Alfred Kastler Physicist Date of Birth: 03.05.1902 Country: Germany

1. Early Life and Education
2. Research and Discoveries
3. Optical Methods for Precise Measurements
4. Double Resonance and Optical Pumping
5. Honors and Awards
6. Personal Life and Legacy

Early Life and Education

Alfred Kastler was born on May 3, 1902, in Guebwiller, Alsace, then part of Germany, to Frederick Kastler and Anna Frey. His childhood curiosity and the vivid experience of a solar eclipse sparked within him an early fascination for the natural sciences. After completing primary school, he joined the Oberrealschule, a secondary school specializing in real-world subjects, later renamed the Bartheldi Lycée after Alsace was incorporated into France following World War I.

In 1920, Kastler entered the École Normale Supérieure in Paris, a prestigious institution for higher education. Upon graduating, he taught physics in lycées (high schools) in Mulhouse, Colmar, and Bordeaux. In 1931, he became an assistant at the University of Bordeaux while pursuing his doctoral research.

Research and Discoveries

Kastler's early research focused on the interaction between light and electrons in atoms. He studied how electrons revolve around the atomic nucleus in distinct orbits while simultaneously spinning on their own axes, similar to tops. Quantum theory dictates that electrons can only occupy specific orbits corresponding to discrete energy levels.

By absorbing energy from incident light, electrons transition to higher energy levels. Upon returning to lower levels, they release the previously absorbed energy by emitting light. Like any electromagnetic radiation, light is composed of quanta of energy called photons. The energy of an absorbed or emitted photon, proportional to the frequency of the absorbed or emitted light, equals the energy difference between the levels involved in the transition.

Each chemical element has its unique set of allowed energy levels in its atoms. Since excited atoms emit light only at frequencies corresponding to energy differences between levels, the emission spectra observed, for example, using a spectroscope, consist of series of colored lines (the line's color corresponds to a visible light frequency). The spectrum not only identifies the element but also yields information about the characteristic energy level scheme of its atoms, revealing their atomic structure.

Optical Methods for Precise Measurements

Kastler realized that conventional spectroscopic techniques had limitations in resolving closely spaced spectral lines. By the late 1940s, sophisticated experiments employed radiofrequency spectroscopy. One such technique, known as atomic beam magnetic resonance, originated with Isidor I. Rabi and his group at Columbia University.

Rabi and colleagues used their method to precisely measure atomic energy levels in the ground state (the state with the lowest energy). The ground state can have multiple magnetic substates that split slightly in the presence of a magnetic field. Thus, by exposing atoms to a magnetic field of appropriately chosen frequency, one could induce transitions between substates. The appropriate frequency of the electromagnetic field corresponds to the photon energy equal to the substate energy difference, lying in the radiofrequency range.

Using carefully arranged magnets and slits, the Columbia group generated narrow beams of atoms occupying just a few magnetic substates, with detectors sensitive to atoms in specific states. When the field was tuned to the correct frequency, changes in the number of atoms reaching the detector indicated that transitions between levels had occurred. By knowing the energy of the photons causing the transitions, Rabi's group could calculate the energy levels associated with the substates. This correspondence between the radiofrequency of the transition-inducing field and the sublevel energy difference is known as Hertzian resonance (named after Heinrich Hertz, who provided the first experimental proof of radio waves). The unit of frequency is now named hertz in his honor.

Double Resonance and Optical Pumping

The atomic beam magnetic resonance method had drawbacks: the average lifetime of an excited state before it decays, emitting its energy and returning to the unperturbed ground state, is very short (about one ten-millionth of a second) and only a small number of atoms undergo the resonance-induced transition.

Kastler, together with his student Jean Brossel, developed techniques in which light was employed to overcome some of the limitations of atomic beam magnetic resonance. This came to be known as the double resonance method.

In this method, a light beam of appropriate frequency excites atoms to a specific energy level. However, not all sublevels are equally populated. Therefore, when the atoms decay back to the ground state, the emitted light distribution becomes anisotropic (unequal in different directions) and partially polarized in each direction. If an electromagnetic field applied to the excited atoms has a frequency (photon energy) necessary to induce transitions between the occupied and unoccupied sublevels, the emitted light undergoes changes in both its spatial distribution and polarization. This change signals that the radiofrequency is tuned to the sublevel energy difference (in resonance with it). Kastler's method provided a means of accurately determining the sublevel structure of excited atomic states.

In 1950, Kastler reported another method called optical pumping, allowing him to shift electrons in atoms from one magnetic sublevel of the ground state to another. In this technique, specially polarized light is directed at an ensemble of atoms. If the ground state has two magnetic sublevels, atoms in one sublevel absorb light and transition to an excited state, while atoms in the other sublevel remain unaffected. Upon emitting radiation and returning to the ground state, atoms populate both the absorbing and non-absorbing levels. The light is said to have "pumped" atoms into the non-absorbing ground state.

Seeking further refinements in their experimental techniques, Kastler and Brossel established a special team within the Physics Laboratory of the École Normale Supérieure in 1951. Over 15 years, their research and collaborations with other scientists contributed to elucidating atomic sublevel structures and advancing the study of quantum-mechanical phenomena.

Honors and Awards

In addition to gaining valuable insights into the sublevel structure of ground states of many atoms, physicists learned to orient atomic nuclei in the vapors of mercury and cadmium in a desired direction. This enabled them to measure some of the nuclei's magnetic properties precisely. Using optical pumping, experimenters could create targets of polarized atoms, which were then bombarded with particle beams in nuclear physics experiments.

Kastler was awarded the 1966 Nobel Prize in Physics "for the discovery and development of optical methods for studying Hertzian resonances in atoms." In presenting the prize, Ivar Waller of the Royal Swedish Academy of Sciences emphasized the transformative nature of Kastler's work and its implications. "A large number of nuclear moments have been determined with high accuracy," Waller noted. "Kastler's ideas concerning optical pumping have played an important role in connection with the development of the laser. Optical pumping has made possible the construction of convenient and very sensitive magnetometers and atomic clocks."

Personal Life and Legacy

After retiring from teaching in 1968, Kastler served as the Director of Scientific Research at the National Centre for Scientific Research until 1972. He married Élise Co-See, a schoolteacher, in 1924. The Kastlers had two sons and a daughter.

A man of exceptional humility and introspection, Kastler nevertheless engaged actively in political events. He voiced his support for Israel, firmly opposed nuclear weapons, and sharply criticized the US role in the Vietnam War. He extended his sympathies to the Algerian independence movement. Kastler passed away on January 7, 1984, in Bandol on the French Riviera.

Besides the Nobel Prize, Kastler received the Holweck Prize of the London Physical Society (1954), the Research Prize of the French Academy of Sciences (1956), the C.E.K. Mees International Medal of The Optical Society of America (1962), and other honors. He became a member of the French Academy of Sciences (1964) and an honorary member of scientific societies in Poland, Germany, Hungary, and Belgium. In 1952, Kastler was made a Chevalier and in 1977 a Commandeur of the Legion of Honor. He held honorary doctorates from the Universities of Louvain, Pisa, and Oxford.