1966 Nobel Prize in Physics
Reason for Award
for the discovery and development of optical methods for studying Hertzian resonances in atoms
Laureates
France
Explanation
Everything around us is made of tiny particles called atoms. The electrons inside an atom jump between fixed energy steps and absorb or emit light when they do so. Dr. Kastler discovered a way to rearrange the electrons with colored light—an idea now called “optical pumping.” Thanks to this trick, the very faint radio-like signals produced by atoms could be heard loud and clear. By listening to those signals, scientists learned much more about how atoms work. The study became the starting point for ultra-precise tools such as advanced clocks and magnetic compasses.
Related Keywords
optical pumping
Optical pumping is an experimental method in which circularly or linearly polarized light is shone on atoms or molecules to intentionally bias their spin states. The technique drives the otherwise nearly equal thermal population of magnetic sublevels into a non-uniform distribution, enabling population inversion or high polarization. Kastler’s work founded this method and opened the road to applications in masers, lasers, atomic clocks, MRI and many other areas. Today it is indispensable for laser cooling of ultracold atoms and for initializing qubits in quantum technologies. By providing control over light–matter interactions, optical pumping has also influenced disparate fields such as semiconductor spintronics and fusion-plasma diagnostics.
metastable state
A metastable state is an excited state that does not immediately decay to the lower energy ground state and therefore has a comparatively long lifetime. Metastable states produced by optical pumping can last from milliseconds to seconds, making them ideal for precision measurements. Kastler exploited this longevity to amplify radio-frequency resonance signals and to detect tiny external magnetic fields and energy splittings. Modern atomic clocks and atom interferometers reference transitions involving metastable states to achieve ultra-high accuracy in timekeeping and gravity sensing. In chemistry and nuclear physics, metastable states are likewise key to understanding reaction pathways and decay processes.
Zeeman effect
The Zeeman effect is the splitting of atomic energy levels and spectral lines when an external magnetic field is applied. Because the splitting width is proportional to the magnetic-field strength, it forms the basis of highly sensitive atomic magnetometers. Through optical pumping, Kastler enhanced transitions between Zeeman sublevels, enabling high signal-to-noise detection even in very weak fields. The effect connects diverse fields, from astrophysics—where it probes solar spots and interstellar magnetic fields—to plasma diagnostics in laboratory experiments. The idea of Zeeman splitting also appears in modern solid-state topics such as the quantum Hall effect and the spin-Seebeck effect.
Hertzian resonance
Hertzian resonance refers to the phenomenon in which electromagnetic waves in the radio-frequency or microwave band resonate with internal transitions of atoms or molecules. The term honors Heinrich Hertz, who first demonstrated radio transmission; at the resonant frequency, strong absorption or emission is observed. Using optical pumping to create a population difference, Kastler greatly enhanced this resonance, allowing precise measurement of otherwise elusive transitions. Hertzian resonance can be viewed as a precursor to techniques like EPR (electron paramagnetic resonance) and NMR (nuclear magnetic resonance), providing a spectroscopic read-out of quantum states. In today’s quantum-information science, Hertzian resonance principles underpin studies such as cavity-microwave coupling with superconducting qubits.
atomic clock
An atomic clock keeps time using a specific transition frequency inside an atom and is the most accurate time-measuring device humanity has built. Kastler’s optical pumping enabled strong population inversion in rubidium and cesium atoms, boosting both the signal strength and stability of atomic clocks. As a consequence, Coordinated Universal Time (UTC) today has errors of only a few nanoseconds, supporting GPS, internet communication, and high-frequency finance. Current research into next-generation optical lattice clocks and ytterbium clocks operating in the terahertz range likewise uses optical pumping concepts in their preparation stages. Atomic clocks are also indispensable in cutting-edge research, such as testing general relativity and measuring Earth’s gravitational potential.