Optimizing Parametrized Quantum Circuits With Free-Axis Single-Qubit Gates

Variational quantum algorithms, which utilize Parametrized Quantum Circuits (PQCs), are promising tools to achieve advantage for optimization problems on near-term devices. Their PQCs have been conventionally constructed from parametrized rotational angles of single-qubit gates around predetermined set axes, and two-qubit entangling gates, such as CNOT gates. We propose a method construct PQC by continuous parametrization both the axes its rotation The is based observation that when fixed, optimal rotations can be computed solving system linear equations whose coefficients determined with small computational overhead. further simplified select freely parameters fixed $\pi$ . show free-axis selection has better expressibility against other structural methods measured Kullback-Leibler (KL) divergence. also demonstrate more effective search ground states Hamiltonians condensed matter physics, chemistry combinatorial optimization. Because allows designing without specifying their it may significantly improve handiness PQCs.