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 superior ity 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 Nature | Vol 591 | 25 March 2021 | 535 ©2021 Spri nger Nature Li mited. All rights reserved. ©2021 Spri nger Nature Li mited. All rights reserved. News & views Cloud Cable Dividing Comet tail human cell Mitochondrion Actin Actin wave Experimentally damaged mitochondrion Figure 1 | How mitochondrial organelles are distributed during cell division. were frequently encased in what looked like a cloud of actin filaments, Moore et al.1 report three types of interaction with filaments of the protein restricting the organelles’ mobility. Some mitochondria in the waves instead actin that aid the distribution of mitochondria when human cells divide. Using had comet-tail-like structures made of actin filaments, and these organelles state-of-the-art microscopy, the authors observed that mitochondria can be moved rapidly (red arrow) in random directions. The authors tracked the fate of tethered to cable structures. It has been reported4 previously that some actin labelled mitochondria that were damaged using an experimental technique. The filaments in dividing cells form waves that revolve around the cell (black arrow) actin-mediated activities helped to mix and divide the mitochondria between in a manner reminiscent of a radar display. In these waves, the authors observed the two daughter cells during cell division, resulting in an even distribution of two types of interaction between actin and mitochondria. Mitochondria the damaged organelles. mitotic spindle that is needed to partition the detail than had been reported previously, and Mitochondria contain their own genome, chromosomes as the cell divides, and, later, found that it is independent of the presence which encodes essential proteins required actin filaments assemble a contractile ring that of actin waves. Second, within a wave, mito- for energy generation through a pathway promotes cell separation. chondria are encased by what looks like clouds called the respiratory chain (also known as Organelles are also extensively remodelled of actin filaments that seem to immobilize the electron-transport chain). If there is an during mitosis. Mitochondria often form