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. H„nsch (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
http://www.technology.niagarac.on.ca/courses/tech238g/images/CoherentLight.gif).

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 H„nsch 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 H„nsch 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 http://www.rp-photonics.com/img/comb.gif 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/)

---