Nature Photon. http://doi.org/h6c (2012)

Light shows its wave or particle nature depending on the choice of measurement. Jian-Shun Tang and colleagues demonstrate that this long-recognized principle of complementarity is, in fact, closely related to quantum superposition.

To explore the principle of complementarity, John Wheeler proposed a delayed-choice experiment. In a Mach–Zehnder interferometer, light passes through a beamsplitter and branches into two beams that are then recombined in a second beamsplitter, before reaching one of two detectors. When the second beamsplitter is present, interference between the two paths is detected, betraying the wave nature of light; if, however, that beamsplitter is not there, light's particle nature is apparent. In the delayed-choice experiment, the second beamsplitter can be either inserted or removed immediately after the photons have passed through the first one. This arrangement excludes the possibility that, by having access to some hidden information, the photon cunningly 'assumes' its wave or particle property beforehand.

Tang et al. used a special second beamsplitter that is in a quantum superposition of being present and absent, which allowed them to observe single photons in wave–particle superposition states. The fact that light can behave either as a wave or a particle, or as a superposition of both, redefines the concept of wave–particle duality.