Light as a timekeeper: Accuracy of a single-ion clock pushed to new limits

The measurement of time has been essential in the development of science to understand the laws of nature and make sense of the world around us. The more accurate the second can be measured, the more phenomena and subtle effects can be studied and discovered.

Ion trap

Ion trap used in NRC’s atomic optical clock with 17 digits of accuracy. Credit: Frequency and Time group at the National Research Council of Canada

Time and Frequency experts at NRC’s Measurement Science and Standards (MSS) reported a substantial improvement toward the current definition of the second by exploiting a unique property of their strontium ion optical frequency standard.

The idea was to hold a single ion, isolate it from external perturbations such as movement, collisions, stray fields, etc., and to probe an extremely narrow optical transition. This way, the atom becomes a nearly perfect, isolated, and precise reference of frequency.

However, no matter how perfectly a clock is made, it is impossible to isolate an atom completely from its environment and describe exactly how the ion will respond. But with this experiment, the scientists at MSS were able to gain a better knowledge of the effect of heat on the ion frequency, reducing its uncertainty by a factor of 24. In addition, other perturbations related to ion movement were reduced by a factor in excess of 200. These results pave the way for a clock accuracy 100 times higher than that of the best cesium clocks that currently define the second.

The single-ion standard is the most precise timepiece in Canada: it is currently 10 times more reproducible than the best cesium fountain clocks and is one of the leading optical frequency standards worldwide.

Present-day performances of optical clocks and continual, fast-paced improvements make a strong case in favor of an eventual redefinition of the second with an optical standard. In fact, this work contributes to realizing the decades-old dream of measuring the physical universe at a much higher level of accuracy than ever before, and to opening a new window on such applications as time-keeping, relativity, astronomy, navigation, geodesy, space exploration, and tests of fundamental physics postulates.

At MSS, we look forward to pursuing this fascinating research that impacts the work of many researchers across the world.


The Frequency and Time group at the National Research Council of Canada (NRC) is part of a small group of scientists worldwide who are developing super-accurate clocks based on optical transitions in atoms.  The optical clock developed at NRC is based on a trapped and laser-cooled single-ion of strontium.  This work pushes back the limits of accuracy in time keeping and contributes to the international efforts aimed at redefining the SI second.

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