Photons passing through slits create an interference pattern on a screen.
Each slit acts as a point source. Where waves arrive in phase, they constructively interfere. Where out of phase, they cancel.
The "photon-by-photon" mode shows how individual quantum particles randomly land, yet collectively build the wave pattern.
Interference is the mechanism that makes quantum computing work. When a quantum system can reach an outcome through multiple paths, the probability amplitudes for those paths add together. Paths can reinforce (constructive interference) or cancel (destructive interference), creating patterns impossible in classical probability.
The multi-slit experiment is the canonical demonstration. Single photons pass through multiple slits and land on a detector screen. Each photon arrives at a single point, but over many trials an interference pattern emerges — bright bands where amplitudes add, dark bands where they cancel.
Quantum algorithms exploit interference deliberately: they amplify amplitudes for correct answers and suppress wrong ones. Grover's search and the quantum Fourier transform both rely on precisely engineered interference.