The experiment Quantum eraser experiment

figure 1. crossed polarizations prevent interference fringes

first, photon shot through specialized nonlinear optical device: beta barium borate (bbo) crystal. crystal converts single photon 2 entangled photons of lower frequency, process known spontaneous parametric down-conversion (spdc). these entangled photons follow separate paths. 1 photon goes directly detector, while second photon passes through double-slit mask second detector. both detectors connected coincidence circuit, ensuring entangled photon pairs counted. stepper motor moves second detector scan across target area, producing intensity map. configuration yields familiar interference pattern.

figure 2. introduction of polarizer in upper path restores interference fringes below

next, circular polarizer placed in front of each slit in double-slit mask, producing clockwise circular polarization in light passing through 1 slit, , counter-clockwise circular polarization in other slit (see figure 1). polarization measured @ detector, marking photons , destroying interference pattern (see fresnel–arago laws).

finally, linear polarizer introduced in path of first photon of entangled pair, giving photon diagonal polarization (see figure 2). entanglement ensures complementary diagonal polarization in partner, passes through double-slit mask. alters effect of circular polarizers: each produce mix of clockwise , counter-clockwise polarized light. second detector can no longer determine path taken, , interference fringes restored.

a double slit rotating polarizers can accounted considering light classical wave. experiment uses entangled photons, not compatible classical mechanics.