(BBO) crystal. This crystal converts the single photon into two entangled photons of lower frequency, a process known as spontaneous parametric down-conversion (SPDC). These entangled photons follow separate paths. One photon goes directly to a polarization-resolving detector, while the second photon passes through the double-slit mask to a second polarization-resolving detector. Both detectors are connected to a coincidence circuit, ensuring that only entangled photon pairs are counted. A stepper motor moves the second detector to scan across the target area, producing an intensity map. This configuration yields the familiar interference pattern. Next, a circular polarizer is placed in front of each slit in the double-slit mask, producing clockwise circular polarization in light passing through one slit, and counter-clockwise circular polarization in the other slit (see Figure 1). (Which slit corresponds to which polarization depends on the polarization reported by the first detector.) This polarization is measured at the second detector, thus "marking" the photons and destroying the interference pattern (see Fresnel–Arago laws). Finally, a linear polarizer is introduced in the path of the first photon of the entangled pair, giving this photon a diagonal polarization (see Figure 2). Entanglement ensures a complementary diagonal polarization in its partner, which passes through the double-slit mask. This alters the effect of the circular polarizers: each will produce a mix of clockwise and counter-clockwise polarized light. Thus the second detector can no longer determine which path was taken, and the interference fringes are restored. A double slit with rotating polarizers can also be accounted for by considering the light to be a classical wave. However this experiment uses entangled photons, which are not compatible with classical mechanics. Other applications Quantum erasure technology can be used to increase the resolution of advanced microscopes. Common misconception A common misunderstanding about this experiment is that it may be used to instantaneously communicate information between two detectors. Simple causation, however, precludes foisting "given" information on the observed outcomes. It is important to understand the role of the coincidence detector in this experimental setup. The linear polarizer in the top path is effectively filtering out half the entangled photons, and via the coincidence detector, is filtering out the corresponding photons in the bottom path. The coincidence detector can only function by comparing data from both sensors, making it impossible to use this setup for instant communication. In other words, only a small percentage of the light passing through the