Researchers have measured an optical clock’s ticking with record-breaking accuracy while also showing that the clock can be operated with unprecedented consistency — achievements representing a significant step toward demonstrating that the new generation of optical atomic clocks are accurate and robust enough to be used to redefine the official length of a second. The official length of a second is currently based on microwave atomic clocks. Andrew Ludlow, one of the research team leaders from the National Institute of Standards and Technology (NIST), said proving the true accuracy of these clocks without being limited by today’s definition of a second will require high-quality comparisons directly between various types of optical clocks. Researchers have measured this ytterbium optical clock's ticking with record-breaking accuracy. The new work is a step toward redefining the length of a second based on time kept by an optical clock. Courtesy of Nate Phillips, NIST. “A more accurate definition of a second and a better time-keeping infrastructure would support continuing advances in the timing systems used in a wide range of applications, including communication and navigation systems,” Ludlow said. “It would also provide more precise measurements for exploring physical phenomena that aren’t yet fully understood.” Ludlow noted that optical clocks are likely capable of much higher accuracy, probably 10 to 100× better than what was measured in the study. Clocks work by counting a reoccurring event with a known frequency, such as the swinging of a pendulum. For traditional atomic clocks, the recurrent event is the natural oscillation of the cesium atom, which has a frequency in the microwave region of the electromagnetic spectrum. Since 1967, the International System of Units (SI) has defined a second as the time that elapses during 9,192,631,770 cycles of the microwave signal produced by these oscillations. Optical atomic clocks use atoms such as ytterbium and strontium that oscillate about 100,000× higher than microwave frequencies, in the optical, or visible, part of the electromagnetic spectrum. These higher frequencies allow optical clocks to tick faster than microwave atomic clocks, making them more accurate and stable over time. “The higher frequencies measured by optical clocks generally make it easier to control environmental influences on the atoms,” said researcher Tara Fortier. “This advantage could eventually enable the development of compact optical clock systems that maintain relatively high performance in a wide range of application environments.” To show that time kept with an optical clock is compatible with today’s standard cesium atomic clocks, the researchers converted the frequency of a ytterbium optical atomic clock at NIST to the microwave region and compared it with a collection of measurements from cesium atomic clocks located across the globe. They achieved frequency measurements of the ytterbium optical clock with an uncertainty of 2.1 × 10−16. This corresponds to losing only about 100 seconds over the age of the universe (14 billion years) and sets a new accuracy record for cesium-referenced measurements of an optical clock. Although optical clocks are very accurate, they do tend to experience significant downtimes because of their technical complexity and prototype design. The researchers at NIST used a group of eight hydrogen masers to keep the time when the optical clock wasn’t operational. Masers, which are like lasers that operate in the microwave spectral range, can reliably keep time but have limited accuracy. “The stability of the masers — one of the best local timescales in the world — is one reason why we were able to perform such an accurate comparison to cesium,” said researcher Tom Parker. They further reduced the uncertainty by making 79 measurements over eight months. This is the first time that optical clock measurements have been reported over such a long time period. To better understand the limits of optical clocks, the researchers plan to compare the ytterbium optical clock used in this study with other types of optical clocks under development at NIST. Eventually, the NIST clocks could be compared with optical clocks in other countries to determine which types of clocks would be best for redefining the SI second. The researchers point out that redefining the length of a second is still some years away. Even if it does change, applying the new standard would require technology that better connects and transmits signals from optical clocks around the world in a way that maintains stability and the accuracy of the time. The research was published in Optica, a publication of OSA, The Optical Society. (https://doi.org/10.1364/OPTICA.6.000448).