Stabilizer simulator runs parallel shots even when measure sampling

[chriseclectic]

Informations

• Qiskit Aer version:
• Python version:
• Operating system:

What is the current behavior?

When running a circuit with measure sampling the stabilizer simulation method sets number of shot threads to the available threads.

This leads to different behaviour for this backend when using a snapshot since it will return one snapshot for each parallel thread rather than a single snapshot as with the statevector simulator.

Steps to reproduce the problem

from qiskit import *
circ = QuantumCircuit(1, 1)
circ.h(0)
circ.measure(0, 0)

results = execute(circ, Aer.get_backend('qasm_simulator'),
                  backend_options={'method': 'stabilizer'}).result()
results.results[0].metadata

returns (on my system)

{'measure_sampling': True,
 'method': 'stabilizer',
 'parallel_shots': 12,
 'parallel_state_update': 1}

What is the expected behavior?

It should return

{'measure_sampling': True,
 'method': 'stabilizer',
 'parallel_shots': 1,
 'parallel_state_update': N}

where N is either 1 (since stabilizer doesn't support OpenMP for state update atm), or the available threads if we add OpenMP support to the method.

Suggested solutions

[chriseclectic]

Closed by #355