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
