QUANTUM DOUBLE-SLIT

How the double-slit experiment works

The double-slit experiment is often called the most beautiful in physics because it shows, in one setup, that matter and light behave as both waves and particles. Fire particles — photons, electrons, even whole molecules — one at a time at a barrier with two slits, and each lands as a single dot on the screen, like a particle. But let thousands accumulate and those dots build into a striped interference pattern, the signature of waves. Each particle somehow passes through both slits and interferes with itself.

The strangest part comes when you try to watch which slit each particle takes. The moment you detect the path, the interference pattern vanishes and you get two plain bands, as if the particles became ordinary again. This simulator reproduces exactly that: particles accumulate into fringes, and switching on the which-path detector collapses the pattern.

Experiments to try in this simulation

1. Build the pattern one particle at a time: fire slowly and watch single dots scatter almost at random, then gradually reveal the interference fringes as the count grows — wave behaviour emerging from particle events.

2. The observer effect: turn on the which-path detector and the fringes collapse into two humps. Measuring which slit the particle used destroys the interference — one of the deepest puzzles in quantum mechanics.

3. Change the particle: switch between photons, electrons, and neutrons. Heavier, faster particles have a shorter de Broglie wavelength, so their fringes are finer and closer together.

4. Geometry: widen the slit separation or move the screen and watch the fringe spacing respond, just as it does for classical waves.

The physics

Every particle has a wavelength given by de Broglie's relation, λ = h/p, where h is Planck's constant and p is momentum. That wavelength sets the fringe spacing, exactly as in the classical wave double-slit where bright fringes fall at d·sin θ = mλ. What is genuinely quantum is that the wave here is a probability amplitude: its square gives the chance of finding the particle at each spot, so the pattern is a map of probability, built up one random landing at a time.

The collapse of the pattern under observation reflects that any measurement extracting which-path information destroys the coherence between the two paths — the heart of the measurement problem in quantum mechanics.

Real-world applications

Wave-particle duality is not a curiosity — it is the foundation of modern technology. Electron microscopes exploit the tiny de Broglie wavelength of fast electrons to image things far smaller than light can resolve. The same quantum coherence underlies quantum computing, where qubits rely on superposition much like a particle 'going through both slits.' Neutron and electron diffraction, direct descendants of this experiment, are everyday tools for working out the atomic structure of materials, from metals to proteins.