Introduction to Quantum Superposition

Imagine you're the holder of a special passport that has many possible nationalities at once. Only when border authorities inspect it is it forced to randomly resolve into a single citizenship. While impossible in everyday life, this analogy provides a useful way to imagine one of the strangest ideas in modern physics: quantum superposition.

In quantum mechanics, an electron behaves in a similarly counterintuitive way. Rather than existing in one definite state, it can exist in multiple possible states simultaneously until it is measured. Although this concept challenges our everyday understanding of reality, decades of theoretical work and experimental evidence have shown that superposition is a genuine feature of the quantum world.

Understanding Quantum Superposition

Electrons possess a property known as wave-particle duality, meaning they can display characteristics of both waves and particles. In quantum mechanics, an electron is described by a wave function that represents all the possible states it can occupy simultaneously. This state of multiple possibilities is known as quantum superposition.

Although Austrian physicist Erwin Schrödinger developed the mathematical wave equation that forms the foundation of quantum mechanics in 1925, he famously criticized one interpretation of his own theory through a thought experiment known as Schrödinger's Cat.

The thought experiment imagines a cat sealed inside a box with a mechanism containing poison that has a 50% chance of being released after one hour. Before the box is opened, quantum mechanics suggests the cat is simultaneously both alive and dead because its fate depends on a quantum event that has not yet been observed. Only when someone opens the box does the system appear to "choose" one outcome: the cat is either alive or dead (Schrödinger, 1935).

While no real cat exists in such a state, the experiment illustrates the strange nature of quantum superposition. Importantly, superposition only applies to quantum-scale particles such as electrons and photons—not to everyday objects.

Experimental Evidence

One of the strongest demonstrations of quantum superposition comes from the single-electron double-slit experiment, first performed in 1974 by Italian physicists Pier Giorgio Merli, Gian Franco Missiroli, and Giulio Pozzi.

In this experiment, an electron gun fired electrons one at a time toward an electron biprism—a thin, electrically charged wire that split the electron's wave into two possible paths before the waves recombined (Rosa, 2012). The electrons then struck a detector screen.

Each electron appeared on the screen as a single dot, behaving like an individual particle. However, after thousands of electrons had been detected, the dots gradually formed alternating bright and dark bands known as an interference pattern. Such a pattern can only be produced by waves interfering with themselves, indicating that each electron had effectively traveled along both paths simultaneously before reaching the detector.

This remarkable result provides strong evidence that electrons exist in a superposition of multiple paths rather than following only one definite route.

Even more striking were later versions of the experiment in which detectors were placed near the slits to determine which path each electron traveled. Once these detectors interacted with the electrons and recorded path information, the interference pattern disappeared entirely (Jaroszkiewicz, 2017). Instead, the electrons behaved like ordinary particles traveling through only one path.

These observations demonstrate that obtaining information about a quantum system fundamentally changes its behavior. In other words, measuring the electron forces the superposition to collapse into a single observable state.

Conclusion

As Nobel Prize-winning physicist Richard Feynman famously remarked, "I think I can safely say that nobody understands quantum mechanics" (Feynman, 1965). Despite remaining one of the most counterintuitive ideas in science, quantum superposition has repeatedly been confirmed through experiments such as the single-electron double-slit experiment.

Superposition reminds us that nature behaves in extraordinary ways at the smallest scales, challenging our intuition and expanding our understanding of reality. Although much about the quantum world remains mysterious, exploring concepts like superposition offers a fascinating glimpse into the fundamental principles that govern our universe.

Writer: Blen Dagnachew

Editor: Menna

References

Feynman, R. (1965). The Character of Physical Law. MIT Press.

Jaroszkiewicz, G. (2017). Quantum eraser experiments. In Cambridge University Press eBooks (pp. 179–197). https://doi.org/10.1017/9781316477182.015

Rosa, R. (2012). The Merli–Missiroli–Pozzi two-slit electron-interference experiment. Physics in Perspective, 14(2), 178–195. https://doi.org/10.1007/s00016-011-0079-0

Schrödinger, E. (1935). Die gegenwärtige Situation in der Quantenmechanik [The present situation in quantum mechanics]. Naturwissenschaften, 23(48–50), 807–849. https://doi.org/10.1007/BF01491891