Entangled photons are basically particles of light with interlinked properties, meaning that the properties of one photon depend on those of a second photon. The study of the interactions that take place between entangled properties may eventually reveal the fundamental concepts of quantum physics. The National Institute of Standards and Technology recently developed a new method through which entangled photon pairs can be created and studied.

By pulsing a beam of light through a loop of optical fiber, photon particles with the same wavelength can be created. Each has a trajectory opposite to the other, meaning that they can interact in a 'four-wave mixing' process. Four-wave mixing represents a process through which the two photons of light traveling in opposite directions interact in order to shift wavelengths. Thus, while before the interaction the two had the same wavelength, after it one will shift its color to red and the other to blue.

As they travel through the looped optical fiber, the entangled photons eventually reach the end of the loop. On one side, the pair experiences vertical polarization, while at the other horizontal polarization, the electrical field of the electromagnetic wave oscillates in a horizontal plane. Because it is practically impossible to determine which way the photon pair took, the team had to appeal to the laws of quantum physics, which say that in fact the entangled pair experiences two different states at the same time! Photons are both polarized vertically and horizontally.

In the real world, the macroscopic world as we know it, this is impossible. One photon must be chosen for measurement, at which point the photon must decide which state it will exhibit.

A little history

The entanglement phenomenon was first postulated by German physicists Albert Einstein, who called it the 'spooky action at distance' since it seemed that two particles can determine the state of the other through faster-than-light interactions - process forbidden by the Theory of Relativity - and this is probably one of the phenomenons that drove Einstein away from the quantum mechanics world. Quantum entanglement often takes physicists into the deep waters of 'local realism', stating that one process occurring in one place cannot have an immediate effect in another place - locality - and the particles must have preexisting properties even before they are measured - realism.

It is clear that locality and realism cannot take place at the same time, but what if they are experienced alternatively? Realism ultimately implies nonlocality, meaning that the simple measurement of a particle cannot influence other particles in different locations of space-time. Photon polarization effect seems to disagree with predictions made by the Nonlocal Hidden Variables of realism, however they accurately describe that of quantum mechanics, thus ruling out certain aspects of the NLHV theory.