Hyper-Entangled Photon Pairs

Physicists at the University of Illinois at Urbana-Champaign have demonstrated for the first time the entanglement of two objects not merely in one aspect of their quantum natures, such as spin, but in a multitude of ways.

Entanglement is the quantum affinity between or among particles (such as atoms or photons) in which the measurement of some property for one particle automatically and instantaneously determines the corresponding property of the other particle.

Take the case of two photons entangled with respect to polarization, the orientation of the electric field associated with the photon. The photons, until detected, have no spin orientation; this is the principle of quantum indeterminacy. Indeed, both photons are said to be in a superposition of arbitrary -- but parallel -- polarization states. Consequently, each photon has a 50 percent likelihood of being measured to have any polarization state -- e.g., +45 or -45 degrees. If now one photon's polarization is measured to be +45, then its entangled twin will surely also be polarized along +45, owing to the way the photons are made in this setup.

One of the chief hopes of entanglement research is to exploit the superposition idea and the entanglement idea for performing unusually fast quantum computation. In the Illinois experiment, two photons, produced in a "down-conversion" process whereby one photon enters an optical crystal and sunders into two lesser-energy correlated daughter photons, are entangled not just in terms of polarization, but also in a number of other ways: energy, momentum, and orbital angular momentum (see PNU 721).

Actually, the photon pair can be produced in either of two crystals, and the uncertainty in the production details of the individual photons is what provides the ability to attain entanglement in all degrees of freedom.

Is it better to entangle two particles in ten ways or ten particles in two ways? They're probably equivalent, says Paul Kwiat, leader of the Illinois group, but for the purpose of quantum computing or communication it might be of some advantage if multiple quantum bits (or qubits) of information can be encoded in a single pair of entangled particles. Kwiat (217-333-9116, kwiat@uiuc.edu) says that his lab detects a record two million entangled photon pairs per second with ample determination of numerous properties, allowing a complete characterization of the entanglement produced.

Barreiro et al., Physical Review Letters, upcoming article