Fundamental Symmetry Tested Using Antihydrogen

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that fluctuations in the levels of wild-type p53 Tampa, Florida 33612, USA. measured with great accuracy in the 1880s, are observed in nerve regeneration12. Thus, the e-mails: [email protected]; before being quantitatively explained in the authors’ findings might have repercussions [email protected] 1910s. The structure of the atom was then that reach beyond the field of cancer research at the heart of the formulation of quantum

to regenerative medicine. Perhaps therapies 1. Amit, M. et al. Nature 578, 449–454 (2020). mechanics and in the generalization of this that modulate the activity of p53 will have a 2. Magnon, C. et al. Science 341, 1236361 (2013). theory to relativistic (fast-moving) particles future role in aiding the repair or regenera- 3. Venkataramani, V. et al. Nature 573, 532–538 (2019). in the 1920s. And it was the unexpected dis- 4. Venkatesh, H. S. et al. Nature 573, 539–545 (2019). 9 tion of neurons, an outcome that would make 5. Zeng, Q. et al. Nature 573, 526–531 (2019). covery of an energy gap between the 2S and a profound difference to the lives of people 6. Yu, K. et al. Nature 578, 166–171 (2020). 2P1/2 excited states of hydrogen by the physicist who have neurodegenerative diseases or other 7. Bieging, K. T. et al. Nature Rev. Cancer 14, 359–370 (2014). Willis Lamb in 1947 that motivated the devel- 8. Zhang, L. et al. J. Exp. Clin. Cancer Res. 38, 53 (2019). types of nerve injury. 9. Lin, C. S. et al. Int. J. Cardiol. 184, 9–13 (2015). opment of quantum electrodynamics — the 10. Kandoth, C. et al. Nature 502, 333–339 (2013). theory that describes the interactions between Marco Napoli and Elsa R. Flores are in the 11. Bykov, V. J. N. et al. Nature Rev. Cancer 18, 89–102 (2018). particles and light. 12. Krishnan, A. et al. Eur. J. Neurosci. 43, 297–308 (2016). Department of Molecular Oncology, H. Lee This energy gap, known as the Lamb shift, Moffitt Cancer Center and Research Institute, This article was published online on 12 February 2020. exists in both hydrogen and antihydrogen. It originates mostly from quantum fluctua- Atomic physics tions, whereby particle–antiparticle pairs spontaneously emerge in empty space and then instantly annihilate each other. How- ever, its magnitude is subtly affected by, for Fundamental symmetry example, the charge radius (the spatial extent of the charge distribution) of the proton or tested using antihydrogen antiproton, the weak nuclear force and, poten- tially, currently unknown phenomena that Randolf Pohl could be the source of the matter–antimatter asymmetry in the Universe. The breaking of a property of nature called charge–parity–time The current work was carried out using symmetry might explain the observed lack of antimatter in the ALPHA experiment at CERN, Europe’s the Universe. Scientists have now hunted for such symmetry particle-physics laboratory near Geneva, Switzerland. A facility called the Antiproton breaking using the antimatter atom antihydrogen. See p.375 Decelerator delivers antiprotons to this experiment, with a source of radioactive sodium providing positrons. Every few min- One of the greatest mysteries in modern utes, 90,000 cold trapped antiprotons and

physics is why the Universe seems to contain 2P3/2 3 million positrons are mixed in a sophisti- mostly matter and almost no antimatter. This Lamb cated charged-particle trap. This process observation could be explained if a property shift Fine-structure yields about 20 cold antihydrogen atoms 2S splitting of nature called charge–parity–time (CPT) that are then confined to a neutral-atom trap symmetry is violated. Under CPT symmetry, made from superconducting magnets. These 2P the physics of particles and their antiparticles 1/2 antihydrogen atoms can be stored10 for at is identical. A tiny violation of CPT symmetry Previous least 60 hours, and production cycles can be work during the Big Bang could, in principle, be repeated to obtain hundreds of such atoms.

responsible for the lack of antiparticles in Energy The aim of the present study was to the Universe. On page 375, the ALPHA Collab- measure the energy differences between the 1 This oration reports high-precision spectroscopic work 1S ground state and the 2P1/2 and 2P3/2 excited measurements of antihydrogen — an atom states of antihydrogen (Fig. 1). The ALPHA comprising an antiproton and a positron (the Collaboration used an approach called laser antiparticle of an electron). The authors find spectroscopy, which involved injecting pulses that the gaps between energy levels in anti 1S of laser light into the antihydrogen trap. This hydrogen are in excellent agreement with injection caused atoms to transition from the

those measured previously in ordinary hydro- Figure 1 | Lowest-energy states of antihydrogen. 1S state to the 2P1/2 or 2P3/2 state and to subse- gen2–4, placing strong constraints on potential The ALPHA Collaboration1 carried out high- quently decay back to the 1S state. Atoms that CPT violation. precision spectroscopic measurements of ended up in a different magnetic substate of Tests of CPT symmetry using individual antihydrogen — the antimatter counterpart of the 1S state from the one in which they started particles — such as neutral kaons5, positrons6 hydrogen. Specifically, the team determined the were expelled from the magnetic neutral-atom and antiprotons7,8 — have shown no sign of CPT energy differences between the 1S ground state and trap. These antihydrogen atoms then annihi- violation. However, studies of antihydrogen the 2P1/2 and 2P3/2 excited states of antihydrogen. lated on contact with regular atoms in the walls They used these results to estimate the fine- might probe the influence of factors that were of the ALPHA apparatus to produce particles structure splitting (the 2P –2P energy gap). They not explored in previous tests. 1/2 3/2 called charged pions. also combined their previous determination11 of Hydrogen is the simplest atom, and its the energy gap between the 1S and 2S states with The ALPHA Collaboration plotted the properties can be calculated with impressive number of observed charged pions as a their current measurement of the 1S–2P1/2 energy precision. For more than a century, the study function of the frequency of the laser light. difference to infer the Lamb shift (the 2S–2P1/2 of this atom has been the driving force behind energy gap). The authors found that all of these They then used the positions of the two

groundbreaking ideas about the structure of results are in agreement with the corresponding peaks in these plots to infer the 1S–2P1/2 and

matter. The optical spectrum of hydrogen was ones for ordinary hydrogen. (Drawing not to scale.) 1S–2P3/2 energy differences in antihydrogen.

News & views These differences agree with the ones In retrospect measured in ordinary hydrogen at the level of 16 parts per billion. The authors used their results to estimate the fine-structure splitting

(the 2P1/2–2P3/2 energy difference) in anti hydrogen, with an uncertainty of 0.5%. This 30 years of the iron value is again in good agreement with the one for ordinary hydrogen. hypothesis of ice ages In 2018, the ALPHA Collaboration measured the energy gap between the 1S and 2S states in Heather Stoll antihydrogen11 to one part in 1012. In the current work, the authors combined this result In 1990, an oceanographer who had never worked on climate with their measurement of the 1S–2P1/2 energy science proposed that ice-age cooling has been amplified by difference to provide an estimate of the Lamb shift in antihydrogen. This value has an uncer- increased concentrations of iron in the sea — and instigated an tainty of 11% (or 3.3%, when the fine-structure explosion of research. splitting in ordinary hydrogen is used in the analysis). Over the past few years, high-precision laser Thirty years ago this month, John Martin of the Southern Ocean around Antarctica is spectroscopy of antihydrogen has become proposed a solution to one of the biggest starved of iron, and therefore relatively low in possible, and the ALPHA Collaboration has mysteries of Earth’s climate system: how was biomass, despite having abundant nitrates and achieved spectacular progress. An examina- nearly one-third of the carbon dioxide in the phosphates. But during glacial times, strong tion of several transitions in antihydrogen atmosphere (about 200 gigatonnes of carbon) winds over cold, sparsely vegetated conti- would enable targeted tests of CPT symme- drawn into the ocean as the planet entered the nents could have transported large amounts try, quantum electrodynamics and the stand- most recent ice age, then stored for tens of of iron-bearing dust into this ocean (Fig. 1b). ard model of particle physics. For example, thousands of years, and released again as the Martin reasoned that this dust could have fer- a measurement of the Lamb shift with an ice sheets melted? These large natural cycles in tilized marine ecosystems and strengthened 4 uncertainty of less than one part in 10 would atmospheric CO2 levels (Fig. 1a) were revealed the biological pump, so that more carbon was allow the antiproton charge radius to be deter- in 1987 by an analysis of ancient air bubbles transferred into the deep ocean, lowering 12 mined . Moreover, improved measurements trapped in the first long ice cores taken from atmospheric CO2 levels. of the energy gap between magnetic substates the Antarctic ice sheet1. Martin recognized Around the time of publication, evidence for in antihydrogen would provide detailed infor- that iron was a key ingredient that could high dust delivery during glacial periods had mation about the magnetic structure of the have transformed the surface ocean during just emerged from studies of deep Antarctic antiproton13. glacial times. His landmark iron hypothesis2, ice cores4. But there were no reliable measure- The laser used for spectroscopy in the published in Paleoceanography, described a ments of dissolved iron in the Southern Ocean current work will, in the future, be used for feedback mechanism linking climatic changes that could confirm that its surface waters are cooling of antihydrogen by inducing 1S–2P1/2 to iron supply, ocean fertility and carbon iron-starved in modern times, or data support- and 1S–2P3/2 transitions. Such cooling would storage in the deep ocean. ing the proposal that delivery of iron-rich dust greatly improve the achievable precision in all Two hundred gigatonnes is a lot of carbon would make a difference to ocean productiv- spectroscopy experiments on antihydrogen. to periodically withdraw from and release to ity. It was clear, however, that large patches of In addition, ultracold antihydrogen can be the atmosphere. In the 1980s, a handful of the world’s ocean had much lower quantities used to study the effect of gravity on these models (see ref. 3, for example) had shown of biomass than would be expected on the atoms14. Cold antihydrogen thus promises that an increase in biomass production in polar basis of the concentrations of key nutrients many cool results. ocean regions was the most effective process such as nitrates and phosphates. But many for removing so much atmospheric carbon. researchers argued that this was due to natural Randolf Pohl is at the Institute of Physics, Photosynthetic organisms in the surface ocean overgrazing of algae by herbivores5.

University of Mainz, 55128 Mainz, Germany. convert CO2 from the atmosphere into bio- The idea that modern algal growth e-mail: [email protected] mass, much of which is subsequently broken is limited by iron availability had, in fact, 6 down into CO2 again by other organisms and been proposed in the 1930s, but had been 1. The ALPHA Collaboration. Nature 578, 375–380 (2020). returned to the atmosphere. But part of the incorrectly discounted by oceanographers — 2. Hagley, E. W. & Pipkin, F. M. Phys. Rev. Lett. 72, 1172–1175 biomass sinks into the deep ocean, which who had measured plenty of iron in seawater (1994). therefore effectively serves as a large storage samples collected from the waters around 3. Parthey, C. G. et al. Phys. Rev. Lett. 107, 203001 (2011). 7 4. Bezginov, N. et al. Science 365, 1007–1012 (2019). reservoir of dissolved CO2. This mechanism their iron ships . Martin was one of the first 5. Schwingenheuer, B. et al. Phys. Rev. Lett. 74, 4376–4379 of CO2 removal is called the biological pump. oceanographers to implement painstaking (1995). 6. Dehmelt, H., Mittleman, R., Van Dyck, R. S. Jr & However, biomass production requires not procedures to avoid the contamination of Schwinberg, P. Phys. Rev. Lett. 83, 4694–4696 (1999). only CO2, but also other nutrients to build samples and to determine that iron con- 7. Hori, M. et al. Nature 475, 484–488 (2011). lipids, proteins and enzymes. Researchers centrations in the north Pacific Ocean were 8. Smorra, C. et al. Nature 550, 371–374 (2017). 7 9. Lamb, W. E. Jr & Retherford, R. C. Phys. Rev. 72, 241–243 were struggling to ascertain how the ocean’s extremely low , certainly low enough to curtail (1947). abundance of key nutrients, such as nitrates biomass production. 10. Capra, A. & ALPHA Collaboration. Hyperfine Interact. 240, or phosphates, might have increased dur- Despite the initial scepticism that greeted 9 (2019). 8 11. Ahmadi, M. et al. Nature 557, 71–75 (2018). ing glacial times to fuel a stronger biological the iron hypothesis, 12 separate experiments 12. Crivelli, P., Cooke, D. & Heiss, M. W. Phys. Rev. D 94, pump. were carried out between 1993 and 2005 in 052008 (2016). Martin argued that iron is another nutri- which around 300–3,000 kilograms of dis- 13. Zemach, A. C. Phys. Rev. 104, 1771–1781 (1956). 14. The ALPHA Collaboration & Charman, A. E. ent that limits the biological pump. He sug- solved iron were injected into small patches Nature Commun. 4, 1785 (2013). gested that the modern marine ecosystem of the Southern Ocean, the equatorial Pacific