This is quantum superposition

The photon is on both paths

This is quantum superposition: the photon is on both paths. But if you search for it, it is only on one path. Try yourself to think of a sensible explanation of this behaviour. For a century now, we have all been trying. This occurs when contradictory properties coexist in a certain sense, implying that an object can exist in one place while simultaneously being elsewhere. Heisenberg's assertion that 'the electron no longer has a trajectory' implies that the electron is not confined to a singular location; rather, it is, in a sense, present in both places at once. This peculiar behavior, termed the 'principle of superposition' by Dirac, serves as a foundational concept in quantum theory.

However, it's crucial to note that quantum superposition itself is not directly observable. What we perceive are the outcomes or consequences of superposition, termed 'quantum interference.' In essence, we witness the effects of superposition rather than directly observing it. To illustrate, consider the work conducted at the Innsbruck laboratory a distinguished experimental physicist renowned for his groundbreaking work with quantum phenomena. He has contributed significantly to areas such as quantum computing, quantum cryptography, and quantum teleportation. Although intricate, an example from his laboratory encapsulates the confusion physicists experience.

Quantum superposition: the photon is on both paths

What makes quantum phenomena so peculiar?
The mere fact that electrons adhere to specific orbits and transition between them doesn't seem like a catastrophic event.
The unusual nature of quantum phenomena is rooted in a phenomenon known as quantum superposition.

Scientists eventually uncover truth

In this setup, there is a table featuring various optical instruments, including a small laser, lenses, prism mirrors for beam separation and integration, and photon detectors. A weak laser beam, comprising a small number of photons, is divided into two parts, creating distinct paths one on the 'right' and the other on the 'left.'

These paths eventually reunite before diverging again and culminating in two detectors, one oriented 'up' and the other 'down.'A stream of photons undergoes separation into two paths by a prism, then reunites and is subsequently divided again. What I observed was intriguing: when I obstructed either the left or right path with my hand, half of the photons reached the up detector, and the other half reached the down detector. Conversely, when both paths were unobstructed, all photons landed in the down detector, with none in the upper on.

This phenomenon is known as quantum interference. Blocking one path with my hand causes photons initially traveling on the other path to divert to the down detector. In contrast, when both paths are unhindered, all photons converge to the down detector. The question of how my hand's placement in one path influences photons from the other path to move to the down detector remains unanswered.

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Half of the photons

A peculiar aspect emerges; if half of the photons reach the up detector when one path is free, one might logically expect that, with both paths free, half the photons should also reach the upper detector. However, this is not the case � none do. The disappearance of photons from the upper detector when both paths are open exemplifies quantum interference. Interference occurs between the two paths � the one on the left and the one on the right. When both paths are open, a unique phenomenon unfolds, absent when photons traverse only one of these paths: the photons en route to the upper detector vanish.

According to Schrodinger's theory, the wave function of each photon splits into two parts or wavelets. One wavelet follows the right path, while the other takes the left.

In recognition of its importance, the 2022 Nobel Prize in Physics was awarded to Alain Aspect, John F. Clauser and Anton Zeilinger for their experiments with entangled photons, which established the violation of Bell inequalities and pioneered quantum information science

On only one side

Upon reuniting, the wave reconstitutes and takes the lower path. Blocking one path prevents the ? wave from reconstituting, altering its behavior. While such wave behavior is not uncommon, as seen in light and ocean waves, we never directly observe the wave; individual photons appear on only one side, either right or left.

Each photon acts as if it passed through both trajectories, akin to waves, yet when observed, it is always located on just one path. This state is known as quantum superposition, where the photon exists in a superposition of two configurations: one on the right and one on the left. Consequently, the photon no longer proceeds upward, as it would along a single path.

There's more to the mystery. If I measure which path the photon takes, the interference vanishes. Merely observing alters the outcome. The paradox lies in the fact that if I don't observe the photon's path, it consistently concludes below. However, if I do observe, it can end up above. Remarkably, a photon can end up above even when unobserved, suggesting that the act of observation alters its trajectory. This implies that the photon changes its path due to the mere anticipation of observation, even without direct visual confirmation.

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The theory does not tell us how the electron moves during a leap.
It only tells us what we see when it leaps. Why?

Yet, when observed. Quantum entanglement

Textbooks on quantum mechanics assert that when we observe a photon's path, it wave entirely shifts to a specific path. Whether or not we observe it, the wave 'collapses,' converging to one path the moment observation occurs. This is the enigma of quantum superposition: the photon is, in a manner of speaking, on both paths. Yet, when observed, it is exclusively on one path. This perplexing behavior has challenged comprehension for a century. Quantum Entanglement: A Crucial Difference Between Classical and Quantum Theories of Physics. Quantum entanglement is a unique property of quantum mechanics, where the state of one particle cannot be described independently from the other.

This property is a key difference between classical and quantum theories of physics. In recognition of its importance, the 2022 Nobel Prize in Physics was awarded to Alain Aspect, John F. Clauser and Anton Zeilinger for their experiments with entangled photons, which established the violation of Bell inequalities and pioneered quantum information science.