Alfred Kastler was born in Guebwiller, Alsace, which was then part of Germany, on May 3, 1902. He followed his early studies at the school in his native town, and continued at the Oberrealschule of Colmar, which became the Lycee Bartholdi in 1918, when Alsace was returned to France.
He entered the École Normale Superieure in 1921, and left in 1926 to teach in a lycée. He taught for 5 years, first in the Mulhouse lycée, then in those of Colmar and Bordeaux. The next stage of his career was in higher education: assistant at the Bordeaux Faculty of Science from 1931 to 1936, lecturer at Clermont-Ferrand from 1936 to 1938, professor at Bordeaux from 1938 to 1941. In 1941, in the midst of the German occupation, Georges Bruhat asked him to come to Paris to help him in establishing physics teaching at the Ecole Normale Superieure. The post was provisional, but was confirmed by the allocation of a chair in a personal capacity at the Paris Faculty of Sciences in 1952.
His mathematics teachers at the Colmar Lycée, Fröhlich from Bavaria and Edouard Greiner from Alsace, were the first to awaken his interest in science. This predilection became consolidated in the special mathematics class held by Mahuet and Brunold, who helped Kastler to gain entry to the École Normale Superieure by the side entrance, so to speak. In the stimulating and friendly atmosphere of this college, the teacher Eugène Bloch (who came from the upper Rhine and who subsequently disappeared without trace in Auschwitz) initiated his students into the concepts of Bohr's atom and quantum physics, and drew Kastler's attention to Sommerfeld's book on atomic structure and spectral lines. This book introduced him to the principle of the conservation of momentum applied by A. Rubinowicz to the exchange of energy between atoms and radiation. This principle was to guide the whole of Kastler's research, beginning with his thesis up to the most recent investigations of the Parisian team.
Alfred Kastler was in 1931 appointed assistant to Pierre Daure, professor at the Bordeaux Faculty of Science. His teaching duties were then less onerous, and Kastler was able to devote all his free time to research, aided by Professor Daure who initiated him into experimental spectroscopy. For many years, he worked in the field of optical spectroscopy, particularly on atomic fluorescence and Raman spectroscopy. [In I937 he became interested in the luminescence of sodium atoms in the upper atmosphere; after establishing that the D line of the twilight sky could be absorbed by sodium vapour, and after some studies at Abisko where twilight is prolonged, he was able to demonstrate in cooperation with his colleague Jean Bricard, that this line is polarized, as it must be if the emission mechanism is one of optical resonance produced by solar radiation.]
During the years of the occupation, French scientists were virtually isolated from the outside world. In 1945, it was possible to send pupils to other western countries, so that they could bring their knowledge of the most recent developments in scientific progress up to date. Among them was Jean Brossel, who returned in 1951 in possession of a mass of information gained under Francis Bitter at M.I.T.
Under the influence of Gorter, Rabi had very successfully applied certain methods to the investigation of atoms in their fundamental state. In 1949, Bitter suggested extending these same methods to the excited states of atoms. Brossel and Kastler together then proposed the " double resonance method ", which combines optical resonance with magnetic resonance.
While Brossel was at M.I.T., between 1949 and 1951, he carried out pioneer work along these lines on the excited state of the mercury atom. At the same time, Kastler was supplementing the method by the technique of "optical pumping", which makes it possible to apply "optical methods for studying the microwave resonances" to the fundamental states of atoms.
After 1951, Kastler worked in collaboration with Jean Brossel in Paris to perfect all these methods. Among the young men and women at the École Normale, which nurtures the intellectual elite, they found their research workers. Their theses represent the various stages in their collective work which has been awarded the Nobel Prize, and of which some account is given in Kastler's Nobel lecture.
Kastler taught as Francqui Professor at the University of Louvain during the year 1953-1954, he hold honorary doctorates from the University of Louvain (1955), Pisa (1960), and Oxford (1966), and he was decorated by the University of Liége.The French and Polish Societies of Physics and the American Society of Optics have elected Kastler to honorary memberships. In 1962, the latter society awarded him the first Mees medal bearing the inscrip tion "Optics transcends all boundaries". In 1954, the British Physical Society awarded him the prize commemorating Fernand Holweck, who disappeared tragically in 1941. Kastler was made a member of the Royal Flemish Academy of Belgium in 1954, and of the Paris Academy of Sciences in 1964; in 1965, the National Centre for Scientific Research awarded him their gold medal, at the same time as his friend and colleague Louis Néel.
In Decermber 1924 Kastler married Elise Cosset, a former pupil of the École Normale Supérieure. By working as a history teacher in secondary schools she made it possible for her husband to devote to research all the leisure time left to him by his own teaching duties. They have three children: Daniel, born in 1926, Mireille born in 1928, and Claude-Yves born in 1936. They have all married, there are now six grandchildren, whose ages range from 14 years to 10 months. Daniel is a Professor of Physics at the Faculty of Science in Marseilles, he is working on theoretical physics problems; Mireille is an ophthalmologist in Paris, and Claude-Yves teaches Russian at the Arts Faculty in Grenoble.
Presentation Speech by Professor Ivar Waller, member of the Nobel Committee for Physics
Your Majesty, Royal Highnesses, Ladies and Gentlemen.
When, shortly after 1930, Alfred Kastler embarked upon a scientific career, he concentrated his attention on problems connected with light scattering. He used novel methods to analyse this phenomenon, which had already been studied by projecting light emitted by certain atoms into a chamber containing the same kind of atoms. The illuminated atoms are thus excited by the light to a higher energy level. When a resonance effect of this kind is produced, strong fluorescence is emitted by the excited atoms as they return to the ground state.
The phenomenon received close attention a little earlier, particularly after it was found that the fluorescence is strongly polarized by placing a polarizer between the lamp and the resonance chamber. Another observation was that this polarization was considerably influenced by a magnetic field acting on the illuminated atoms.
Kastler made an important contribution to our understanding of these phenomena. He studied the relationships between the spatial orientation of the atoms and the polarization of their radiation, and thus laid the foundations of the work that is today honoured with the Nobel Physics prize.
The starting point of the work was research into Hertzian resonances. These are produced when atoms interact with radio waves or microwaves, i. e. with electromagnetic radiation having a frequency at least a thousand times lower than visible light. Such waves are therefore well suited to the study of fine details in spectra, which, though observable by optical spectroscopy, could not be measured with satisfactory precision by this method. Hertzian resonances were first used for this purpose - and with success - in 1938, by Rabi following Gorter's suggestion. Rabi was able to measure, with high precision, the splitting of energy levels into a number of sublevels, a phenomenon that is produced in the presence of a magnetic field and that is due to the orientation of the atoms in space. The hyperfine structure is another kind of small subdivisions, associated with the magnetic and electric moments of magnetic nuclei. On the basis of his exact measurements, Rabi was in a position to calculate these nuclear moments with great precision.
Aided by Jean Brossel, first his pupil and later close co-worker, Kastler was the first to propose a method of investigating Hertzian resonances by optical methods, indicating the possibility of exciting selectively magnetic sublevels from excited states by polarized light having the resonance frequency. If a high-frequency oscillating magnetic field is applied, Hertzian resonance will be induced when the ratio of this frequency to an applied constant magnetic field is suitably chosen. Hertzian resonances tend to equalize the population of the magnetic sublevels, and inconsequence influence the observed polarization of the fluorescence. In practice, the resonance chamber in the process described earlier is surrounded by a coil carrying a current of radio or microwave frequency.
The experiment was carried out some years later by Brossel in collaboration with the American physicist Bitter. To extend the use of Hertzian resonances to excited states Bitter had already suggested combining optical and Hertzian resonances, but he did not propose a method of accomplishing this aim. He called the Brossel-Kastler method double optical resonance.
New profound analysis of the atomic processes connected with the scattering of resonance radiation led Kastler to the method of optical pumping, which he proposed in 1950. In this method the atoms are illuminated with resonance radiation, which is as a rule circularly polarized. According to Kastler, the atoms returning to the ground state concentrate in certain sublevels and assume preferential orientations in space if the experiment is conducted under appropriate conditions. The use of this method should allow orientation of both atoms and atomic nuclei. The experiment was actually performed two years later by Brossel, Kastler and Winter.
Double resonance and optical pumping permit very sensitive detection of Hertzian resonances, because such resonances provoke easily observable optical effects. These methods are therefore based on a different principle than ESR or NMR spectroscopy; in contrast to the latter methods, they can be applied to materials having very low density. The methods were systematically developed by Kastler in collaboration with Brossel and with a large number of young and brilliant researchers, and the investigations bear witness to the extraordinary fertility and the numerous possibilities of application of this approach.
As an important example of the phenomena involving excited states studied by double resonance in Kastler's laboratory, I shall mention the narrowing of spectral lines with increasing gas pressure within the resonance chamber.
Experiments on optical pumping were at first done with atomic beams. They led to extensive experimental and theoretical investigations of the simultaneous interactions of several quanta of an oscillating magnetic field with atoms. An important improvement in the method of pumping was obtained when the attempts to conduct these experiments on the vapour in the resonance chamber proved successful. Some very interesting work was done on the relaxation of atoms back to the disordered state after pumping, which provided information on the mechanism acting in interatomic collisions and in collisions between atoms and the walls of the container.
In the last few years, Cohen-Tannoudji has conducted research of extreme general importance, again in Kastler's laboratory, by studying the broadening and displacement of energy levels in pumped atoms, caused by their interactions with an electromagnetic field.
A large number of nuclear moments have been determined with high precision. Kastler's ideas about optical pumping played an important part in the development of the laser. Optical pumping has permitted the construction of easy to use and very sensitive magnetometers as well as atomic clocks.
Professor Alfred Kastler. Through your discoveries, made partly in collaboration with your erstwhile pupil Jean Brossel, you have set a seal upon the great French tradition in optical science. Your methods have been perfected and have been successfully applied to a large number of fundamental problems by yourself and by the team of eminent young scientists attracted by the illustrious reputation of your laboratory. You have consistently acknowledged the research of your colleagues with characteristic generosity and personal modesty.
I ask you, Professor Kastler, to receive the Nobel Prize for Physics from the hands of His Majesty the King.
Alfred Kastler's speech at the Nobel Banquet in Stockholm, December 10, 1966 (in French)
Sire, Altesses Royales, Mesdames, Messieurs,
Permettez-moi de vous exprimer ce soir, non pas seulement la gratitude d'un homme, mais celle d'une équipe dont vous avez distingué l'oeuvre collective et que m'a puissamment aidé à former mon collègue et ami Jean Brossel.
Permettez-moi également de vous dire la gratitude de l'Ecole Normale Supérieure qui nous a fourni nos chercheurs, jeunes gens et jeunes filles, ainsi que celle de ma petite patrie, l'Alsace, et de ma grande patrie, la France, déjà très honorée, l'an passé, par le choix que vous avez fait des trois prix Nobel de Médecine.
C'est après une longue éclipse que le nom de la France reparaît au Palmarès des prix Nobel de Physique, et je me sens très ému d'avoir été l'objet de votre choix. En effet, l'effort de notre pays dans le domaine de la recherche scientifique - en dépit de l'impulsion donnée par Jean Perrin - fut souvent timide et insuffisant. En outre, notre jeune élite intellectuelle a été décimée au cours de la première guerre mondiale, et c'est toute une génération de chercheurs qui a été fauchée sur les champs de bataille. Chaque année, la cérémonie du 11 Novembre qui nous réunit autour du Monument aux Morts, face à l'interminable liste d'élèves de l'Ecole Normale morts au champ d'honneur, nous permet d'apprécier l'immensité du sacrifice.
De nouvelles générations s'efforcent actuellement de combler ce grand vide ainsi que celui produit par l'isolement de la dernière guerre. Nous ne cessons de demander pour elles plus de crédits, de locaux, de moyens de collaborer avec les chercheurs des autres pays, dans la confiance et l'amitié.
Depuis la fin de la dernière guerre, un certain nombre de Français sont déja venus en Suède. Je me rappelle volontiers le plaisir que j'ai eu à aller, il y a 18 ans, dans le Grand Nord, au bord du Torneträsk, à Abisko, pour y étudier les spectres du ciel crépusculaire. Je revois cette veille de Noel 1948 où mon train s'enfonçait dans la nuit polaire et où je contemplais, émerveillé, à chaque station, le grand sapin de Noël dont les lumières illuminaient les ramures givrées des bouleaux de la forêt en une éclatante féerie. La nuit suivante nous donna le grandiose spectacle d'un ciel embrasé par une aurore boréale. Quelques semaines plus tard, fin Janvier, ce fut l'apparition du soleil qui inonda de joie le visage de nos compagnons de voyage dont deux lapons, dans le train qui nous menait vers l'église de bois et les mines de Kiruna.
C'est alors que j'ai compris ce que signifie pour vous la lumière et avec quelle ferveur, encore au milieu des ténèbres, vous saluez la renaissance des jours et la venue du printemps, ce printemps symbolisé pour moi par la merveilleuse statue de jeune fille, oeuvre de mon ami Gunnar Nilsson, qui orne la place de l'école de Farsta, dans la banlieue de Stockholm.
Permettez-moi maintenant de revenir à la France et aux semaines que je viens de vivre. Si je n'avais connu le prestige dont jouit le prix Nobel, les centaines de témoignages d'amitié que j'ai reçus et dont la plupart attendent encore une réponse, m'auraient convaincu de l'extraordinaire rayonnement qui entoure le nom d'Alfred Nobel. Et ce n'est pas le moindre de ses mérites que d'avoir, en encourageant le développement des activités humaines, été à l'origine de la formation, sans cesse renaissante, de ce réseau de sympathie qui, par lettres et télégrammes, entoure chaque mois de novembre notre planète souvent déchirée.
Alfred Nobel, en associant, dans son testament, aux prix de sciences et de médecine un prix Nobel de la Paix, n'a-t-il pas voulu léguer à ceux qui sont élevés à la dignité de prix Nobel une grande et grave responsabilité : celle de veiller, d'oeuvrer et de lutter pour que la Science, source de bien et de mal, ne soit pas détournée vers des oeuvres de destruction et de mort, mais pour qu'elle serve à des oeuvres constructives, pour qu'elle aide à l'épanouissement de la vie, et qu'en un siècle ou elle rapproche les hommes vivant aux antipodes, elle leur apporte aussi plus de lumière et le bien suprême : la Paix.
Afin que le cri des chrétiens du Moyen Age " Noël, Noël " qui signifie " Joie " devienne le cri de tous les hommes.
Tack så mycket.
From Nobel Lectures, Physics 1963-1970.