What is the type of spontaneous parametric down-conversion (SPDC) used in the following quantum eraser experiment?
One of the facts that need clarification is whether the generation of entangled photons implicitly means that such an experiment must use type-II SPDC BBO crystal or whether the design of the experiment allows freedom of choosing between type-I and type-II.
First, a photon is shot through a specialized nonlinear optical device: a beta barium borate (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.
- Figure 2. Introduction of polarizer in upper path restores interference fringes below