Nearly 30 years ago, Hong, Ou and Mandel observed what Richard Feynman referred to as "The heart of quantum mechanics": the two-photon interference effect. This observation marked the beginning of a new quantum era, where the peculiarity of quantum physics may now be used to our advantage to outperform classical computations, securely communicate information, simulate highly complex physical systems and increase the sensitivity of precise measurements. In the inception of quantum mechanics, a series of seminal experiments were performed showing the superposition principle. For instance, the double-slit experiment with photons or electrons, shows the interference of a single particle with itself, revealing the "blurring" of the quantum wavefunction prior to measurement. Such sort of experiments open up fundamental and philosophical questions regarding the non-local (and contextual) nature of quantum mechanics. Although, for the case of particles, superposition and interference remain counterintuitive and surprising; when applied to classical waves, they become instinctive and common. Thus, quantum phenomena arising from the wave-particle duality does not encapsulate the whole essence of quantum weirdness. In the search of the most quantum phenomenon, interference of "quantum paths" was observed in 1987 by Hong, Ou and Mandel in their seminal experiment on two-photon interference, which was independently formulated for a lossless beam-splitter by Fearn and Loudon. This type of interference is exquisitely quantum in nature and has absolutely no analogue in classical physics. It is precisely this separation from classical to quantum physics that gave the Hong-Ou-Mandel effect a distinct advantage in many applications ranging from quantum computation to sensing, and has motivated physicists to study two-particle interference for fermionic and bosonic quantum objects. The Hong-Ou-Mandel type of interference has so far been observed with massive particles, among others, such as electrons, atoms and plasmons, demonstrating the extent of this effect to larger and more complex quantum systems. A wide array of novel applications to this quantum effect is to be expected in the future.

中文翻译：

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两个光子干涉：洪欧曼德尔效应

近 30 年前，Hong、Ou 和 Mandel 观察到了 Richard Feynman 所说的“量子力学的核心”：双光子干涉效应。这一观察标志着一个新的量子时代的开始，在这个时代，量子物理学的特殊性现在可以用于我们的优势，以超越经典计算、安全地传递信息、模拟高度复杂的物理系统并提高精确测量的灵敏度。在量子力学诞生之初，进行了一系列开创性的实验，展示了叠加原理。例如，光子或电子的双缝实验显示了单个粒子与自身的干涉，揭示了测量之前量子波函数的“模糊”。这类实验提出了关于量子力学的非局部（和上下文）性质的基本和哲学问题。尽管对于粒子来说，叠加和干涉仍然是违反直觉和令人惊讶的；当应用于经典波浪时，它们变得本能和普遍。因此，由波粒二象性引起的量子现象并没有概括量子怪异的全部本质。在寻找最量子现象的过程中，Hong、Ou 和 Mandel 于 1987 年在他们关于双光子干涉的开创性实验中观察到“量子路径”的干涉，该实验由 Fearn 和 Loudon 独立制定用于无损分束器。这种类型的干涉本质上是精致的量子，在经典物理学中绝对没有类似物。正是这种从经典物理学到量子物理学的分离，使得 Hong-Ou-Mandel 效应在从量子计算到传感的许多应用中具有明显的优势，并促使物理学家研究费米子和玻色子量子物体的两粒子干涉。迄今为止，已经在大质量粒子（例如电子、原子和等离子激元）中观察到 Hong-Ou-Mandel 类型的干涉，证明了这种效应对更大和更复杂的量子系统的影响程度。这种量子效应的大量新应用有望在未来出现。并激励物理学家研究费米子和玻色子量子物体的两粒子干涉。迄今为止，已经在大质量粒子（例如电子、原子和等离子激元）中观察到 Hong-Ou-Mandel 类型的干涉，证明了这种效应对更大和更复杂的量子系统的影响程度。这种量子效应的大量新应用有望在未来出现。并激励物理学家研究费米子和玻色子量子物体的两粒子干涉。迄今为止，已经在大质量粒子（例如电子、原子和等离子激元）中观察到 Hong-Ou-Mandel 类型的干涉，证明了这种效应对更大和更复杂的量子系统的影响程度。这种量子效应的大量新应用有望在未来出现。