Entanglement, as a crucial feature of quantum systems, is essential for various applications of quantum technologies. High-dimensional entanglement has the potential to encode arbitrary large amount of information and enhance robustness against eavesdropping and quantum cloning. The orbital angular momentum (OAM) entanglement can achieve the high-dimensional entanglement nearly for free stems due to its discrete and theoretically infinite-dimensional Hilbert space. A stringent limitation, however, is that the phase-matching condition limits the entanglement dimension because the coincidence rate decreases significantly for high-order modes. Here we demonstrate relatively flat high-dimensional OAM entanglement based on a spontaneous parametric down conversion (SPDC) from an ultrathin nonlinear lithium niobite crystal. The difference of coincidences between the different-order OAM modes significantly decreases. To further enhance the nonlinear process, this microscale SPDC source will provide a promising and integrated method to generate optimal high-dimensional OAM entanglement.
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Generation of nonlinear frequency combs in χ(3) optical microresonators has attracted tremendous research interest during the last decade. Recently, realization of the microcomb owing to χ(2) optical nonlinearity in the microresonator promises new breakthroughs and is a big scientific challenge. Moreover, it is of high scientific interest that the presence of both second- and third-order nonlinearities results in complex cavity dynamics. In particular, the role of χ(3) nonlinearity in the generation of the quadratic microcomb is still far from being well understood. Here, we demonstrate the interaction between the second- and third-order nonlinearity in the lithium niobate microresonator, which can provide a new way of phase matching to control the mode-locking condition and pulse number for the quadratic microcomb. Our results verify that the Kerr nonlinearity can benefit the quadratic microcomb. The principle can be further extended to other material platforms to provide more manipulation methods for comb generation based on χ(2) nonlinearity at mid-infrared.
The generation of a quadratic soliton in a pulse-pumped microresonator has attracted significant interest in recent years. The strong second-order nonlinearity and high peak power of pumps offer a straightforward way to increase efficiency. In this case, the influence of the third-order nonlinearity effect becomes significant and cannot be ignored. In this paper, we study the quadratic soliton in a degenerate optical parametric oscillator driven synchronously by the pulse pump with third-order nonlinearity. Our simulations verify that the robustness of quadratic soliton generation is enhanced when the system experiences a perturbation from pump power, cavity detuning, and pump pulse width. These results represent a new way of manipulating frequency comb in resonant microphotonic structures.
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