RESEARCH NEWS & VIEWS the Sun’s surface, could have diluted the star’s outer convective zone. The demonstration of a connection between a host star’s metallicity and the presence of its planets would have significant implications for exoplanet searches.■ Wick Haxton is in the Physics Department, University of California, Berkeley, and the

Nuclear Science Division, Lawrence Berkeley National Laboratory, California 94720, USA. e-mail: [email protected] Borexino Collaboration. Nature 512, 383–386 (2014). Bahcall, J. N. J. R. Astron. Soc. Can. 94, 219–227 (2000). Bethe, H. A. Phys. Rev. 55, 436–456 (1939). Davis, R. Jr, Harmer, D. S. & Hoffman, K. C. Phys. Rev. Lett. 20, 1205–1209 (1968). 5. Fukuda, Y. et al. Phys. Rev. Lett. 77, 1683–1686 (1996). 1. 2. 3. 4.

AP P LIED PHYSICS

Hybrid sensors ring the changes An improved design for a class of magnetometer greatly increases the sensitivity of these devices — and might be the vanguard of a new generation of hybrid sensors that combine different types of signal to increase sensitivity. JÖRG WRACHTRUP & AMIT FINKLER

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hen seventeenth-century sailors undertook their dangerous journeys around the Cape of Good Hope or to the Spice Islands, they had an invaluable navigational tool on board: the compass. Only with the aid of this, the most precise measurement device of those times, were they able to accomplish their daring feats. Since then, precision measurements of magnetic fields have been key drivers of basic science and of a whole wealth of technologies with applications ranging from navigation to the medical sciences. Writing in Advanced Mater­ials, Forstner et al.1 report a vastly improved design for a certain class of magnetometer — a hybrid sensor that measures magnetic fields using an optical signal. Various technologies have been developed to measure magnetic fields. The record holders for sensitivity are devices known as atomvapour magnetometers2. Others can measure the dipolar magnetic interaction between two electrons3, and still others, most notably diamond spin sensors, have a compact design that might allow magnetometers to be made as small as one nanometre across. The central element of Forstner and colleagues’ sensor is a ‘whispering gallery mode’ micro-resonator: a doughnut-shaped device that traps light waves and is known for its strong optical confinement and ultra-high quality factor (a measure of the energy lost from the device; high quality factors correspond to low rates of energy loss). The frequency at which light resonates in the doughnut depends on the doughnut’s geometry. To sense magnetic fields, Forstner et al. combined the resonator with a magnetostrictive material. This material expands in a magnetic field and thereby exerts a force on the cavity system, changing the micro-resonator’s

shape and thus its resonance frequencies. So, by measuring the change in resonance frequencies, the strength of the magnetic field can be determined. The authors placed the magnetostrictive material in the middle of the doughnut, where it does not interfere with optical measurements and is most effective in changing the shape of the resonator (Fig. 1). Because of the high quality factor of the device, tiny distortions of its shape — and therefore tiny changes of magnetic field — can be detected. In a proof-ofprinciple experiment, Forstner and co-workers demonstrated that their device has a magneticfield sensitivity of 100 picotesla (1 picotesla is 10–12 tesla) for measurements of 1 second duration. Other kinds of magnetometer, such as atom-vapour types, are sensitive to much lower fields4,5 (less than 1 femtotesla; 1 femtotesla is 10–15 tesla), but Forstner and co-workers’ approach is three orders of magnitude more

6. GALLEX Collaboration et al. Phys. Lett. B 447, 127–133 (1999). 7. Abdurashitov, J. N. et al. Phys. Rev. C 80, 015807 (2009). 8. Ahmad, Q. R. et al. Phys. Rev. Lett. 89, 011302 (2002). 9. Abe, K. et al. Phys. Rev. D 83, 052010 (2011). 10. Bellini, G. et al. Phys. Rev. Lett. 107, 141302 (2011). 11. Bellini, G. et al. Phys. Rev. Lett. 108, 051302 (2012). 12. Haxton, W. C., Robertson, R. G. H. & Serenelli, A. M. Annu. Rev. Astron. Astrophys. 51, 21–61 (2013).

sensitive than previous hybrid optomechanical sensors6, mostly because of the integration of the magnetostrictive material into the microresonator. Moreover, the researchers extended the frequency range of magnetic fields that can be sensitively detected by the sensor by using the nonlinear properties of the magnetostrictive material. Compared with other, more-sensitive sensor types, the main advantage of Forstner and colleagues’ device is its small size, which is limited to the size of the doughnut (about a few tens of micrometres). Furthermore, the device works at room temperature and tolerates background magnetic fields. This opens up various potential applications, such as navigation without the need for a global positioning system, and the detection of weak biomagnetic fields from neurons. Forstner and co-workers’ achievements follow a recent trend in sensor design: the use of optomechanical sensors, in which optical and mechanical signals are combined to enhance sensitivity. By incorporating advances in nano­photonics and the fabrication of nanoscale structures, optomechanical sensors have been made that perform better than their electromechanical counterparts, mostly because the noise in optical readouts is much lower than in electronic ones. Excellent examples include state-of-the-art force sensors7 and accelerometers8, and a detector of microwave photons9. Hybrid designs based on principles other than optomechanics might also be adopted to

Spectrum analyser

Magnetostrictive material

Optical fibre

Resonator

Light

Figure 1 | An optomechanical magnetometer.  Forstner et al.1 have constructed a magnetic-field sensor in which a doughnut-shaped ‘whispering gallery mode’ resonator device surrounds a piece of magnetostrictive material. Light is guided through a tapered optical fibre into the sensor, where it is trapped by the resonator. In the presence of a magnetic field, the magnetostrictive material expands, distorting the resonator and changing the frequency at which light resonates in the device. The change of resonance frequency is measured by a spectrum analyser and used to determine the strength of the magnetic field.

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NEWS & VIEWS RESEARCH improve the sensitivity of sensors. Consider diamond spin sensors. Rather than using optical resonances, diamond spin sensing relies on the precision measurement of transitions between resonance states that are associated with a quantum property of electrons: spin. Such sensors can be responsive to various quantities, including magnetic10 and electric fields11, temperature12, pressure13 and strain14. But the sensitivity of these devices — particularly for temperature and strain — is limited by the nature of the ground-state spin wave function of the electrons used in the sensing mechanism13. It has been proposed15 that this problem could be overcome by constructing a hybrid sensor using the same magnetostrictive mater­ ial as that used by Forstner and colleagues. However, the material would function in exactly the opposite way to that reported by these authors: a strain (or an electric field) would generate a magnetic field, which is sensitively detected by the electron spins, which in turn are read out optically. The resulting diamond hybrid sensor is predicted to be about 1,000 times more sensitive for pressure, force or electric field than diamond spins alone, and would retain excellent spatial resolution. Hybrid sensors thus seem to be an upcoming theme in sensor technology. An important further step will be to use advances in quantum technology to achieve the limits of accuracy.

The resulting quantum hybrid sensors could potentially revolutionize sensor technology in various disciplines, enabling unprecedented opportunities in technology and basic science. ■ Jörg Wrachtrup and Amit Finkler are at the Third Institute of Physics, University of Stuttgart, Stuttgart 70569, Germany. e-mail: [email protected]

50 Years Ago

1. Forstner, S. et al. Adv. Mater. http://dx.doi. org/10.1002/adma.201401144 (2014). 2. Budker, D. & Romalis, M. Nature Phys. 3, 227–234 (2007). 3. Kotler, S. Akerman, N., Navon, N. Glickman, Y. & Ozeri, R. Nature 510, 376–380 (2014). 4. Kominis, I. K., Kornack, T. W., Allred, J. C. & Romalis, M. V. Nature 422, 596–599 (2003). 5. Lee, S.-K., Sauer, K. L., Seltzer, S. J., Alem, O. & Romalis, V. Appl. Phys. Lett. 89, 214106 (2006). 6. Forstner, S. et al. Phys. Rev. Lett. 108, 120801 (2012). 7. Gavartin, E., Verlot, P. & Kippenberg, T. J. Nature Nanotechnol. 7, 509–514 (2012). 8. Krause, A. G., Winger, M., Blasius, T. D., Lin, Q. & Painter, O. Nature Photon. 6, 768–772 (2012). 9. Bagci, T. et al. Nature 507, 81–85 (2014). 10. Taylor, J. M. et al. Nature Phys. 4, 810–816 (2008). 11. Dolde, F. et al. Nature Phys. 7, 459–463 (2011). 12. Neumann, P. et al. Nano Lett. 13, 2738–2742 (2013). 13. Doherty, M. W. et al. Phys. Rev. Lett. 112, 047601 (2014). 14. Ovartchaiyapong, P., Lee, K. W., Myers, B. A. & Bleszynski Jayich, A. C. Nature Commun. 5, 4429 (2014). 15. Cai, J., Jelezko, F. & Plenio, M. B. Nature Commun. 5, 4065 (2014).

‘Obituary: Prof. James Franck’ — I remember his famous lecture … which gave strong support to Bohr’s new theory of the atom. Einstein said to me: “It’s so lovely, it makes you cry!” … In 1920 Franck was called to a chair in Göttingen … many honours … have come his way, among them the Nobel Prize in Physics in 1926 jointly with Hertz … Göttingen saw Franck’s happiest and most fruitful period … Those happy days ended when Hitler came to power. When the racial laws were announced, Franck published a courageous open letter in which he resigned his chair … He was the most lovable of men because he loved people; kindness shone from his eyes. There must be many beside myself who now feel they have lost one of their best friends. Lise Mietner

From Nature 29 August 1964

BE H AVIO URAL ECO LO GY

Love thy neighbour

100 Years Ago

A theoretical model suggests that the cause of female-driven extra-pair mating lies in the spreading of male interests among neighbouring families, creating powerful incentives for male cooperation and concomitant benefits for females. BEN C. SHELDON & MARC MANGEL

B

irds have long served as paragons of contented coupledom, and with apparent justification, given that more than 90% of bird species breed in socially monogamous pairs, with the male and female seemingly cooperating to rear offspring. But appearances can be deceptive. In fact, in the majority of bird species tested — data are available for more than 200 species — a substantial proportion (sometimes exceeding 50%) of offspring are sired by a male other than that providing parental care. A huge, but rather fruitless1, effort to explain the evolution of female extrapair mating behaviour in birds has focused on potential genetic benefits to females. Now, writing in PLoS ONE, Eliassen and Jørgensen2 offer an alternative explanation: that extra-pair mating by females creates an incentive for males to cooperate with their neighbours, generating a

form of collective good that may be a powerful evolutionary force. The development in the 1980s of genetic markers to assign parentage revolutionized our understanding of mating patterns in many organisms, but nowhere was the effect greater than in birds3. Before this, most birds seemed rather uninteresting from the perspective of sexual selection, because it was expected that social monogamy, reinforced by the need for rapidly growing offspring to receive extensive care from both parents, would limit the extent to which sexual selection could influence fitness (in evolutionary terms, the likelihood of survival and reproductive success). The realization that extra-pair paternity could, in principle, greatly increase the fitness variation among males relative to females, hence creating the potential for strong sexual selection, led to a vigorous interest in testing the causes and consequences of extra-pair copulation

The Trail of the Sandhill Stag by E. T. Seton; Wild Game in Zambezia by R. C. F. Maugham — The first of the books before us aims rather at being a work of art than of natural history … Never since the days of melancholy Jacques was such an outpour of sentiment upon a stricken deer. To those who like this mood the book may be recommended, for it is curiously wrought and daintily embellished … The second book on the list strikes quite another strain … But the list of stores required by two persons for a trip of two months is startling. It almost shakes one’s confidence in the author to learn that he cannot go into camp with a friend for two months without a dozen tins each of lobster and salmon, two dozen tins of sausages, and three dozen tins of fruit in syrup. From Nature 27 August 1914

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Applied physics: Hybrid sensors ring the changes.

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