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The Weirdest Truth: “Incomplete” Quantum Physics

The truth of our universe occasionally perplexes us due to its counter-intuitive characteristics. The wave-particle duality of light, the fundamental theory of Quantum Physics, is a typical example. Additionally, in 1982, the advent of a new concept, “Quantum Erasure,” made even scientists doubtful.

We can regard the inherent property of light, duality, as a dark box having two types of balls inside, a wave-like and a particle-like ball. To grasp the ball in the box, we need to wear special gloves correlating to each ball respectively. For example, if there is a “mark” on the glove, we can seize the particle-like ball, but if not, we can seize the wave-like ball. Although the two properties are inherent in light, it cannot exhibit both wave-like property and particle-like property at the same time.

It is Young’s double-slit experiment that first attested to the duality of light. He produced an interference pattern of light incident on a double-slit and verified the disappearance of interference when the slit that light went through was identified.

One team of physicists, M. O. Scully and Kai Drühl, suggested a modified double-slit experiment called Quantum Erasure. According to their suggestion, we can recover the interference pattern when we “erase” the mark of the trajectory of the photon. It is as if the grasped ball transforms from a particle to a wave immediately when we remove the “mark” on the glove in the dark box.

Do we remove the trajectory information of light by pushing the delete button? Quantum erasure aroused people’s curiosity, but people should be familiar with how physicists record a physical mark on the photon in order to understand the experiment. For example, they change the polarization of light to physically record the trajectory on the photon itself. 

According to the study by the Walborn research team in 2002, after attaching a different polarizer to each slit, researchers could identify which slit a photon went through by unlike polarization of the incident photon. Then, the interference pattern disappeared. After that, researchers added another polarizer between a double-slit and a screen. Then, they observed the interference pattern recovery. The key to this observation is the loss of physical record due to adding an additional polarizer.

The current Quantum theory is unable to predict Quantum erasure. It is why physicists are now reconsidering Einstein’s assertion about incomplete Quantum theory. Quantum erasure humbles us with the realization that we still have much to delve into nature.

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