Fire particles one at a time and watch interference patterns build up—turn on decoherence to destroy quantum behavior
The double-slit experiment is perhaps the most famous demonstration of quantum mechanics. It reveals the fundamental wave-particle duality: particles like electrons and photons behave as waves when not observed, creating interference patterns impossible for classical particles. Yet when we try to determine which slit they pass through, the interference vanishes and we see two classical piles.
The top panel shows the quantum wavefunction Ψ(x,y) as it propagates through space. Brightness represents the probability density |Ψ|², while color hue encodes the phase. When the wave hits the barrier with two slits, it splits into two coherent components that overlap and interfere beyond the slits. Watch the interference pattern emerge as the wave packets from each slit meet and combine.
The bottom panel shows the accumulated detection probability at a distant screen—what you'd see if you placed a detector array there. With γ = 0 (no decoherence), you see the classic interference fringes: alternating bright and dark bands. The bright fringes occur where the two paths interfere constructively (phases add), and dark fringes where they interfere destructively (phases cancel).
Increase the decoherence rate γ and watch the fringes fade. Decoherence represents uncontrolled entanglement with the environment—imagine tiny air molecules scattering off the particle and recording which-path information. As γ increases, the quantum coherence between the two paths is destroyed, and the interference pattern transitions to a classical sum of two Gaussian humps (one per slit). This is not wave function collapse; it's the emergence of classicality from quantum dynamics.
When a particle approaches the slits, it doesn't "choose" one—it enters a superposition state |Ψ⟩ = (|slit 1⟩ + |slit 2⟩)/√2. This is a genuine quantum state, not ignorance about which slit it went through. The two components have a definite phase relationship that determines the interference pattern. Change the slit separation and you change this phase difference across the screen, altering the fringe spacing.
Decoherence makes the paths distinguishable by entangling the particle with environmental degrees of freedom. Mathematically, the pure state |Ψ⟩ becomes a mixed state ρ. The off-diagonal elements of ρ (the coherences) decay exponentially at rate γ. When the coherences vanish, there's no interference—the system behaves classically. This explains why we don't see everyday objects in superposition: they decohere almost instantly.
Feynman called the double-slit experiment "impossible, absolutely impossible, to explain in any classical way" and said it contains "the only mystery" of quantum mechanics. Everything else—uncertainty, entanglement, measurement—flows from this one fact: quantum systems exist in superpositions and exhibit interference.
The role of decoherence is equally profound. It shows that classicality is not fundamental but emergent. The universe doesn't have a classical layer and a quantum layer; it's quantum all the way up. Large, warm, interacting systems look classical because they decohere on timescales far shorter than we can observe. The double-slit experiment, with controllable decoherence, lets you turn the "quantumness" dial and watch the transition from wave to particle behavior in real time.
Experiments have verified this interference for electrons, neutrons, atoms, and even large molecules like buckyballs (C₆₀) and beyond. The pattern is always the same: coherence yields interference; decoherence or measurement destroys it. Nature doesn't care how big or complex the particle is—the rules of quantum mechanics apply universally.