MPS::apply_multi_qubit_gate can crash in some circumstances

[aromanro]

Informations

• Qiskit Aer version: Latest development version
• Python version: Not relevant as qiskit aer was used from C++ when noticing it
• Operating system: Windows 11

What is the current behavior?

I was testing the MPS simulator with some randomly generated circuits... with some I've got crashes. This one I noticed after fixing https://github.com/Qiskit/qiskit-aer/issues/2291

The call chain was: AerState::apply_measure -> AerState::flush_ops -> Base::apply_ops -> MatrixProductState::State::apply_op -> MPS::apply_diagonal_matrix -> MPS::apply_matrix -> MPS::apply_multi_qubit_gate.

It crashed on this line: https://github.com/Qiskit/qiskit-aer/blob/582407a3f8ef5070c850fb4c417c53c1db1f5998/src/simulators/matrix_product_state/matrix_product_state_internal.cpp#L798

is_diagonal was true (as it should be by looking at the call chain), mat had 1 row and 8 columns (being a diagonal matrix, only the diagonal is given, so it's actually a vector)... but at that line it's used with mat(row, col) which is generating the crash.

Steps to reproduce the problem

Again, this might be quite tough to reproduce, I was generating some random circuits with more than 100 ops to test some things and maybe one in 100 circuits crashed.

What is the expected behavior?

No crash.

Suggested solutions

The problem lines should probably be replaced by something like:

      if (row == col)
        new_mat(new_vec[row], new_vec[row]) = is_diagonal ? mat(0, row) : mat(row, row);
      else
        new_mat(new_vec[row], new_vec[col]) = is_diagonal ? 0. : mat(row, col);

... or maybe a vector new_mat should be constructed if is_diagonal is true