Code ergonomics: Create SparseObservable from QkObs

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Issue 类型
功能
描述清晰度
基本清楚
活跃度
冷清
技术栈
c, cpp
领域
api, backend-api-design

调研方向

Start at the SparseObservable API and inspect the existing constructors alongside the QkObs accessors used in the proposal. Implement the conversion entry point so it preserves qubit count, coefficients, boundaries, bit terms, and indices, then verify the multi-bit-term example and the error path from qk_obs_term.

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描述

What should we add?

Hello, I wanted to propose an alternative from method for creating SparseObservable objects.

The idea would be to receive a C QkObs pointer where it's extracted its data in order to construct a new SparseObservable object. This would be beneficial if it's wanted to construct observables where the terms have more than one bit_term.

I believe the implementation could be something like the following:

static SparseObservable from_obs(QkObs *obs)
{
    QkComplex64 *c_coeffs = qk_obs_coeffs(obs);
    size_t *c_boundaries = qk_obs_boundaries(obs);
    
    size_t terms_len = qk_obs_num_terms(obs);

    std::vector<size_t> boundaries(terms_len + 1);
    std::vector<std::complex<double>> coeffs(terms_len);
    
    std::vector<QkBitTerm> bit_terms;
    reg_t indices;

    QkObsTerm term;
    for (size_t i = 0; i < terms_len + 1; i++)
    {
        boundaries[i] = c_boundaries[i];
        if (i < terms_len)
        {
            if (qk_obs_term(obs, i, &term) != QkExitCode_Success) {
                throw std::runtime_error("Error while obtaining term from observable.");
            }
            coeffs[i] = std::complex<double>(term.coeff.re, term.coeff.im);
            for (size_t j = 0; j < term.len; j++)
            {
                bit_terms.push_back(term.bit_terms[j]);
                indices.push_back(term.indices[j]);
            }
        }
    }

    return SparseObservable(qk_obs_num_qubits(obs), coeffs, bit_terms, indices, boundaries);
}

And the usage could be something like the following:

size_t num_qubits = 3;
QkObs *obs = qk_obs_zero(num_qubits);

QkBitTerm zz[2] = {QkBitTerm_Z, QkBitTerm_Z};
uint32_t zz_q[2] = {0, 1};
QkObsTerm zz_term = {{1, 0.}, 2, zz, zz_q, num_qubits};
qk_obs_add_term(obs, &zz_term);

QkBitTerm x[1] = {QkBitTerm_X};
uint32_t x_q[1] = {2};
QkObsTerm x_term = {{1, 0.}, 1, x, x_q, num_qubits};
qk_obs_add_term(obs, &x_term);

auto cpp_obs = SparseObservable::from_obs(obs);

qk_obs_free(obs);
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