For contributions to the quantum theory of optical coherence, and to the development of laser-based precision spectroscopy, including the optical frequency comb technique.
The winners of this year’s Nobel Prize in Physics are Roy Glauber (Harvard), John Hall (NIST) and Theodor Hänsch (Max-Planck-Institute for Quantum Optics), recognizing their fundamental and practical achievements in quantum optics. Glauber’s work, for which he wins half the prize, established the theoretical basis of quantum optics. Hall and Hänsch, who share the other half of the prize, have developed laser-based precision spectroscopy to such a high degree of accuracy that it is now a vital technique in pushing the limits of practical devices such as atomic clocks, and even in determining the constancy of fundamental ‘constants’.
Exploring the properties of light has always been a driving force for progress in physics. Albert Einstein’s study of the photoelectric effect — which earned him the 1921 Nobel Prize — provided one of the first bridges between the classical and the quantum world. The realization that light is quantized also led to a profound understanding of the physics of atomic and molecular spectra — an early example is Niels Bohr’s theory of the hydrogen spectrum.
Later in the twentieth century, the quantization of electromagnetic fields gained firm theoretical ground in the framework of quantum electrodynamics, and experiments performed by Robert Hanbury Brown and Richard Twiss1 — in which they showed that photons in a beam of narrow spectral width tend to arrive in correlated pairs — proved that the quantum nature of optical fields had to be factored in if the process of light detection were to be described correctly. It was Glauber2 who, in 1963, established the notion that, once a photon is absorbed, the state of the field is changed and therefore influences the next absorption event. Glauber’s theory, and its application to both coherent and incoherent light sources, constitute the beginning of the field of modern-day ‘quantum optics’.
Hall and Hänsch’s contribution lies in creating and measuring frequency with unprecedented accuracy — work that has made possible the observation of finer and finer details of atomic spectra, beyond fine and hyperfine structures, down to subtle quantum electrodynamic phenomena such as the Lamb shift. Key to such precision is the stability of the laser light sources involved. Hall and colleagues3 succeeded in achieving stability below the level of one Hertz by locking the laser frequency to sharp interference fringes from a passive interferometer, using electronic feed-back. Further development of the technique by groups led by Hall and Hänsch has since brought optical laser spectroscopy to the same level of precision (10-15) as that achieved in microwave atomic clocks.
To advance the capabilities of such devices even further, the wavelength of frequency-stabilized laser sources has to be defined with high accuracy. To do so, it is necessary to bridge the large range between optical frequencies and the frequencies of the best atomic clocks (based on microwave transitions in caesium). Hänsch4 developed a method based on trains of phase-locked ultrashort laser pulses that produce, in the frequency domain, comb structures that span wide frequency intervals. Earlier ‘frequency measurement’ schemes only worked at a few selected frequencies, but this ‘optical frequency comb’ makes precision measurements possible at any frequency.
Hanbury Brown, R. & Twiss, R. Q. Correlation between photons in two coherent beams of light. Nature 177, 27–29 (1956)
Glauber, R. J. Photon correlations. Phys. Rev. Lett. 10, 84–86 (1963)
Salomon, C., Hils, D. & Hall, J. L. Laser stabilization at the millihertz level. J. Opt. Soc. Am. B 5, 1576–1587 (1988)
Eckstein, J. N., Ferguson, A. I. & Hänsch, T. W. High-resolution two-photon spectroscopy with picosecond light pulses. Phys. Rev. Lett. 40, 847–850 (1978)