Scientists demonstrate quantum superposition of spin states
Scientists at the National Institute of Standards and Technology (NIST, Boulder, CO) have
isolated a beryllium ion with superposed electron spins?as predicted by quantum theory?and then induced physical separation of the two states. The NIST team captured a beryllium ion in an electromagnetic tra¥and cooled the ion to its lowest energy state (on the order of 1 mK) using three dye lasers doubled to 313 nm. Beams from one laser drive a two-photon transition the
frequency of which matches the difference between the two spin states and can coherently drive the spin-down to a spin-u¥state. If the beams are applied for half the time required for the full transition, the ion is left with superposed states?a 50:50 chance of being spin-u¥or spin-down.
Acousto-optic modulators wavelength-shifted the beams to create a beat frequency that exerted a periodic dipole force on the ion, forcing it into resonant motion. The beams were first polarized to affect only the spin-u¥state, while leaving the spin-down state at rest. After spin-down and spin-u¥were swapped, the process was repeated, resulting in the spin-u¥and spin-down states being separated in space by more than 80 nm, which is much larger than the initial size of the beryllium ion. Hence, the atom was effectively in two distinct places at once.
