A Coherent Life Emil Wolf Died in June 2018 at the Age of 95
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comment A coherent life Emil Wolf died in June 2018 at the age of 95. The father of optical coherence theory was at the University of Rochester for nearly 60 years. A memorial in August at the university attracted more than 150 attendees from around the world. P. Scott Carney and Joseph Eberly mil Wolf, who joined the University saving his life with a 3 am phone call to of Rochester in 1959 and was the come get on the truck leaving Paris. With EWilson professor of optical physics and the exception of his brother Karel and one professor of optics, made an enormous, cousin, none of his extended family survived sustained and lasting impact in optics (Fig. 1). the Holocaust. He published over 400 papers across As a refugee in Britain, Wolf finished eight decades and edited 60 volumes of high school and attended Bristol University the book series Progress in Optics. This is on scholarship where he earned a BSc impressive certainly, but doesn’t explain in mathematics and physics and a PhD why he is seen as a genuinely transformative in mathematics (Fig. 2). His advisor, figure. A glance through textbooks of optics E. H. Linfoot, was soon named director written in the three decades of the 1930s of the Cambridge Observatory and Wolf through the 1950s gives the impression followed him as a post-doc to Cambridge. of a field characterized by rules of thumb, In Cambridge, Wolf would travel to descriptions of instruments, ancient London for meetings of the Optical Society accomplishments, and no significant future, where he met both Leonard Mandel and either theoretical or technological, beyond Dennis Gabor, then at Imperial College photography. Today, one easily says that in London. Gabor and Wolf were fast friends. 1960 the laser remade optics, opening an Holography would eventually make entirely fresh landscape to explore. But Wolf an appearance in Principles of Optics. got there first. He was already making optics Wolf would maintain an interest in the into a discipline of fundamental science in information carried by scattered fields for the 1950s. The book Principles of Optics1,2, the rest of his career, perhaps most famously which Wolf co-authored with Max Born leading to his paper ‘Three-dimensional (Nobel Prize winner perhaps best known for structure determination of semi-transparent contributions to the foundation of quantum objects from holographic data’ in 19694, mechanics), has long been standard reading a field that would come to be called for students of physics and researchers in diffraction tomography. Fig. 1 | Emil Wolf (1922–2018), who co-authored optics alike. When Max Born approached Gabor for the famous book Principles in Optics with Max Born Wolf’s breakthrough conception was a suggestions for a co-author for his book on and made major advances in the field of optics balance between two partly contradictory optics, Gabor introduced Wolf. This led to including diffraction, coherence, spectroscopy observations — the certainty that optics a move to Edinburgh and a collaboration and scattering, is pictured here on a street in April was not radiometry, meaning that light between Born and Wolf that would produce 1950 with Ann Ward (whom he married in 1951). was primarily, fundamentally, about Principles of Optics1,2. Born initially had Credit: Image courtesy of Bruno Wolf optical fields, not optical beams, and the some reservations. He had previously had realization that the optical field was not itself an assistant, Klaus Fuchs, who turned out observable, presenting oscillations much too to be an atomic spy. Wolf would say that rapid to be followed by any known detector. stroke, classical coherence theory emerged, Born probably had to question whether he All of optics then represented a challenge crystal clear. should hire a Wolf after a fox. Nonetheless, to find how light could be field-based Born to Jewish parents Josef and Pavla Gabor’s recommendation won through and but observable. Wolf in Prague in 1922, Wolf became a a collaboration began that would indelibly Wolf showed the challenge could be refugee at the age of 16 following the 1939 mark the world of optics, and would be met with field correlation functions. Nazi invasion of Czechoslovakia. He made treasured by Wolf for the rest of his life. His understanding of the powerful role his way illegally from Prague to Paris where When Born retired and moved back to correlations play and the way they underlie he found work with the Czech government Germany, Wolf moved to the University all of observable optics led to a series of in exile. When Paris fell, he was evacuated of Manchester, held a series of teaching seminal publications, one titled ‘Optics in along with the Czech government officials positions and had some support under terms of observable quantities’ published in to London. The evacuation was made in an Imperial Chemical Industries (ICI) 19543. This paper provided the principle that extremis and decades later Wolf would take Fellowship. During this period, Wolf would unified a disorganized zoo of correlation care to credit Julius Firt, a Czech journalist do some of his most impactful work. He effects and parameters of optics. In one also with the government in exile, for made a new formulation of the focusing NATURE PHOTONICS | VOL 12 | NOVEMBER 2018 | 637–639 | www.nature.com/naturephotonics 637 comment after years of work, Born asked Wolf what celebrated the 50th anniversary of the was taking so long. Wolf explained he was famous Jaynes–Cummings paper published working on an exposition of coherence in 196310. The 11th CQO is being prepared theory. According to Wolf, Born responded for summer 2019. “Wolf, who in the world apart from you is In the 1970s, Wolf brought the machinery interested in coherence theory?” he had developed for coherence theory Wolf prevailed on him to wait for the to bear on radiometry. He provided the chapter to be done and shortly after the foundations of radiometry and a means to publication of the book, the arrival of the generalize it to arbitrary sources and states laser ensured that many people were indeed of coherence. With Girish Saran Agarwal, very interested in coherence theory. Six he made significant inroads in quantum more editions and nearly 60 years later, optics and the mathematical underpinnings they still are. The transition of the book’s of a statistical optical field theory. In the publisher from Pergamon1 to Cambridge2 1980s, he identified a mechanism for shifts was a minor triumph Wolf liked to recall. of spectral lines other than the Doppler He had discovered that Pergamon, under shift, culminating in his 1987 paper ‘Non- Robert Maxwell, had been cheating them cosmological redshifts of spectral lines’11. by a miscounting of sales. Cooperating with This so-called Wolf shift was observed Born’s son in London, Wolf brought suit in and posed as a possible mechanism for Britain, fought Maxwell in court, and won. anomalous spectral shifts in certain Robert Hopkins, director of the Institute astronomical observations. of Optics at the University of Rochester also Wolf continued to produce notable work noticed Wolf’s work in coherence. Hopkins throughout the 1990s and 2000s. One of Fig. 2 | A graduation portrait of youthful Emil Wolf, wanted to build strength in the area and the authors of this Comment, P.S.C., was a probably taken on 29 June 1945 when he received was on his way to England for a conference. graduate student in his group in the mid- his BSc in mathematics and physics from Bristol Wolf’s ICI Fellowship was running out and 1990s. Wolf was remarkable as an advisor for University. Credit: Image courtesy of Bruno Wolf he was anxious to find a position. Hopkins many reasons, the most remarkable of which wrote asking for a meeting, but the meeting was his accessibility. His students saw and nearly didn’t happen. The letter from interacted with him at length every day. We problem that would take into account energy Hopkins got misfiled by a secretary and was would usually have coffee or lunch with him conservation, polarization and the real shape only discovered by Wolf as he was searching and this would sometimes stretch well into of lenses. The results were published in the for a missing chapter of Principles of Optics the afternoon, talking together about the Proceedings of the Royal Society of London5,6 also misplaced by the same secretary. science we were working on and where we and the second paper, with a student, Recalling the events7, he said, “It was all a were going with it. Bernard Richards, is still referred to by matter of luck, particularly that phone call Long-time colleague and collaborator practitioners simply as ‘Richards and Wolf’. in Paris at three in the morning saying to get Taco Visser recalled at the memorial a It remains his most cited journal paper. on the lorry, the truck. It just shows you how classic reaction to a negative referee report. Also in Manchester, in 1954, he published much luck there is in life. First to get out of Wolf began going through all the things in Il Nuovo Cimento his famous paper on Paris and then to get to America.” Of course, that were wrong with the report, becoming coherence theory and optical observables3. the meeting did happen and he came to the more and more vehement, his tone rising, Wolf’s accomplishment in coherence Institute the next year. going to the blackboard and flourishing theory was noted by many, including Henry Shortly after arriving in Rochester, he chalk on slate. Finally, Visser said, “Emil, Lipson and his assistant Brian Thompson.