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.