To place the recent work by Ye and co-workers in context, it is useful to recall the basic ingredients for a fundamental frequency standard. Improvements in optical clocks can also lead to new technologies, including GPS systems with subcentimeter accuracy, deep space navigation, more secure telecommunications, new methods of exploring oil and gas reserves, and more accurate means of detecting tectonic motion. Already, optical clocks are being proposed as new tools to detect gravitational waves, to place bounds on the variation of fundamental constants, and to understand the very nature of time itself. Phenomena that were once impossible to study or questions about nature that seemed rhetorical suddenly morph into lines of experimental research. Given that clocks already have such astonishingly high levels of precision, why strive for more? To a scientist, the answer is simple: With each improvement come new vistas for scientific exploration and discovery. Their device, which is based on thousands of ultracold Sr atoms confined in a 3D optical lattice, surpasses the previous “best” optical clock by a factor of 1.4 in precision. 5 × 1 0 − 1 9 -equivalent to an imprecision of 100 ms over the estimated lifetime of the Universe. A team led by Jun Ye at JILA in Colorado has now set a new record among optical atomic clocks by demonstrating a strontium (Sr) clock with a relative precision of 2. Alkaline-earth frequency standards are almost a thousandfold more precise than the cesium atomic clocks that are currently used to define the second. These elements have ultranarrow transitions at optical frequencies, providing stable and precise frequency standards for measuring time, as does the quartz oscillator in a wristwatch. Today’s most precise atomic clocks are based on strontium, ytterbium, and other alkaline-earth (or alkaline-earth-like) atoms. They also removed the deleterious effects of interactions between the Sr atoms by cooling the atoms into a quantum degenerate state, in which each optical lattice site was occupied by only one atom. The Ye team was able to correct for these variations by mapping them spatially. The Sr transition frequency, here represented by a clock face, varies across the lattice, which reduces the precision of an ensemble measurement of the frequency. APS/ Alan Stonebraker Figure 1: The optical atomic clock developed by Ye and colleagues consists of around 10,000 Sr atoms in a 3D optical lattice (green).
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