Rydberg blockade

Rydberg blockade between neutral atoms held in traps several microns apart has now been demonstrated and exploited to create a quantum-entangled state. Both feats are considered significant steps in the quest for quantum computing with neutral atoms. Blockade refers to the inhibition of excitation in one part of a system by the prior excitation of another part. And the excitation in question is the raising of alkali atoms to high Rydberg states-—that is, states in which the valence electron is excited to a high principal quantum number. The atoms interact strongly enough at micron separations for Rydberg excitation of one to prevent the excitation of the other. A group at the University of Wisconsin–Madison has demonstrated Rydberg blockade between two rubidium atoms held in optical traps 10 μm apart. And a group at the Université Paris–Sud and the Institut d'Optique in France used Rydberg blockade between Rb atoms held 4 μm apart to create an entangled state of the kind one would need for a quantum logic gate. The figure shows that under laser excitation in the Paris experiment, the entangled two-atom state (blue curve) oscillated more rapidly than a lone atom (red curve) between ground and Rydberg states. (E. Urban et al., Nat. Phys., in press, doi:10.1038/nphys1178; A. Gaëtan et al., Nat. Phys., in press, doi:10.1038/nphys1183 . — Bertram Schwarzschild