Whatever happened to one particle would thus immediately affect the other particle, wherever in the universe it may be. Einstein called this "Spooky action at a distance."
Amir D. Aczel, Entanglement, The Greatest Mystery In Physics.
When a photon (usually polarized laser light) passes through matter, it will be absorbed by an electron. Eventually, and spontaneously, the electron will return to its ground state by emitting the photon. Certain crystal structures increase the likelihood that the photon will split into two photons, both of them with longer wavelengths than the original. Keep in mind that a longer wavelength means a lower frequency, and thus less energy. The total energy of the two photons must equal the energy of the photon originally fired from the laser (conservation of energy).
When the original photon splits into two photons, the resulting photon pair is considered entangled.
The process of using certain crystals to split incoming photons into pairs of photons is called parametric down-conversion.
Normally the photons exit the crystal such that one is aligned in a horizontally polarized light cone, the other aligned vertically. By adjusting the experiment, the horizontal and vertical light cones can be made to overlap. Even though the polarization of the individual photons is unknown, the nature of quantum mechanics predicts they differ.
To illustrate, if an entangled photon meets a vertical polarizing filter (analagous to the fence in Figure 4.4), the photon may or may not pass through. If it does, then its entangled partner will not because the instant that the first photon's polarization is known, the second photon's polarization will be the exact opposite.
It is this instant communication between the entangled photons to indicate each other's polarization that lies at the heart of quantum entanglement. This is the "spooky action at a distance" that Einstein believed was theoretically implausible.
Experiments have shown that Einstein may have been wrong: entangled photons seem to communicate instantaneously. Figure 5.1 illustrates how to create entangled photons.
Figure 5.1. Photon Entangler Device.
- An ultraviolet laser sends a single photon through Beta Barium Borate.
- As the photon travels through the crystal, there is a chance it will split.
- If it splits, the photon will exit from the Beta Barium Borate as two photons.
- The resulting photon pair are entangled.
An ultraviolet laser is used because the laser light has a high frequency. A high frequency implies a greater chance of splitting into two entangled photons.
Figure 5.2 is an enhanced photograph of a photon that has split into an entangled pair.
Figure 5.2. Entangled Photons.