slit. By decreasing the brightness of the source sufficiently, individual particles that form the interference pattern are detectable. The emergence of an interference pattern suggests that each particle passing through the slits interferes with itself, and that therefore in some sense the particles are going through both slits at once. This is an idea that contradicts our everyday experience of discrete objects. A well-known thought experiment, which played a vital role in the history of quantum mechanics (for example, see the discussion on Einstein's version of this experiment), demonstrated that if particle detectors are positioned at the slits, showing through which slit a photon goes, the interference pattern will disappear. This which-way experiment illustrates the complementarity principle that photons can behave as either particles or as waves, but cannot be simultaneously observed to be both a particle and a wave. However, technically feasible realizations of this experiment were not proposed until the 1970s. Which-path information and the visibility of interference fringes are hence complementary quantities, meaning that information about a photon's path can be observed, or interference fringes can be observed, but they cannot both be observed at once. In the double-slit experiment, conventional wisdom held that observing the particles' path inevitably disturbed them enough to destroy the interference pattern as a result of the Heisenberg uncertainty principle. However, in 1982, Scully and Drühl found a loophole around this interpretation. They proposed a "quantum eraser" to obtain which-path information without scattering the particles, but introducing uncontrolled phase factors to them (by spontaneous parametric down-conversion or SPDC). Rather than attempting to observe which photon was entering each slit (thus disturbing the photons), they proposed to "mark" them with information that, in principle at least, would allow the photons to be distinguished after passing through the slits. Lest there be any misunderstanding, the interference pattern does disappear when because the phase cannot be measured when the photons are so marked. However, the phase is measurable if the which-path information is further manipulated after the marked photons have passed through the double slits to obscure the which-path markings. And the interference pattern reappears that extracted according to the phase values. Since 1982, multiple experiments have demonstrated the validity of the so-called quantum "eraser". A simple quantum-eraser experiment A simple version of the quantum eraser can be described as follows: Rather than splitting one photon or its probability wave between two slits, the photon