Two Photon Interference: The Hong-Ou-Mandel Effect.

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.