Lithium Niobate Electro-Optic Photonic Processor for Variational Quantum Eigensolver.

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.