Variational Preparation of Entangled States in a System of Transmon Qubits

The conventional method for generating entangled states in qubit systems relies on applying precise two‐qubit entangling gates alongside single‐qubit rotations. However, achieving high‐fidelity entanglement demands high accuracy in two‐qubit operations, requiring complex calibration protocols. In this work, a minimally calibrated two‐qubit iSwap‐like gate, tuned via straightforward parameter optimization (flux pulse amplitude and duration) is used, to prepare Bell states and GHZ state experimentally in systems of two and three transmon qubits. Integration of this gate into a variational quantum algorithm (VQA) bypasses the need for intricate calibration while maintaining high fidelity. The proposed methodology employs variational quantum algorithms (VQAs) to create the target quantum state through imperfect multiqubit operations. Furthermore, a violation of the Clauser–Horne–Shimony–Holt (CHSH) inequality for Bell states is experimentally demonstrated, confirming their high fidelity of preparation.