2025/12/01 by Skachkov, Dmitry, Englund, Dirk R., Leuenberger, Michael N.
#FOS: Physical sciences #Materials Science (cond-mat.mtrl-sci) #Quantum Physics (quant-ph)
paper · doi:10.48550/arxiv.2512.01874
Spontaneous parametric downconversion (SPDC) and four-wave mixing in χ(2) and χ(3) media underpin most entangled-photon sources, but direct generation of higher-order entangled multiphoton states by n-th order parametric downconversion remains extremely challenging because conventional materials exhibit tiny high-order nonlinearities. Here we show that single-layer Nb3Cl8, an excitonic Mott insulator on a breathing Kagome lattice, supports exceptionally large nonlinear susceptibilities up to seventh order. Many-body GW--Bethe--Salpeter and time-dependent BSE / Kadanoff--Baym simulations yield resonant χ(2)--χ(7) for monolayer Nb3Cl8, with |χ(4)| and |χ(5)| surpassing values in prototypical transition metal dichalcogenides by 5--9 orders of magnitude. We trace this enhancement to flat bands and strongly bound Frenkel excitons with ferroelectrically aligned out-of-plane dipoles. Building on experimentally demonstrated 1× N integrated beam splitters with arbitrary power ratios, we propose an on-chip architecture where each output arm hosts an Nb3Cl8 patch, optionally gated by graphene to tune the complex n-photon amplitudes. Using the ab-initio χ(3) and χ(4) values, we predict that three-photon GHZ3 and four-photon cluster-state sources in this platform can achieve n-photon generation rates up to ∼ 108 and ∼ 106 times larger, respectively, than silica-fiber- and MoS2-based implementations with comparable geometry. We derive the quantum Hamiltonian and explicit n-photon generation rates for this platform, and show how suitable interferometric networks enable electrically and spectrally tunable GHZ, W, and cluster states based on genuine high-order nonlinear processes in a 2D excitonic Mott insulator.