MPS simulator has global bond dimension and singular values truncation threshold

[aromanro]

Informations

• Qiskit Aer version: Latest development version
• Python version: Doesn't matter, applies to all
• Operating system: Doesn't matter, applies to all

What is the current behavior?

The Matrix Product State simulator has bond dimension and singular values truncation threshold as static values.
See here:

qiskit-aer/src/simulators/matrix_product_state/matrix_product_state_tensor.hpp

Line 182 in e26970a

static uint_t max_bond_dimension_;

The problem with this is that it affects all MPS simulators one creates.

Steps to reproduce the problem

Create a MPS simulator with default settings, execute a complex enough circuit in it, save its results.

Create two MPS simulators, set a low bond dimension and a big truncation threshold on one (which would introduce quite big errors for many circuits). Execute the same complex circuit from above with both. Compare results among themselves and with the ones previously saved.

What is the expected behavior?

Each MPS simulator should have its own settings.

Suggested solutions

Make them non static?

[aromanro]

Making those values non-static is not that easy, since there are a lot of static calls in the MPS_Tensor class. A solution would be to store the values elsewhere and pass them as parameters where they are relevant (basically the call to MPS_Tensor::Decompose).