Reinforcement Learning for Parameterized Quantum State Preparation: A Comparative Study

We extend directed quantum circuit synthesis (DQCS) with reinforcement learning from purely discrete gate selection to parameterized quantum state preparation with continuous single-qubit rotations $R_x$, $R_y$, and $R_z$. We compare two training regimes: a one-stage agent that jointly selects the gate type, the affected qubit(s), and the rotation angle; and a two-stage variant that first proposes a discrete circuit and subsequently optimizes the rotation angles with Adam using parameter-shift gradients. Using Gymnasium and PennyLane, we evaluate Proximal Policy Optimization (PPO) and Advantage Actor--Critic (A2C) on systems comprising two to ten qubits and on targets of increasing complexity with $λ$ ranging from one to five. Whereas A2C does not learn effective policies in this setting, PPO succeeds under stable hyperparameters (one-stage: learning rate approximately $5\times10^{-4}$ with a self-fidelity-error threshold of 0.01; two-stage: learning rate approximately $10^{-4}$). Both approaches reliably reconstruct computational basis states (between 83% and 99% success) and Bell states (between 61% and 77% success). However, scalability saturates for $λ$ of approximately three to four and does not extend to ten-qubit targets even at $λ=2$. The two-stage method offers only marginal accuracy gains while requiring around three times the runtime. For practicality under a fixed compute budget, we therefore recommend the one-stage PPO policy, provide explicit synthesized circuits, and contrast with a classical variational baseline to outline avenues for improved scalability.