Double-slit experiment

The double-slit experiment sends particles, such as photons or electrons, at a barrier with two narrow openings and records where they land on a screen behind it. When both slits are open and no attempt is made to tell which slit each particle passed through, the particles build up an interference pattern of bright and dark bands, the signature of waves. Yet each particle arrives as a single localized impact, the signature of a particle. It is the clearest demonstration of wave-particle duality.

#What is observed

The experiment can be run so that particles pass through one at a time, with long gaps between them. Each lands at a single point, seemingly at random. As thousands accumulate, the individual dots assemble into the striped interference pattern. This rules out any explanation in which particles interfere with one another, because only one particle is present at a time. Instead each particle's wave function passes through both slits and interferes with itself, and the squared amplitude of the combined wave gives the probability of the particle landing at each point on the screen.

If one slit is closed, the interference vanishes and the particles pile up in a single band behind the open slit. With both slits open the pattern is not the sum of the two single-slit patterns, which is the hallmark of quantum Superposition.

#Which-path information

The decisive quantum feature appears when a detector is added to record which slit each particle goes through. As soon as reliable which-path information becomes available, the interference pattern disappears and the particles behave like classical objects passing through one slit or the other. This is not merely a matter of a clumsy detector bumping the particle; it holds even for gentle, in-principle measurements, and it can be arranged and reversed in delicate variants. Acquiring which-path information and observing interference are complementary: gaining one erases the other, a trade-off tied to the uncertainty principle. The transition from a superposition of paths to a definite outcome connects the experiment directly to the measurement problem (Stanford Encyclopedia of Philosophy).

#History

Thomas Young performed the original double-slit experiment with light around 1801, providing strong evidence for the wave theory of light. Its quantum significance emerged after Louis de Broglie proposed matter waves in 1924 (Nobel Foundation, de Broglie) and electron diffraction was observed. Over the twentieth century versions were carried out with single electrons, neutrons, atoms, and even large molecules, all showing interference, confirming that duality is a general property of matter and not limited to light.

#Feynman's assessment

Richard Feynman opened his lectures on quantum mechanics with the double-slit experiment, presenting it as the phenomenon that captures everything strange about the theory. He described it as containing "the only mystery" of quantum mechanics and argued that its behavior cannot be explained in any classical way (Feynman Lectures, Vol III). The remark has become the standard way of framing why the experiment is regarded as central to quantum mechanics.

#Significance

The double-slit experiment is the canonical teaching example of quantum behavior. It shows superposition, interference, the probabilistic Born rule, and the role of measurement in a single setup, and it demonstrates that observation is not a passive act at the quantum scale. The same interference of probability amplitudes that it displays is the resource that quantum computing exploits.