++) has an unsigned int index and num_qubits is also unsigned int so num_qubits - 1 is actually not negative, but a big number.

Steps to reproduce the problem

As described above, just call AerState::probabilities() after configuring and initializing a MPS simulator.

What is the expected behavior?

Not crashing and working as expected.

Suggested solutions

Probably the MPS::find_centralized_indices could check and avoid such a case with empty qubits, as for the correct functionality, one way would be adding something like

  for (uint_t i = 0; i < num_of_qubits_; ++i)
    op.qubits.push_back(i);

in AerState::probabilities().