8/13/2019 Tonomura in Memoriam

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Akira Tonomura(19422012)

Physicist who pioneered electron holography.

Akira Tonomura changed the fieldof fundamental physics throughmicroscopy. Like botanist RobertBrown before him, he opened up a newworld to observation. In the nineteenthcentury, Browns microscope revealedBrownian motion and the cell nucleus.In the twentieth and twenty-first,Tonomuras has shown us basic princi-ples of the quantum landscape and itsapplications.

Over decades, Tonomura developedthe extremely stable, phase-coherent

electron beams needed for good holo-graphic imaging, a technique that ena-bles measurement of both the intensityand the phase of transmitted electrons.This allowed many thought experi-ments of quantum mechanics to bedone in practice, and revealed detailsof magnetic and electric fields at thenanoscale. Tonomura used electronholography to illuminate the wavepar-ticle duality of electrons and to measurethe magnetic fields of superconductorsand other quantum effects under chal-lenging conditions, earning him heroicstatus among electron microscopists.His achievements meant he was tipped sev-eral times for a Nobel prize. He died of pan-creatic cancer in early May, at the age of 70.

Tonomura spent some of his early child-hood in Hiroshima, Japan. Fortunately,his family moved away from the city twomonths before the fateful morning in earlyAugust 1945 when the nuclear bomb wasdropped. Soon after graduating in phys-ics from the University of Tokyo in 1965,Tonomura joined the central research labo-ratory of the Hitachi Corporation in Tokyo and, with crucial encouragement from thedistinguished electron microscopist HiroshiWatanabe, began his long career.

At that time, physicist Dennis Gabor,working at the engineering company BritishThomson-Houston, based in London, hadalready proposed the idea of using holog-raphy to increase the resolution of electronmicroscopes, and the technique had passedsome feasibility tests. The most immediateand spectacular applications of holographywere in the field of light optics, in which itwas used to create stunning three-dimen-sional images. This led to a Nobel prize forGabor. Electron holography could advanceno further until the invention of the electronbiprism (a positively charged filament that

causes the electron paths on either side ofit to cross) by Gottfried Mllenstedt at theUniversity of Tbingen in Germany, whereTonomura worked briefly in 19734. Usingthis technology, Tonomura created the first

practical electron holography microscopefor Hitachi in 1978.

Tonomuras microscope proved crucial insettling a controversy over a bizarre quan-tum phenomenon. The AharonovBohm(AB) effect states that the phase of anelectrons wavefunction can be shifted by anearby magnetic field, even if the electrondoesnt pass through that field. This idea sitsuneasily with the classical theories that wereused to develop practical electron optics.Early experiments hinted at confirmationof the AB effect, but critics argued thatstray fields might have caused the observedphase shift. So Tonomura placed a ringmagnet inside a superconducting sheathto eliminate any stray magnetic fields, andencased that within a copper layer to stopa passing electron beam from entering themagnetic field region. He then used elec-tron holography to confirm the predictedphase shift between electron paths insideand outside the ring. This conclusive andelegant experiment of 1986 finally silencedthe critics, and was immediately recognizedbeyond the world of electron microscopy asa remarkable tour de force.

Few electron holography microscopesoutpaced the resolution of ordinary electron

microscopes as Gabor had envisaged, butTonomuras use of holography to detectelectron phases allowed him to pioneer anddominate the techniques practical applica-tion. In 1989, he scored a major success by

imaging a magnetic vortex emergingfrom a superconducting film, and builton that with observations of vortices in various metallic and ceramic supercon-ductors. Tonomura and others mappedmagnetic fields in small particles, inmagnetic tape and, most recently, in theskyrmion lattice a periodic arrange-

ment of magnetic vortices generated bya complex structure of electron spins.In quantum computing, observationsof superconducting vortices are keyto investigating the behaviour of thesepotential qubits.

Tonomura combined stubborn per-sistence and experimental skill withimagination and excellent communica-tion skills. He presented material withscrupulous care in publications andseminars. In a memorable Royal Institu-tion lecture in 1994, he filled the strictlyallotted one-hour time slot almost to thesecond. His images of magnetic phe-

nomena were so striking that they were oftenon the cover of major journals. An Internet video of his version of the classic double-slit experiment continues to demonstratefor many the central mystery of quantummechanics (see go.nature.com/722hph).It shows how electrons travelling througha biprism arrive at a detector one by one,as particles, but over time build up a waveinterference pattern.

Tonomuras stellar reputation and powersof persuasion helped to secure financialsupport from the Japanese governmentfor his ambitious ideas. In 2010, he wasawarded the largest grant for an individ-ual research project in the countrys his-tory (see Nature 464, 966967; 2010). Hebecame seriously ill a year later. In the spiritof Gabors original idea for boosting resolu-tion, the project aims to use high-voltageelectron holography to create 3D imagesof electron wavefunctions. Its future nowdepends on securing a leader as inspiringas Akira Tonomura.

Archie Howie is a physicist and electronmicroscopist in the Cavendish Laboratory,University of Cambridge, UK.e-mail: ah30@cam.ac.uk

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