NbCl Exhibits Seventh-order Nonlinearities, Enhancing On-chip Multiphoton Entangled States by 5–9 Orders of Magnitude

The generation of complex entangled states, essential for advanced quantum technologies, currently faces limitations due to the weak nonlinear properties of materials used to create multiple photons simultaneously. Dmitry Skachkov, Dirk R. Englund from the Massachusetts Institute of Technology, and Michael N. Leuenberger from the University of Central Florida, report a significant breakthrough by demonstrating exceptionally strong nonlinear responses in a single layer of niobium chloride, an excitonic Mott insulator. Their work reveals that this material supports nonlinearities up to seventh order, far exceeding those found in commonly used materials by as much as nine orders of magnitude, and opens the door to creating brighter and more efficient sources of entangled photons for applications like quantum computing and communication. By combining theoretical calculations with established integrated optics techniques, the team proposes a pathway to building on-chip devices capable of generating complex multi-photon states, such as GHZ and cluster states, at rates significantly higher than existing technologies.

Excitonic Mott Insulators for Quantum Photonics

Research into novel materials is driving advancements in quantum photonics, with a focus on creating efficient sources of non-classical light for integrated circuits. Scientists are investigating excitonic Mott insulators, materials where strong interactions between electrons lead to unique optical properties, offering potential for manipulating light in new ways. This work centers on harnessing these properties to build compact and scalable quantum photonic devices, essential for future quantum technologies. The research team has focused on a family of materials, including niobium trichloride, niobium tribromide, and niobium triiodide, which possess a unique layered structure known as a breathing kagome lattice.

These materials exhibit flat bands in their electronic structure, enhancing interactions and leading to strong nonlinearities, crucial for efficient light manipulation. Integrating these materials with lithium niobate waveguides promises the creation of compact and efficient nonlinear optical devices. The investigation employs sophisticated computational methods, starting with first-principles calculations based on density functional theory, to determine the electronic structure and optical properties of these materials. More advanced calculations, utilizing many-body perturbation theory, accurately model the behavior of electrons and their interactions, revealing the material’s optical absorption and nonlinear responses.

These simulations predict and optimize the performance of potential devices. Calculations demonstrate that these breathing kagome materials exhibit remarkably strong nonlinear optical responses due to their flat bands and excitonic Mott insulating behavior. This translates to efficient generation of higher harmonics of light, a key requirement for many quantum photonic applications. The optical properties of these materials can be tuned by adjusting the layer thickness or applying external stimuli, offering further control over their behavior. Some of these materials, like tantalum triiodide, exhibit multiferroic behavior, simultaneously displaying magnetic and electric order, which could be exploited to control optical properties. The observation of chiral valley locking in tantalum triiodide opens possibilities for valleytronics, a field focused on manipulating information using electron valleys. This research is pushing the boundaries of materials science and photonics, paving the way for future quantum technologies.

Nb3Cl8 Generates High-Order Entangled Photons

Scientists have pioneered a new approach to generating entangled photons by investigating single-layer niobium trichloride, a material exhibiting exceptionally large nonlinear optical responses, extending up to the seventh order. This addresses a fundamental challenge in quantum photonics, where generating higher-order entangled states is often limited by weak nonlinearities in conventional materials. The study demonstrates that niobium trichloride surpasses the performance of commonly used materials, like transition metal dichalcogenides, by a significant margin in terms of nonlinear response. To comprehensively characterize the material’s properties, scientists employed density functional theory combined with many-body perturbation theory, specifically the GW-Bethe-Salpeter equation approach.

This enabled precise calculation of the ground-state electronic structure and quasiparticle band structures, revealing the presence of flat bands and strongly bound excitons with ferroelectrically aligned dipoles, which are key to the enhanced nonlinearity. The team utilized sophisticated computational techniques to accurately model the material’s electronic behavior. Researchers propagated the one-particle density matrix under a time-dependent electric field, solving the governing equation to determine the macroscopic polarization. This method allowed for extraction of harmonic components from the polarization response, enabling precise determination of susceptibilities up to seventh order.

The team confirmed convergence of the calculated susceptibilities and identified the crucial role of the excitonic ground state in amplifying the nonlinear response. Analysis of the spatial distributions of excitonic states provided further insight into the underlying mechanisms driving the enhanced nonlinearity. The study predicts that three-photon GHZ and four-photon cluster-state sources based on this platform can achieve generation rates significantly larger than those achievable with current technologies, demonstrating the potential for advancements in integrated quantum photonics.

Giant Nonlinear Optics in Niobium Trichloride

Scientists have discovered that a single-layer material, niobium trichloride, exhibits exceptionally large nonlinear optical susceptibilities, extending up to the seventh order. This breakthrough stems from the unique electronic and excitonic properties of the material, specifically its behavior as an excitonic Mott insulator with strongly bound excitons and ferroelectrically aligned dipoles. Calculations reveal that niobium trichloride possesses nonlinear coefficients that surpass those of monolayer molybdenum disulfide and bulk nonlinear crystals at comparable photon energies. The research team employed a combination of advanced computational techniques, including density functional theory, many-body GW calculations, and the Bethe-Salpeter equation, to quantify the material’s nonlinear response.

These calculations accurately model the electronic structure and excitonic properties of niobium trichloride, enabling the prediction of its nonlinear optical coefficients. The team investigated two pump protocols, a continuous wave and an ultrashort pulse, to probe the material’s response and extract the susceptibilities. Measurements of the nonlinear susceptibilities demonstrate remarkably high values, significantly larger than those observed in other two-dimensional materials and bulk nonlinear crystals. The team predicts that this material can enable the creation of highly efficient entangled photon sources.

This enhancement is attributed to the combination of flat electronic bands, localized excitons, and the ferroelectric alignment of exciton dipoles within the monolayer structure. The researchers demonstrate that this unique combination compensates for the material’s atomic thickness, paving the way for programmable multiphoton entangled sources on a chip. Calculations combining many-body perturbation theory and time-dependent approaches reveal these susceptibilities are significantly, by five to nine orders of magnitude, greater than those found in commonly used materials like molybdenum disulfide and bulk nonlinear crystals. This enhancement stems from the material’s unique electronic structure, specifically the presence of flat bands and strongly bound excitons with aligned electrical dipoles. Building on this discovery, the team developed a theoretical framework to describe the generation of multiple entangled photons through a process called spontaneous parametric downconversion. They propose an integrated on-chip architecture utilizing this material, combined with established beam splitters, to create a platform for generating complex entangled states, including GHZ and cluster states.