Atomic Clocks Compared with Astounding Accuracy

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Tropical ecosystems are large contributors 10. Ahlström, A., Schurgers, G., Arneth, A. & Smith, B. 351–372 (2005). to the global terrestrial carbon sink10, but they Environ. Res. Lett. 7, 044008 (2012). 14. Lawrence, D. M. et al. J. Adv. Model. Earth Syst. 11, 11. Norby, R. J. et al. New Phytol. 209, 17–28 (2016). 4245–4287 (2019). are notoriously under-studied. Field obser- 12. Lugli, L. F. et al. New Phytol. 230, 116–128 (2021). 15. Yu, L., Ahrens, B., Wutzler, T., Schrumpf, M. & Zaehle, S. vations are scarce and few manipulation 13. Ainsworth, E. A. & Long, S. P. New Phytol. 165, Geosci. Model Dev. 13, 783–803 (2020). experiments — such as CO2 enrichment or nutrient additions — have been carried out in Metrology these ecosystems11,12. Below-ground processes are particularly challenging to assess in the tropics, where the effects of multiple nutrient scarcities often come into play12. Terrer Atomic clocks compared and colleagues’ study provides a promising framework that can be elaborated to describe with astounding accuracy diverse plant–soil interactions in various terrestrial ecosystems in the future. Rachel M. Godun CO2-enrichment experiments generally last for just a few years, or just over a decade Three atomic clocks based on different atoms have been 13 at most . Such timescales are unlikely to compared with record accuracy. The findings bring a capture the effects of elevated CO2 levels on redefinition of the second a step closer and aid the search for plant mortality, plant-species composition and soil-carbon turnover time, all of which dark matter — an elusive component of the Universe. See p.564 can affect the sequestration of carbon by ecosystems in different ways in the longer term. Mechanistic understanding gained from The remarkable accuracy of atomic clocks absorbed when an atom changes from one experiments about the coupling between makes them excellent instruments for time- energy state to another. Clocks based on dif- carbon and nutrient cycling can, however, keeping and other precision measurements. ferent atoms run at different rates, and the be integrated into computational models. On page 564, the Boulder Atomic Clock Opti- term ‘optical clock’ refers to one that runs And this will allow us to constrain estimates cal Network (BACON) Collaboration1 reports at an optical frequency. Three of the world’s of the size of the terrestrial carbon sink in the extremely accurate comparisons of three best optical clocks are the aluminium-ion and coming decades. The interactions between world-leading clocks in Boulder, Colorado, ytterbium clocks at NIST and the strontium plants and their associated soil fungi, as well housed at the National Institute of Standards clock at JILA. The measured frequencies of as other crucial below-ground agents and and Technology (NIST) and the JILA research all three clocks are estimated to be correct processes such as microbial communities, institute. The authors show how their clock to within a fractional uncertainty of 2 parts are already stirring up modelling efforts14,15. comparisons provide insights into funda- in 1018 or better2–4. This level of uncertainty Terrer and colleagues’ study now invites mental physics and represent substantial could, in principle, allow the clocks to keep researchers to test hypotheses about the progress towards redefining the second in time so accurately that they would gain or processes that drive coordinated above- and the International System of Units (SI). lose no more than one second over the age below-ground responses to rising CO2 levels. Atomic clocks ‘tick’ at a rate determined of the Universe. Such optical clocks would be Such studies could be a real step forwards in by the frequency of light that is emitted or 100 times more accurate than caesium clocks5. our understanding of the fate of the terrestrial carbon sink.

Ana Bastos is in the Department of Biogeochemical Integration, Max Planck Institute for Biogeochemistry, Jena 07745, Aluminium-ion Ytterbium Strontium clock clock clock Germany. Katrin Fleischer is in the Department of Biogeochemical Signals, Max Planck Institute for Biogeochemistry, NIST Jena 07745, Germany. JILA e-mails: [email protected]; 1.5-km [email protected] free-space link

1. Terrer, C. et al. Nature 591, 599–603 (2021). 3.6-km 2. Ciais, P. et al. in Climate Change 2013: The Physical optical-fibre link Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (eds Stocker, T. F. et al.) Ch.6, 465–570 (Cambridge Univ. Press, 2014). 3. Drake, B. G., Gonzàlez-Meler, M. A. & Long, S. P. Annu. Rev. Plant Physiol. Plant Mol. Biol. 48, 609–639 (1997). Figure 1 | Comparing a network of optical clocks. The BACON Collaboration1 operated a network of three 4. Walker, A. P. et al. New Phytol. 229, 2413–2445 (2021). 5. Terrer, C., Vicca, S., Hungate, B. A., Phillips, R. P. & atomic clocks in Boulder, Colorado. The network consisted of an aluminium-ion clock and an ytterbium clock, Prentice, I. C. Science 353, 72–74 (2016). housed at the National Institute of Standards and Technology (NIST), and a strontium clock, located at the 6. Tian, H. et al. Global Biogeochem. Cycles 29, 775–792 JILA research institute. Data were transmitted between the two facilities through a 3.6-kilometre optical-fibre (2015). link and in the form of laser pulses through the air along a 1.5-km ‘free-space’ link. The authors used this set-up 7. Fleischer, K. et al. Nature Geosci. 12, 736–741 (2019). 8. Bossio, D. A. et al. Nature Sustain. 3, 391–398 (2020). to compare the three atomic clocks with unprecedented accuracy — an achievement that has implications for 9. Jiang, M. et al. Glob. Change Biol. 26, 5856–5873 (2020). fundamental physics and the future of international timekeeping. (Adapted from Fig. 1 of ref. 1.)

There is therefore a desire to redefine the SI causes the frequency of an atomic clock to of the world around us to be obtained at second in terms of an optical-clock frequency depend on its altitude. Consequently, the unprecedented resolution. Examples of such and to move away from the current definition height difference between two remote clocks investigations include testing Einstein’s theory based on caesium. But before such a redefini- can be determined by measuring their differ- of relativity at ever more stringent levels13, and tion is possible, scientists must build confi- ence in frequency. At the level of measurement searching for possible changes in the values of dence in the reproducibility of optical clocks uncertainty achieved in the latest work, clock physical constants8. Regardless of the appli- through a series of clock comparisons. The comparisons could resolve centimetre-sized cation — whether in redefining the SI second, target accuracy for these comparisons is at height differences. Therefore, clocks could surveying or fundamental physics — the better the level of parts in 1018 to clearly demonstrate provide new tools for long-term environmen- the clock comparisons, the greater the impact. the superiority of optical clocks over caesium tal monitoring of, for example, ice sheets or And with the current accuracy limits being set clocks5. ocean levels. by technical issues, rather than fundamental Clock comparisons are carried out by The BACON Collaboration demon- limits, the prospects for even better measure- measuring ratios of the optical-clock fre- strated another intriguing application of ments in the future are extremely promising. quencies using instruments called femto clock comparisons. The authors used the second-frequency combs. Until now, the best clock-frequency ratios to search for signs Rachel M. Godun is in the Department of Time comparisons between optical clocks based of possible interactions between atoms and and Frequency, National Physical Laboratory, on different atoms6–11 have been at the level dark matter — an elusive substance predicted Teddington, Middlesex TW11 0LW, UK. of parts in 1017. The BACON Collaboration to account for most of the matter in the Uni- e-mail: [email protected] presents measurements of optical-frequency verse. According to current understand- ratios reaching uncertainties at the level of ing, atoms do not interact with dark matter 18 parts in 10 , bringing the redefinition of the through electromagnetic forces. However, if 1. Boulder Atomic Clock Optical Network (BACON) SI second a step closer. these interactions were to exist, they would Collaboration. Nature 591, 564–569 (2021). 2. Brewer, S. M. et al. Phys. Rev. Lett. 123, 033201 (2019). Such frequency-ratio measurements are no cause tiny changes in atomic-clock frequen- 3. Bothwell, T. et al. Metrologia 56, 065004 (2019). mean feat, and are equivalent to determining cies. The authors detected no such changes in 4. McGrew, W. F. et al. Nature 564, 87–90 (2018). the distance from Earth to the Moon to within their experiment, but a null result is still use- 5. Riehle, F., Gill, P., Arias, F. & Robertsson, L. Metrologia 55, 188–200 (2018). a few nanometres. Therefore, exceptional ful. Combined with previous data, the finding 6. Rosenband, T. et al. Science 319, 1808–1812 (2008). care is required to control any sources of revealed that the maximum strength of any 7. Nemitz, N. et al. Nature Photon. 10, 258–261 (2016). frequency offset. The authors compared the electromagnetic interactions between atoms 8. Lange, R. et al. Phys. Rev. Lett. 126, 011102 (2021). 9. Origlia, S. et al. Phys. Rev. A 98, 053443 (2018). aluminium-ion and ytterbium clocks at NIST and a particular type of dark matter was nearly 10. Dörscher, S. et al. Metrologia 58, 015005 (2021). and the strontium clock at JILA over several ten times lower than previously determined. 11. Takano, T., Mizushima, R. & Katori, H. Appl. Phys. Express months to check reproducibility and reduce Networks of optical clocks can also be used 10, 072801 (2017). 12. Mehlstäubler, T. E., Grosche, G., Lisdat, C., Schmidt, P. O. statistical uncertainties. They found that the to explore many other aspects of physics, & Denker, H. Rep. Prog. Phys. 81, 064401 (2018). day-to-day variation in the ratios was slightly because their precision allows measurements 13. Delva, P. et al. Phys. Rev. Lett. 118, 221102 (2017). larger than expected after accounting for all known effects. This observation suggests Cell biology the existence of unknown effects, which are intrinsically hard to quantify. Nevertheless, the authors developed a statistical model to account for these uncertainties and to enable a Mitochondria are mixed rigorous assessment of the total measurement uncertainty. during cell division The BACON Collaboration used redundant elements throughout the network of optical Till Klecker & Benedikt Westermann clocks to check for any sources of bias. In particular, the authors linked the clocks at Organelles called mitochondria have essential roles in the cell NIST and JILA in two ways (Fig. 1). They used and must be inherited successfully as it divides. It turns out a 3.6-kilometre optical-fibre link — a tried- that three types of interaction with filaments of actin protein and-tested method for connecting signals between optical clocks at the required level mix and partition mitochondria during cell division. See p.659 of uncertainty. And, in what they believe to be a world first, they compared the clocks using a 1.5-km ‘free-space’ link by sending laser In 1855, the German physician Rudolf Virchow be inherited, too. On page 659, Moore et al.1 pulses through the air between NIST and JILA coined the phrase Omnis cellula e cellula — all describe this process in an unprecedented along a straight line joining the two institutes. cells come from cells. In other words, cells arise level of detail. They found that the two types of connection from the growth and division of existing cells. Two major constituents of the cytoskeleton provided a similar level of uncertainty, apart The genetic information stored in chromo (a network of proteins that determines cellu- from when data could not be obtained from somes is passed on to the next generation dur- lar architecture) are responsible for cellular the free-space link during a snowstorm. ing cell division in a highly ordered process dynamics. These are microtubules, structures The authors’ demonstration that high- called mitosis. Biologists have spent many that serve as tracks for long-distance transport accuracy clocks can be connected by free- decades deciphering the molecular choreo of organelles; and actin filaments, which medi- space links, without needing an optical-fibre graphy of this fascinating process, but much ate transport over short distances and enable infrastructure, is exciting because it opens up less attention has been given to the inheritance shape changes at the cell’s outer boundary in possibilities for applications outside the labo- of organelles called mitochondria. These are a region called the cortex. The cytoskeleton ratory, such as land surveying12. According to essential for energy metabolism and, because is drastically remodelled during cell division. Einstein’s theory of relativity, Earth’s gravity they cannot be generated de novo, they must Microtubules build a structure called the