Investigators at the University of Illinois at Urbana-Champaign and at the University of Queensland in Brisbane, Australia, have used paired nonlinear crystals to prepare photon pairs that are entangled in all degrees of freedom: polarization, spatial mode and energy-time. Hyperentanglement may have applications in quantum bits of information to be encoded on a photon pair.

In their experiment, 351-nm radiation from an argon-ion laser pumped contiguous 0.6-mm-thick BBO crystals to produce 702-nm photons by Type I parametric down-conversion. By focusing the pump beam to a waist size of 90 μm at the thin crystals, they maximized polarization and spatial-mode entanglement. Testing against a Bell inequality, they found that the resulting photon pairs exceeded the classical limit by more than 20 standard deviations for each degree of freedom.

A report on the work will appear in an upcoming issue of Physical Review Letters.