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The magic of quantum optics: How do two identical photons quietly cooperate in a beam splitter?
In the field of quantum physics, there is a phenomenon that leaves scientists speechless. This is the famous Hong–Ou–Mandel effect. What it shows is not simple light interference, but profound cooperation and collaboration phenomena in the quantum world. In 1987, scientists Hong, Ou and Mandel demonstrated this effect for the first time at the University of Rochester, opening the way to a new understanding of quantum optics.
When two identical photons enter a 1:1 beam splitter at the same time, whether they are reflected or transmitted, they will exit in exactly the same way, which cannot be explained in classical physics.
When two photons overlap, their behavior can have unimaginable consequences. This phenomenon is not accidental but depends on their physical properties. In an ideal 1:1 beam splitter, each photon has a 50% probability of being transmitted or reflected. However, when two photons are identical, they will always appear on the same path. Simply put, the behavior of a beam splitter straddles the line between quantum and classical: in the quantum realm, this means that two photons "cooperate" with each other, a phenomenon that cannot be seen in classical optics.
To fully understand this phenomenon, we first need to understand the nature of photons and how beam splitters work. When a photon enters a beam splitter, it has two possible behaviors: it is either reflected or transmitted. However, in actual observations, scientists discovered that when two identical photons enter the splitter at the same time, a profound and strange interference phenomenon occurs between the transmission and reflection paths. This phenomenon can be explained by the concept of quantum superposition.
When two photons enter the splitter with the same frequency and phase, the interference will reach maximum only when they completely overlap and their properties are exactly the same. This is the so-called "Hong– Ou-Mandel dip”.
Scientists' experiments show that when two photons overlap perfectly, the coincidence event in the detector will be zero, which is the so-called "Hong-Ou-Mandel dip". This means that when the photons are identical, they will not be observed in the detector at the same time. This phenomenon will disappear as the photon's properties become more discernible, reflecting the fine balance between the photon's properties and behavior.
This phenomenon is not only of great significance in basic research, it also lays the theoretical foundation for quantum computing and quantum communication. The working principle of quantum logic gates is to use this interference effect for effective information processing. What followed was the development of many related applications, such as quantum encryption and quantum communications, which are the most cutting-edge areas of modern technology.
In the laboratory, scientists have successfully observed this quantum interference effect directly using single-photon detectors and used enhanced cameras to clearly record the behavior of single photons. The ability to distinguish individual photons online against a low-noise background makes these experiments possible.
“When two photons appear at the same time, they appear in one of the detectors, rather than in both detectors at the same time. This clearly violates the predictions of classical physics and shows the wonder and flexibility of the quantum world. ”
In addition to its application in quantum computing, the Hong–Ou–Mandel effect is also widely used in the development of optical sensors and other optical devices, which allows us to make full use of quantum technology in sensing, imaging and other fields. Furthermore, by continuing to explore this effect, scientists continue to advance our understanding of quantum physics, leading to the development of quantum optics and photonics.
While our understanding of these effects is improving, many questions remain unanswered. When we face such mysterious quantum phenomena, will it make us interested in further exploration?
