PHYSICS NEWS UPDATE

The American Institute of Physics Bulletin of Physics News

Number 748 October 4, 2005 by Phillip F. Schewe and Ben Stein

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 at

).

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
(http://www.aip.org/pnu/1992/split/pnu082-1.htm), as well as
understanding optical detectors that count single photons at a time
(e.g., http://www.aip.org/pnu/2005/split/720-1.html). Single-photon
detectors are important for applications such as quantum
cryptography (http://www.aip.org/pnu/2000/split/pnu480-1.htm), 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

for
illustration; articles on the technique are at
http://www.aip.org/pnu/1999/split/pnu434-1.htm ,
http://www.aip.org/pnu/2005/split/735-2.html ,
http://focus.aps.org/story/v5/st24,
http://www.physicstoday.org/vol-54/iss-3/pdf/vol53no6p19-21.pdf).
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. (Nobel
Prize website at
http://nobelprize.org/physics/laureates/2005/index.html; Background
information to be available at http://www.physicstoday.org/)