Neutral atoms held in optical traps are promising candidates for qubits in a quantum computer, with the atoms’ hyperfine states serving as the computer’s ones and zeros. But creating the necessary entangled states is a challenge, because atoms don't normally interact strongly at long distances. Two research groups, one at the University of Wisconsin and one at the Université Paris-Sud, the Institute d'Optique, and CNRS, recently demonstrated a long-range interaction called Rydberg blockade: When two atoms are separated by several microns, exciting one into a Rydberg state (an energetic state with a large, delocalized wavefunction) prevents the other from being similarly excited. (See Physics Today, February 2009, page 15.) Now, both groups have used Rydberg blockade to entangle the atoms in two hyperfine states. The Paris researchers irradiated both ground-state atoms with a laser pulse to create an entanglement with one atom in a Rydberg state and the other in the ground state. A second pulse coaxed the Rydberg atom back to the ground state, but into a different hyperfine level. The Wisconsin researchers constructed a quantum logic gate called a controlled NOT, or CNOT: a sequence of laser pulses, involving excitations to a Rydberg state, that changes the state of one atom if and only if the other, the control, is in a particular hyperfine state. Applying the CNOT gate when the control atom is in a superposition of states entangles the two atoms. (T. Wilk et al., Phys. Rev. Lett., in press; L. Isenhower et al., Phys. Rev. Lett., in press.) —Johanna Miller
Results matching “rydberg”
The chemists' familiar lineup of bond types has a new member. Joining ionic, covalent, hydrogen, and van der Waals is an unusual ultralong bond between two similar atoms. The new dimers consist of rubidium atoms in their ground state and other rubidium atoms in highly excited Rydberg states. Tilman Pfau of the University of Stuttgart in Germany and his coworkers made the dimers after first trapping Rb atoms under conditions just shy of what’s needed to make a Bose-Einstein condensate. Holding the dimers together is a weak, wavy potential whose ability to bind atoms under or near BEC conditions was predicted in 2000 by Chris Greene, Alan Dickinson, and Hossein Sadeghpour. Enrico Fermi had identified the potential’s underlying interaction back in 1934, well before BECs made their laboratory debut. Because the potential is so wide and weak, it binds the two atoms only if they’re far apart. The dimers are big indeed. At 80 nm, they’re wider than a ribosome, the 5-megadalton macromolecule that translates our DNA into protein, and longer than the transistor gates in the newest, most powerful microprocessors. Pfau’s group made the simplest, most stable of the dimers that Green and company predicted. Other, more exotic dimers from the same family, dubbed trilobites (see image) and butterflies by Greene, are next on Pfau’s to-do list. (V. Bendkowksy et al., Nature , in press; http://arxiv.org/abs/0809.2961.) — Charles Day
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
