Superposition is the ability of a quantum system to be in multiple states at the same time until it is measured.
Because the concept is difficult to understand, this essential principle of quantum mechanics is often illustrated by an experiment carried out in 1801 by the English physicist, Thomas Young. Young's double-slit experiment was intended to prove that light consists of waves. Today, the experiment is used to help people understand the way that electrons can act like waves and create interference patterns.
For this experiment, a beam of light is aimed at a barrier with two vertical slits. The light passes through the slits and the resulting pattern is recorded on a photographic plate. When one slit is covered, the pattern is what would be expected: a single line of light, aligned with whichever slit is open.
Intuitively, one would expect that if both slits are open, the pattern of light will reflect two lines of light aligned with the slits. In fact, what happens is that the photographic plate separates into multiple lines of lightness and darkness in varying degrees.
What is being illustrated by this result is that interference is taking place between the waves going through the slits, in what, seemingly, should be two non-crossing trajectories. Each photon not only goes through both slits; it simultaneously takes every possible trajectory en route to the photographic plate.
In order to see how this might possibly occur, other experiments have focused on tracking the paths of individual photons. Surprisingly, the measurement in some way disrupts the photons' trajectories and somehow, the results of the experiment become what would be predicted by classical physics: two bright lines on the photographic plate, each aligned with the slits in the barrier. This has led scientists to conclude that superposition cannot be directly observed; one can only observe the resulting consequence, interference.
In computing, the concept of superposition has important implications for the way information will be processed and stored in the future. For example, today's classical computers process information in bits of one or zero, similar to a light switch being turned on or off. The quantum supercomputers of tomorrow, however, will process information as qubits -- one, zero or a superposition of the two states.