Method get_expectation_vector

Core/QuantumMachine/OriginQVM.cpp:1688–1749

}

void QVM::get_expectation_vector(QProg prog, const QHamiltonian& hamiltonian, const QVec& qv, vector_d& measure_probably)
{
	directlyRun(prog);
	auto qstate = getQState();
	initState(qstate);

	auto _parity_check = [](size_t number)
	{
		bool label = true;
		size_t i = 0;
		while ((number >> i) != 0)
		{
			if ((number >> i) % 2 == 1)
				label = !label;

			++i;
		}
		return label;
	};


	for (size_t i = 0; i < hamiltonian.size(); i++)
	{
		auto component = hamiltonian[i];
		if (component.first.empty())
		{
			measure_probably[i] = 1.0;
			continue;
		}

		QProg qprog;
		Qnum vqubit;
		for (auto iter : component.first)
		{
			vqubit.push_back(qv[iter.first]->get_phy_addr());
			if (iter.second == 'X')
				qprog << H(qv[iter.first]);
			else if (iter.second == 'Y')
				qprog << RX(qv[iter.first], PI / 2);
		}

		directlyRun(qprog);
		prob_vec pmeasure_vector;
		_pGates->pMeasure(vqubit, pmeasure_vector);

		double expectation = 0;
#pragma omp parallel for reduction(+:expectation)
		for (auto i = 0; i < pmeasure_vector.size(); i++)
		{
			if (_parity_check(i))
			{
				expectation += pmeasure_vector[i];
			}
			else
			{
				expectation -= pmeasure_vector[i];
			}
		}