Ions and neutral atoms held in electromagnetic traps are two of many candidates that may one day become the qubits in a quantum computer: Their hyperfine states could serve as the computer’s ones and zeroes. Ions interact via long-range Coulomb forces, which can facilitate creation of the entangled states that are the prerequisite for quantum computation. But that same Coulomb interaction gives rise to collective motions that can disrupt a qubit array. Atoms aren’t susceptible to such disruptions. But they’re also more difficult to entangle.
Last year two research groups independently demonstrated a long-range interaction, called Rydberg blockade, between trapped neutral atoms; they published their results in a pair of back-to-back papers. (See Physics Today, February 2009, page 15.)
One group, led by Mark Saffman and Thad Walker at the University of Wisconsin-Madison, showed the blockade in its simplest form: When two atoms were separated by several microns,...
