Anderson localization in a wave-mechanical system. (Top) Extended states in a 3D lattice without disorder (shown in two-dimensional cross section). The blue colored lattice represents the potential of the lattice and the red surface shown above depicts the envelope of the particle wave function, which extends throughout the lattice. (Middle) Disorder enters the lattice structure through the addition of noise. This leads to the localization of particle wave packets in the potential as depicted by the peaks in the particle wave function. (Bottom) The setup used by Chabé et al. to measure the Anderson transition. A one-dimensional periodic potential is created by an optical standing wave. A cloud of cold atoms (orange) is exposed to the potential, whose intensity is pulsed, with the pulse height modulated by a periodic function of three incommensurate frequencies.