Quantum Eraser - The Experiment

The Experiment

First, a photon is shot through a specialized nonlinear optical device: a beta barium borate (BBO) crystal. This action leads to what is known as spontaneous parametric down conversion (SPDC), i.e., it converts the single photon into two entangled photons of lower frequency. From then on these entangled photons follow separate paths. One photon goes directly to a detector, which sends information of the received photon to a coincidence counter, a device that notes the nearly simultaneous reception of a photon in each of two detectors so that it can count how many pairs of entangled photons have made it through the apparatus and exclude the influence of any photons that enter the apparatus without having become entangled. When the coincidence counter is signaled of the arrival of the partner photon it increments its count. A timer is set up so that it signals a stepper motor to move the second detector on a regular basis so that it can scan across the range of positions where interference fringes could be detected. Meanwhile, the second entangled photon is faced with the double-slit, whereupon it proceeds by two paths to the second detector, which sends information of a received photon to the coincidence counter. At this point, the coincidence counter has been told that both entangled photons of the original pair have been detected and that fact is added to its record along with the position currently held by the second detector. After a predetermined amount of time has passed, the detector will be moved by the tractor to examine another location. This apparatus will eventually yield the familiar interference pattern, because nothing has interfered with the disturbance that propagates through two paths after meeting the two slits and getting split up.

Next, in an attempt to determine which path the photon took through the double slits, a quarter wave plate (QWP) is placed in front of each of the double-slits that the second photon must pass through (see Illustration 1). These crystals will change the polarization of the light, one producing "clockwise" circular polarization and the other producing its contrary, thus "marking" through which slit and polarizer pair the photon has traveled. Subsequently, the newly polarized photon will be measured at the detector. Giving photons that go through one slit a "clockwise" polarization and giving photons that go the other way a "counter- clockwise" polarization will destroy the interference pattern.

The next progression in the setup will attempt to bring back the interference pattern by placing a polarizer before the detector of the entangled photons that took the other path out of the beta barium borate crystal (see Illustration 2). Because pairs of photons are entangled, giving one a diagonal polarization (rotating its plane of vibration 45 degrees) will cause a complementary polarization of its entangled pair member. So from this point on, the photons heading down toward the double slits will meet the two circular polarizers after having been rotated. And when photons enter either circular polarizer "half way off" from their original orientation, the result will be that on each sub-path half will be given one kind of circular polarization and half will receive the other polarization. The end result is that half the photons emerging from each circular polarizer will be "clockwise" and half will be "counter-clockwise." It will then be impossible to look at the polarization of a photon and know by which path it has come. Each component of an original wave-function will interfere with itself. And at this stage the interference fringes will reappear.