Figure 1
APS/Alan Stonebraker

Figure 1: Various forms of the double-slit interference experiment have allowed physicists to explore what can—and can’t—be said about the history of a quantum particle. (Top) The classic double-slit experiment with photons. (Middle) The double slit experiment with lenses placed in front of the slits. (Bottom) The nested Mach-Zehnder interferometer experiment performed by Danan et al.