German physicist, who with US physicists Roy J Glauber and John L Hall shared the 2005 Nobel Prize in Physics for his contributions to the development of laser-based precision spectroscopy.
In 1970, Hänsch developed a new type of laser that was capable of producing light pulses of exceptionally high spectral resolution. These lasers emitted photons that had almost identical energies, with a precision of close to one in a million. This system was used to measure the frequency of the Balmer spectral line of hydrogen to a higher precision than was possible before this development. Hänsch continued to develop high-precision laser systems until the 1990s, when he collaborated with Hall to perfect a method of extremely accurate measurement known as the optical frequency comb technique. The system was able to measure the frequency of the Lyman line of hydrogen to an accuracy of fifteen digits and was applicable to light of all colours. The precision lasers and the frequency comb technique laid the foundation for the development of laser-based precision spectroscopy. The accuracy of the frequency comb technique makes possible the measurement of the stability of the constants of nature over time.
Hänsch was born in Heidelberg, Germany. He moved to the USA in 1970 to teach and carry out research at Stanford University, California. He was awarded his PhD in physics by the University of Heidelberg, Germany, in 1969. He held the position of professor at Stanford University from 1975 to 1986. He returned to Germany in 1986 where he became director of the Max Planck Institut für Quantenoptik (quantum optics) in Garching and professor of experimental physics and laser spectroscopy at the Ludwig-Maximilians University in Munich.