Lithium Niobate Electro‐Optic Photonic Processor for Variational Quantum Eigensolver

ABSTRACT

Encoding quantum information in high‐dimensional photonic states as a qudit provides a powerful route to resource‐efficient quantum simulation. Among various integrated photonic platforms, lithium niobate on insulator is particularly attractive because it combines low optical loss, strong optical nonlinearity, and high‐speed electro‐optic modulation. Here, we demonstrate an electro‐optically controlled variational quantum eigensolver (VQE) on an integrated lithium niobate ququart processor. Using ququart encoding in four path modes and electro‐optic modulation, the processor enables reconfigurable high‐fidelity state preparation and projective measurements. To reduce the number of measurement groups, we implement entangled‐basis‐emulating ququart projective measurements that reproduce the measurement‐grouping role of two‐qubit entangled‐basis measurements for fully commuting Pauli operators, without requiring genuine two‐qubit entanglement or entangling gates. Using this approach, we estimate molecular ground‐state energies within the chemical‐accuracy threshold over the measured interatomic‐distance range. We further extend the platform to a chip that integrates a periodically poled lithium niobate photon‐pair source with a ququart photonic processor. These results highlight LNOI photonics as a promising platform for reconfigurable photonic quantum simulation with on‐chip photon‐pair sources.