CRACKING A COMBINATION lock requires fewer tries with some quantum wizardry. For example, a two-bit classical lock might demand as many as four attempts to open it (top). On average, an n-bit lock requires about n/2 tries. Because a quantum lock can be put into multiple states at once, it takes only about steps to open it if Grover's algorithm is used. The authors' experiment corresponds to opening a two-bit quantum lock, which (after suitable preparation) can be set to the right combination in a single step (bottom). The numbers shown on the dial indicate the relative populations measured for each of the four quantum states.