On-chip high-order parametric downconversion in the excitonic Mott insulator Nb$_3$Cl$_8$ for programmable multiphoton entangled states

TL;DR

This paper demonstrates that monolayer Nb$_3$Cl$_8$, an excitonic Mott insulator, exhibits exceptionally large high-order nonlinear susceptibilities, enabling on-chip generation of multiphoton entangled states with significantly higher efficiency than traditional materials.

Contribution

The study reveals the giant high-order nonlinearities in Nb$_3$Cl$_8$ and proposes an integrated platform for efficient, tunable multiphoton entangled state generation using this material.

Findings

Nb$_3$Cl$_8$ shows nonlinear susceptibilities up to seventh order, surpassing other materials by 5-9 orders of magnitude.

Predicted generation rates for three- and four-photon states are up to 10^8 and 10^6 times higher than existing platforms.

The platform allows for electrically and spectrally tunable high-order entangled photon states.

Abstract

Spontaneous parametric downconversion (SPDC) and four-wave mixing in $\chi^{(2)}$ and $\chi^{(3)}$ 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 Nb$_3$Cl$_8$, 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 $\chi^{(2)}$--$\chi^{(7)}$ for monolayer Nb$_3$Cl$_8$, with $|\chi^{(4)}|$ and $|\chi^{(5)}|$ 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$\times N$ integrated beam splitters with arbitrary power ratios, we propose an on-chip architecture where each output arm hosts an Nb$_3$Cl$_8$ patch, optionally gated by graphene to tune the complex $n$-photon amplitudes. Using the ab-initio $\chi^{(3)}$ and $\chi^{(4)}$ values, we predict that three-photon GHZ$_3$ and four-photon cluster-state sources in this platform can achieve $n$-photon generation rates up to $\sim 10^8$ and $\sim 10^6$ times larger, respectively, than silica-fiber- and MoS$_2$-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.