An optical clock based on a pair of calcium ions achieves a given precision more quickly when the ions are entangled.

What time is it? How precisely you can answer this question might depend on how long you are able to measure. Glance at a clock and you’ll first register the positions of the hour and minute hands. Look for longer and you’ll make out the movement of the second hand, improving your precision 60-fold. The most precise timepieces currently available are state-of-the-art optical clocks, and these also return a more precise result the longer that they are interrogated. But for many applications—in satellite navigation systems, for example, where the position of a fast-moving vehicle needs to be determined quickly—the answer must be prompt as well as precise. Now Kai Dietze at the German National Metrology Institute and colleagues have demonstrated a way to use quantum entanglement to halve the measurement time of an ion-based optical clock without compromising its precision [1].

Optical clocks are the technological successors to microwave atomic clocks, which, for nearly 60 years, have defined the International System of Units (SI) unit of time: the second. Microwave atomic clocks have been refined since they were first invented in the 1950s, but now optical clocks are reaching maturity in the sense that several systems reach or exceed the criteria required by the International Bureau of Weights and Measures for redefining the second. Optical clocks could potentially outperform microwave clocks by 4 orders of magnitude, with implications for fundamental physics and geodesy.