Hyper-entangled photon pairs

Entanglement is the quantum connection between or among particles (such as atoms or photons) because of which the measurement of some property for one particle automatically and instantaneously determines the corresponding property of the other particle. One of the chief hopes of entanglement researchers is to exploit the phenomenon for quantum computation. (See Physics Today, April 2003, page 46.) Paul Kwiat and his colleagues at the University of Illinois have now demonstrated the entanglement of two photons in all possible degrees of freedom. The physicists sent light to be “down-converted” in two adjacent nonlinear crystals, producing daughter photons that were simultaneously entangled in polarization, orbital angular momentum, and energy—time. The uncertainty of the precise production details of each photon in each crystal is what permitted the hyper-entanglement to occur. The setup was also used to produce an unusual state in which photon pairs simultaneously displayed classical correlations in...