For two or more qubits to become entangled, they need to interact with one another. Neutral atoms \u201cfeel\u201d one another\u2019s presence via so-called van der Waals forces, which arise from the way one atom responds to fluctuations in the cloud of electrons in another atom nearby. But these feeble forces are felt only when atoms are extremely close together. Manipulating normal atoms to the required precision using light fields just can\u2019t be done.<\/p>\n

As Lukin and his colleagues pointed out in their original proposal back in 2000, the interaction distance can be dramatically increased if we boost the size of the atoms themselves. The more energy an electron has, the further it tends to roam from the atomic nucleus. If a laser is used to pump up an electron into an energy state far greater than those usually found in atoms \u2014 called a Rydberg state after the Swedish physicist Johannes Rydberg, who in the 1880s studied the way atoms emit light at discrete wavelengths \u2014 the electron can roam thousands of times farther out from the nucleus than usual.<\/p>\n

This boost in size enables two atoms held several micrometers apart \u2014 perfectly feasible in optical traps \u2014 to interact.<\/p>\n<\/div>\n

<\/br><\/br><\/br><\/p>\n