Pulses of light comprising around 100,000 entangled photons have been created by physicists in Germany and Russia. The pulses were made in the "squeezed-vacuum" state and the team found that the entanglement should become stronger as the number of photons in the pulse increases. Such pulses could find use in technologies such as quantum cryptography or metrology.
Entanglement is a quantum effect that allows particles such as photons to have a much closer relationship than predicted by classical physics. For instance, two photons can be created experimentally, such that if one is measured to be polarized in the vertical direction, a measurement on the other will reveal the same polarization. This occurs in spite of the fact that a measurement on a single photon will reveal a random value of polarization. While such a correlation can occur in the non-quantum world, quantum mechanics strengthens it to beyond what is expected from classical physics. This misfit between the quantum and classical worlds was described succinctly by the Northern Irish physicist John Bell in 1964 and was confirmed by a series of experiments done in the 1970s and 1980s.
To read more, click here.