We report, for the first time, the observation of spontaneous parametric down-conversion (SPDC) free of phase matching (momentum conservation). We alleviate the need to conserve momentum by exploiting the position-momentum uncertainty relation and using a planar geometry source, a 6  μm thick layer of lithium niobate. Nonphase-matched SPDC opens up a new platform on which to investigate fundamental quantum effects but it also has practical applications. The ultrasmall thickness leads to a frequency spectrum an order of magnitude broader than that of phase-matched SPDC. The strong two-photon correlations are still preserved due to energy conservation. This results in ultrashort temporal correlation widths and huge frequency entanglement. The studies we make here can be considered as the initial steps into the emerging field of nonlinear quantum optics on the microscale and nanoscale.

中文翻译：

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没有动量守恒的纠缠光子的微型产生。

我们首次报告了无相匹配（动量守恒）的自发参量下转换（SPDC）的观察。我们通过利用位置-动量不确定性关系并使用平面几何源（厚度为6μm的铌酸锂）来减轻保存动量的需要。非相位匹配的SPDC为研究基本量子效应开辟了一个新平台，但它也有实际应用。极小的厚度会导致频谱比相匹配的SPDC频谱宽一个数量级。由于能量守恒，仍然保留了强的两光子相关性。这导致超短的时间相关宽度和巨大的频率纠缠。