Here we are, continuing with the topic “Quantum Physics”, and today’s theme is going to be “Quantum Entanglement”.
To understand the Quantum Entanglement, several ideas and words must be explained. The photon is a key concept in physics, and so critical to entanglement that its behaviors must be fully understood.
What is a photon? – A photon is a force carrier particle. Elementary particles (electrons, protons, neutrons, and such) can only interact using force carriers. In the race analogy, photons are like fruit; they are responsible for electromagnetic interactions.
Polarization! – Polarization is the direction in which photons oscillate. In physics, to oscillate means to vary between alternate extremes, often within a set time limit. For a photon, its polarization is the orientation of its axis, which in turn is linked to its direction of motion.
When a photon (usually polarized laser light) passes through matter, it will be absorbed by an electron. Eventually, 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. If an entangled photon meets a vertical polarizing filter, 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.