The api from contrib/runtime does not always work

[aromanro]

Informations

• Qiskit Aer version:

Latest dev version from main branch.

• Python version:

Irrelevant, as it's called from C/C++.

• Operating system:

Windows, Linux, MacOS

What is the current behavior?

Not all api calls work for all simulators. They do work for example for the statevector one, but some fail - throwing an exception - when using the MPS simulator, for example.

Steps to reproduce the problem

Configure the MPS simulator instead of the default.
Call any function that is implemented using state->apply_mc... (I think all implemented like that will fail, since they are not implemented by the MPS simulator). If I recall correctly, there are others that might fail besides those, but I will have to check.

What is the expected behavior?

Calls should succed, applying the operation.

Suggested solutions

There are workarounds, already mentioned in this related issue: #2221

For example aer_apply_x could be implemented using apply_x instead of apply_mcx. Various other gates might not have such an easy change, but still they can be implemented, for example the aer_apply_rx could be implemented with something like:

   const Types::qubits_vector qubits = { qubit };
   const cmatrix_t rx = AER::Linalg::Matrix::rx(theta);
   state->apply_unitary(qubits, rx);

or the more complex ones like 'aer_apply_crx', with something like:

   const Types::qubits_vector qubits = { ctrl_qubit, tgt_qubit };

   cmatrix_t mat(4, 4);
   mat(0, 0) = 1;
   mat(2, 2) = 1;

   const double t2 = theta * 0.5;

   const complex_t i(0., 1.);
   mat(1, 1) = std::cos(t2);
   mat(1, 3) = -i * std::sin(t2);
   mat(3, 1) = mat(1, 3);
   mat(3, 3) = mat(1, 1);

   state->apply_unitary(qubits, mat);

or for `aer_apply_crz':

   const Types::qubits_vector qubits = { ctrl_qubit, tgt_qubit };

   const double t2 = theta * 0.5;
   AER::cvector_t v = { {1, 0}, std::polar(1., -t2), {1, 0}, std::polar(1., t2) };
				
   state->apply_diagonal_matrix(qubits, v);

and so on...

I think I could do the changes, if considered worthy.