Copy results to python data structure inmeasure_sampler is not parallelized

[hhorii]

Informations

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

What is the current behavior?

measure_sampler has big sequential part though sampling is parallelized.

qiskit-aer/src/simulators/circuit_executor.hpp

Lines 1090 to 1113 in 180a0b6

	for (int_t i = all_samples.size() - 1; i >= 0; i--) {
	creg.initialize(num_memory, num_registers);
	// process memory bit measurements
	for (const auto &pair : memory_map) {
	creg.store_measure(reg_t({all_samples[i][pair.second]}),
	reg_t({pair.first}), reg_t());
	}
	// process register bit measurements
	for (const auto &pair : register_map) {
	creg.store_measure(reg_t({all_samples[i][pair.second]}), reg_t(),
	reg_t({pair.first}));
	}
	// process read out errors for memory and registers
	for (const Operations::Op &roerror : roerror_ops)
	creg.apply_roerror(roerror, rng);
	// Save count data
	if (save_creg_to_state)
	state.creg() = creg;
	else
	result.save_count_data(creg, save_creg_memory_);
	}

Steps to reproduce the problem

from qiskit_aer import AerSimulator
from qiskit.circuit.library import QuantumVolume
from qiskit import transpile

simulator = AerSimulator()
c = QuantumVolume(20)
c.h(c.qubits[:])
c.measure_all()

c = transpile(c, simulator)

for shots in [10, 100, 1000, 10000, 100000, 1000000, 10000000]:
    res = simulator.run(c, shots=shots).result()
    print("shots=", shots, res.metadata)

Most of execution time of shots==10000000 is serial execution.

What is the expected behavior?

Most of sampling should be parallelized.

Suggested solutions

Parallelize the above codes.