Frequency and its inverse, time, are the most accurately measured quantities. Historically, improvements in the accuracy of clocks have enabled advances in navigation, communication, and science. Since 1967, the definition of the International System (SI) second has been based on the frequency of a microwave transition in cesium, and present day cesium atomic clocks have a fractional uncertainty near 10-16. Recently, a new type of atomic clock based on optical transitions has been developed, with a current fractional uncertainty near 10-18 (approximately one second divided by the age of the universe), and they are rapidly improving. This talk presents a brief summary of the development of optical atomic clocks, with a focus on the Al~+ quantum-logic clock developed at NIST. We discuss the current state-of-the-art in optical clock performance, and describe new applications in sensing and fundamental physics. Future directions in optical atomic clock research are also considered.
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