in your element Rubidium round-the-clock Iulia Georgescu explains why rubidium is atomic physicists’ favourite chemical element. ubidium (Rb) was discovered in 1861 by physicist Gustav Kirchhoff — Rperhaps better known for his work on electrical circuits — together with chemist Robert Bunsen, in Heidelberg. Using their newly invented spectroscope to investigate the composition of a variety of samples they came across new elements: first caesium (Cs) in mineral water and then rubidium in the mineral lepidolite. Both were named after the colour of their emission lines — sky blue (caesius) and red (rubidius), respectively. Element 37 firmly belongs to group I of LIBRARY PHOTO SOURCE/SCIENCE SCIENCE the periodic table, with characteristics similar to those of the other alkali metals. A soft energy state. When cooled close to absolute still have uncertainties of just 10–12. To put metal with a low melting point (39.3 °C), it zero, bosonic atoms collectively huddle things into perspective, the current record reacts violently with water — even more so into the lowest energy state, forming a is held by the strontium-87 (87Sr) optical than sodium (typically used in high-school ‘Bose–Einstein condensate’ — a single lattice clock at 10−18, meaning that the clock demonstrations) and potassium (already too quantum mechanical entity observable at should be accurate to within a second in dangerous for this purpose) in agreement with the near-macroscopic scale. Cornell and 15 billion years. the increasing energy of the valence electron Wieman shared the 2001 Nobel Prize in That may be a long time on the human as one goes down the column. The hydrogen Physics with Wolfgang Ketterle, who a timescale, but not for rubidium. It only gas produced during the reaction of rubidium few months later made the first sodium has two naturally occurring isotopes, 85Rb with water ignites in air; in fact, rubidium Bose–Einstein condensate. and 87Rb, the latter is radioactive with a itself can spontaneously ignite in air. In the past twenty years, rubidium half-life of 48.8 billion years. It decays Rubidium proved well suited to low- Bose–Einstein condensates have enabled to stable 87Sr, which turns out to be a temperature experiments. In the 1990s amazing progress in our understanding of very useful process as it gives us another cheap commercial laser diodes — such quantum many-body phenomena and our rubidium clock — a geological one, used in as those used in CD players — operating ability to mimic exotic physics, like radiation rubidium–strontium dating. close to the wavelength used to laser-cool emitted from black holes. Ultracold atomic Because rubidium is reasonably rubidium (780 nm) became readily available. gases are also key to technologies such as abundant in the Earth’s crust, to determine The rubidium vapour required for these atomic clocks or sensors (gravitational, the age of a rock one can compare the experiments is generated at accessible rotational, magnetic) with excellent sensitivity ratios of both 87Rb and its decay product temperatures, just above that of the human that exploit quantum mechanical effects. 87Sr to the naturally occurring 86Sr. First body. These factors, together with rubidium’s Discovered together spectroscopically, used by Otto Hahn in the early 1940s, this laser-cooling-friendly atomic energy level caesium and rubidium currently both set rubidium–strontium dating method is very structure, made it the atom of choice for our standards for time. The International reliable for very long timescales, for instance many atomic physics experiments. System of Units’ definition of the second 4.5 billion years — the age of the Earth. In the early summer of 1995 is 9,192,631,770 periods of the transition I hope you now agree that rubidium Eric Cornell and Carl Wieman first frequency between two energy levels of rocks. ❐ managed to cool 87Rb vapour down to 133Cs. For sixty years, since the first atomic 170 nK (pictured), thus observing the clock, caesium has been the primary time IULIA GEORGESCU is a Senior Editor of strange state of matter predicted to arise and frequency standard. Rubidium clocks Nature Physics. near absolute zero temperatures by also make use of a hyperfine transition in the Satyendra Nath Bose and Albert Einstein microwave region, and serve as secondary Correction seventy years beforehand. Any number of standards. However, their low cost, smaller In the In Your Element article ‘Strontium’s bosons — particles obeying Bose–Einstein size and greater stability make rubidium scarlet sparkles’ (Nature Chem. 7, 940; 2015), statistics — can occupy the same quantum clocks ideal for many commercial applications the years in the first paragraph were incorrect. including the Global Positioning System These were corrected in the online versions (GPS). Nowadays, rubidium clocks have been after print on 29 October 2015. miniaturized to the size of a matchbox and Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe 1034 NATURE CHEMISTRY | VOL 7 | DECEMBER 2015 | www.nature.com/naturechemistry © 2015 Macmillan Publishers Limited. All rights reserved.