Hyper-entangled Photon Pairs

Physicists at the University of
Illinois 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% 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 http://www.aip.org/pnu/2005/split/721-3.html).
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)