New Super-Accurate Optical Atomic Clock

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. (Source: N. Phillips, NIST)

Researchers have measured an optical clock’s ticking with record-breaking accuracy while also showing the clock can be operated with unpre­cedented consistency. These achieve­ments represent 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, which is currently based on microwave atomic clocks.

“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,” said Andrew Ludlow, one of the research team leaders from the National Institute of Standards and Tech­nology (NIST), USA. “It would also provide more precise measure­ments for exploring physical phenomena that aren’t yet fully understood.”

“Optical clocks are likely capable of much higher accuracy, probably 10 to 100 times better than what we measured in this work,” said Ludlow. “To prove 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.” 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 times 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 Tara Fortier, a member of the research team. “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 an 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 x 10-16. This corresponds to losing only about 100 seconds over the age of the universe 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 time scales in the world – is one reason why we were able to perform such an accurate comparison to cesium,” said Tom Parker, a member of the research team. They further reduced the uncertainty by making 79 measure­ments over 8 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. (Source: NIST)

Reference: W. F. McGrew et al.: Towards the optical second: verifying optical clocks at the SI limit, Optica 6, 448 (2019); DOI: 10.1364/OPTICA.6.000448

Link: Optical Frequency Measurements Group, National Institute of Standards and Technology, Boulder, USA

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