Quantum Sandbox for a rolling barrel

Confine a quantum particle to a box and its energy can only take discrete values, E_n = n²h²/8mL². The wavefunctions are standing waves with n humps; squaring them gives the probability of finding the particle at each point. Narrowing the box pushes the energy levels dramatically higher — quantum confinement.

Classically a particle with less energy than a barrier is trapped. Quantum mechanically its wavefunction decays inside the barrier but does not vanish, so there is a finite chance it appears on the other side — tunneling. The probability falls off exponentially with barrier width and height, the principle behind scanning tunneling microscopes and nuclear fusion.

The quantum harmonic oscillator — a particle in a parabolic well — has energy levels evenly spaced by ħω, starting at a nonzero zero-point energy of ½ħω. Its wavefunctions are Hermite polynomials times a Gaussian. It models molecular vibrations, phonons, and quantum fields, making it the most important solvable system in physics.

Solving the Schrödinger equation for hydrogen gives the orbitals — the probability clouds where an electron is likely to be found. The quantum numbers n, l, and m set the size, shape, and orientation. Brighter regions are higher probability density.