The 2005 Nobel Prize in Physics

The 2005 Nobel Prize in Physics is devoted to optics, with half of the prize going to Roy J. Glauber of Harvard University for his quantum theory of optical coherence, and one-quarter each going to John L. Hall (JILA, University of Colorado and National Institute of Standards and Technology, Boulder, CO) and Theodor W. Hänsch (Max Planck Institute for Quantum Optics, Garching, Germany; Ludwig-Maximilians-University, Munich, Germany), for their development of ultra-high-precision measurements of light.

In a sense, scientists created lasers before they fully understood their optical properties or could measure their light very precisely. Laser light has radically different properties from the light in a flashlight. For one thing, the light from a laser beam is coherent. If light can be imagined as a wave with peaks and valleys, "coherence" means that the peaks of the various light waves line up in step with one another, or otherwise have some sort of precisely defined, consistent interrelationship (see nice illustration).

Glauber described optical coherence and the detection of laser light in the language of quantum mechanics (for example, by treating electromagnetic fields as being quantized, or having ladder-like steps of possible energies). Helping to create the burgeoning field of quantum optics, Glauber’s theory provided understanding of quantum "noise," jittery and unavoidable fluctuations in the properties of light. This in turn provides information on the limits of measuring light (PNU 82), as well as understanding optical detectors that count single photons at a time (e.g., PNU 720). Single-photon detectors are important for applications such as quantum cryptography (PNU 480), the ultimate form of secure transmission which is already in use today.

Meanwhile, Hall and Hänsch developed techniques for measuring the frequency of light to what is currently 15 digits of accuracy. These frequency-measurement techniques helped scientists to devise fundamental definitions of physical units (for example, Hall and others helped to redefine one meter as the distance that light travels in 1/299,792,458 seconds). Measuring optical frequency has also helped to test Einstein’s theory of special relativity to record-breaking levels of precision. In addition, optical-frequency measurements have made possible tabletop experiments that search for new physics, such as the question of whether the fine structure constant, the quantity that determines the inherent strength of the electromagnetic force, is changing over time.

Hall and Hänsch are cited in particular for the recent development of the "optical frequency comb technique," in which ultrashort pulses of light create a set of equally spaced frequency peaks resembling a comb (see illustration ; articles on the technique are in PNU 434, PNU 735, Physical Review Focus, and Physics Today). The combs can be used to measure other optical frequencies with unprecedented precision and ease (and with much smaller equipment than previously possible). They enable better atomic clocks which in turn can make the Global Positioning System more precise.

The Nobel Prize Web site
Background information to be available at Physics Today