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arXiv · 2610.11026

Demonstration of subspace-search variational quantum eigensolver using photonic orbital angular momentum qudits

Kang-Min Hu·Myung-Hyun Sohn·Yong-Su Kim·Kwang Jo Lee·Hyun Woo Kim·Hyang-Tag Lim
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Abstract

Accurate excited state energies are essential for interpreting fluorescence and photochemical processes, yet variational quantum eigensolvers (VQEs) on noisy intermediate-scale quantum (NISQ) hardware have largely been confined to ground states. Subspace-search VQE (SSVQE) offers a NISQ-friendly route to excited states without ancilla-based overlap measurements, however it has lacked an experimental demonstration since it requires the coherent preparation and manipulation of multiple orthogonal basis states under a shared variational unitary. Here we realize SSVQE on a photonic platform by encoding orthogonal qudit states in the orbital angular momentum of single photons and optimizing a shared variational circuit. We estimate excited state energies for a four-dimensional $\rm{HeH^+}$ molecule and an eight-dimensional Heisenberg chain, thereby demonstrating scalable excited state eigensolving in a high-dimensional Hilbert space. Our results establish photonic qudits as a resource-efficient pathway to excited state quantum simulation.

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Kang-Min Hu, Myung-Hyun Sohn, Yong-Su Kim, Kwang Jo Lee, Hyun Woo Kim, Hyang-Tag Lim. 2026-10-08. Demonstration of subspace-search variational quantum eigensolver using photonic orbital angular momentum qudits. https://doi.org/10.1038/s41534-026-01380-2

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