[sci]ON THE NATURE OF LIGHT

[Sir Frederick]

ON THE NATURE OF LIGHT
T. Brown et al (University of St. Andrews, UK) discuss the nature
of light. What is light? This question might at first sight seem
an odd one to ask -- light is all around us and generally taken
for granted. But any attempt to really get to grips with the
nature of light takes one on a fascinating journey into the heart
of physics. Light can be thought of as a wave consisting of very
fast oscillations of an electric field. A typical light wave may
have a wavelength of 800 x 10^(-9) meters, which, bearing in mind
the speed of light [3 x 10^(8) meters per second], gives a
frequency for the wave of 3.75 x 10^(14) hertz. This means that
one cycle of the electric field in the light wave takes place in
just 2.7 x 10^(-15) seconds, or 2.7 femtoseconds. In most
situations, this fast variation in the electric field is too
rapid to be noticed, and what is observed rather is the envelope
function that modulates the underlying fast carrier variation.
Laser systems provide the ideal tool to investigate the
properties of light. The light beams produced by a laser are
"coherent"; that is, a fixed phase relationship exists in the
output, in contrast to light encountered in every day life. Some
modern laser systems are designed to produce light in the form of
very short and regularly spaced pulses rather than in the more
familiar continuous wave or "always-on" format. The pulse
periodicity is governed by the physical size of the laser, and
the output is a sequence of abrupt short pulses. The pulse
duration is short compared with the pulse repetition rate, and
the average power from such systems is thus relatively low, but
the peak power of the pulses is several orders of magnitude
higher.
-----------
Science 2001 293:1265
-----------
SCIENCE-WEEK 21 Dec 2001 http://scienceweek.com
-------------------
Related Background:
EXPERIMENTAL PHYSICS: FIRST EFFECTIVE ZERO-VELOCITY LIGHT
Experiments involving the control of light pulses in a quantum
mechanical regime hold great promise for a future technology of
quantum computing involving optical information storage and
transmission. In 1999, L.V. Hau et al succeeded in slowing light
to a velocity of 30 meters per second in an ultracold sodium gas.
Now the same laboratory reports effectively stopping light
completely for an interval of 1 millisecond before releasing the
pulse to resume normal velocity. Essentially, the phenomenon
involves "storing" the light pulse in the quantum states of the
atoms, with the light pulse reconstituted for propagation at a
later time.
... ... C. Liu et al (4 authors at 2 installations, US) report
observations of halted light pulses, the authors making the
following points:
1) The authors point out that "*electromagnetically induced
transparency" is a quantum interference effect that permits the
propagation of light through an otherwise opaque atomic medium. A
"coupling laser" is used to create the interference necessary to
allow the transmission of resonant pulses from a "probe laser".
This technique has previously been used to slow and spatially
compress light pulses by 7 orders of magnitude, resulting in
their complete localization and containment within an atomic
cloud.
2) The authors report the use of electromagnetically induced
transparency to bring laser pulses to a complete stop in a
magnetically trapped cold cloud of sodium atoms (approximately 11
million sodium atoms at 0.9 microkelvins). Within the spatially
localized pulse region, the atoms are in a *superposition state
determined by the amplitude and phases of the coupling and probe
laser fields. Upon sudden turn-off of the coupling laser, the
compressed probe pulse is effectively stopped, and *coherence
information initially contained in the laser fields is "frozen"
in the atomic medium for up to 1 millisecond. When the coupling
laser is turned back on, the probe pulse is regenerated: the
stored coherence is read out and transferred back into the
radiation field. The authors present a theoretical model from
which it is concluded that the system is self-adjusting to
minimize dissipative loss during the "read" and "write"
operations. The authors state: "We anticipate applications of
this phenomenon for quantum information processing."
... ... In a commentary on this work, Eric A. Cornell (University
of Colorado, US) states: "The key fact here is that as the pulse
of light penetrates into the dense region of the ultracold atomic
cloud, it turns into a "quantum coherence pattern" printed on the
sodium atoms -- the information in the light beam becomes stored
in the quantum phase relationships within the internal atom
states. In the final limit, when the pulse comes to a dead stop,
all the photons have been "imprinted" (absorbed in a fully
reversible way) into the coherence pattern. Later, when the
coupling light is turned back on, the information contained in
the pattern is read out and converted back into propagating
photons that accelerate to the conventional speed of light as
they come to the edge of the atom sample."
-----------
Nature 25 Jan 01 409:461,490
-----------
Notes:
... ... *electromagnetically induced transparency: In his
commentary, Cornell states: "The key to slowing light is the
presence of a second laser beam, the so-called 'coupling' pulse.
Distinguishable from the propagating (or 'probe') pulse by its
polarization, the coupling light delicately adjusts the internal
energy levels of the atoms, suppressing their ability to absorb
the probe light -- in effect, a single absorption level is split
into two levels that cancel each other out. This phenomenon is
known as 'electromagnetically induced transparency'.
... ... *superposition state: In this context, the general idea
is that the cloud of sodium atoms (335 microns by 55 microns in
this experiment), under these laser conditions, behaves almost as
a single quantum mechanical entity: the quantum states of the
atoms are superposed into a single wave function for the entire
system.
... ... *coherence information: In quantum physics, coherence
involves the locking of phase differences between wave functions:
the wave functions of two or more particles are said to be
coherent if the phase difference between their wave functions
remains constant. In general, a perfectly coherent system of
particles can be described by a single macroscopic wave function.
In general, in optics, the term "coherence" refers to the
existence of a correlation between the phases of two or more
waves. In this context, the term "coherence information" refers
to information contained in and dependent upon the coherence of
the laser pulse.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 16Feb01
-----------
SCIENCE-WEEK 21 Dec 2001 http://scienceweek.com
-------------------
Related Background:
IN FOCUS: ON THE HISTORY OF THE PHYSICS OF LIGHT
"The revival of the wave theory of light, begun by Thomas Young
(1773-1829), is one of the most important features of the history
of the 19th century. Young pointed out that the dividing of a
beam of light into a refracted ray at the interface between two
mediums was to be expected from the wave theory but had not been
satisfactorily explained on the corpuscular theory. In 1801, he
presented to the Royal Society a paper 'On the Theory of Light
and Colors', in which he proposed the principle of the
interference of two wave trains as an explanation of Newton's
rings and the colors of thin plates. From Newton's measurements
of the thickness of the air layers necessary to produce the
several colors, Young was enabled to compute wavelengths. In
subsequent papers, he described the interference fringes which he
had observed by placing hairs on silk threads in front of a
narrow slit illuminated from the rear; he announced the change of
phase on reflection; he explained diffraction bands by the
principle of interference; and he showed that the spacing of
these bands gave values of the wavelength agreeing with those
obtained from Newton's rings and that, therefore, both phenomena
must be due to a common cause... But the dogmatic spirit in
regard to scientific matters was not yet dead. Young's paper
aroused a storm of protest, even of derision and abuse. He was
attacked not by the church, as was Galileo, but by some of his
scientific, or, more probably, pseudoscientific contemporaries.
His chief assailant was Henry Brougham, afterward Lord Chancellor
of England, who 'reviewed' Young's papers in the _Edinburgh
Review_. The nature of Brougham's attack is indicated by the
following quotation: 'We wish to raise our feeble voice against
innovations that can have no other effect than to check the
progress of science and renew all those wild phantoms of the
imagination which Bacon and Newton put to flight from her temple.
We wish to recall philosophers to the strict and severe methods
of investigation.' Although Young replied at length in a
privately published pamphlet, it was a long time before public
opinion was willing to receive his theories with an open mind."
-----------
F.K. Richtmyer et al: Introduction to Modern Physics (5th ed.)
(McGraw-Hill, New York 1955, p.33)
[Editor's note: Thomas Young (1773-1829) was an infant prodigy
who matured into an adult prodigy. At Cambridge, he was called
"Phenomenon Young". He studied and practiced medicine, but failed
at it because of the apparent absence of a "suave bedside
manner". But while still a medical student, he was the first to
discover the manner in which the lens of the eye changes shape
(accommodation) in focusing on objects at differing distances.
And it was Thomas Young who, in 1801, explained astigmatism as
resulting from irregularities in the curvature of the cornea. The
early rejection of Young's wave theory of light by his British
contemporaries was due more to chauvinism than to reason: the
particle theory of light was essentially British; the wave theory
of light was essentially French. At the age of 41, Young
abandoned both medicine and physics and devoted himself to an
analysis of the Rosetta Stone. In 1818, he produced a classic
paper on Egypt that laid the groundwork for the later definitive
analysis of the Rosetta Stone by Jean Francois Champollion (1790-
1832).]
-----------
ScienceWeek 2001 26 Jan
-----------
SCIENCE-WEEK 21 Dec 2001 http://scienceweek.com
-------------------
Related Background:
EXPERIMENTAL PHYSICS:
ON EXCEEDING THE VELOCITY OF LIGHT
The term "absolute refractive index" (refractive constant)
refers to the ratio of the speed of electromagnetic radiation in
free space to the speed of the radiation in a particular medium,
with the refractive index varying with the wavelength of the
radiation. (The term "relative refractive index" refers to ratio
of the speed of electromagnetic radiation in one medium to that
in an adjacent medium.)
In general, the term "dispersion" refers to the
decomposition of a beam of white light into colored beams that
spread out to produce a spectrum. More specifically, dispersion
is concerned with descriptions of the variation of refractive
index with wavelength.
Ordinary (normal) dispersion is exhibited by most
transparent substances, which show increasing refractive index
with decreasing wavelength, the variation being greater at
shorter wavelengths.
The term "anomalous dispersion" refers to marked changes in
the curve relating refractive index to wavelength when the
wavelength is near the absorption bands of the material. On the
longer wavelength side of the absorption band, the refractive
index is high; on the shorter wavelength side of the absorption
band, the refractive index is low. Hence, the adjective
"anomalous".
An experimental "light pulse" has a finite duration, and in
theory (the so-called "bandwidth theorem") this requires an
infinite number of waves of different frequency to be added
together. The shorter the desired pulse, the larger the bandwidth
of frequencies that must be used. Theoretically, all light pulses
are therefore formed by a packet of waves of different frequency,
each of which has a different amplitude and phase. The speed of
individual waves is called the "phase velocity", and the velocity
at which the peak of the wave packet propagates is called the
"group velocity". In a vacuum, the phase and group velocities are
identical, but in a highly absorbing or dispersive medium the
phase and group velocities are usually different. A so-called
"negative group velocity" results when the phases of the
different frequency components are shifted by the medium through
which they travel in a way such that the wave packet they form at
the exit is brought forward in time compared with the same pulse
traveling through a vacuum.
During the past two decades, using various techniques,
researchers have demonstrated the transmission of certain light
pulses over short distances with apparent velocities greater than
the speed of light (c =~ 3 x 10^(8) meters per second) in a
vacuum, the effect known as "superluminal light propagation". The
experimental conditions are extremely specific, and the results
difficult of general interpretation. But despite ballyhoo in the
popular media concerning these experiments, the rule in physics
remains intact: no mass can travel faster than (c), and no
information or signal can be transmitted faster than (c). Any
violation of this rule would imply a violation of both Einstein's
theory of special relativity and the principle of causality --
and the rule has not yet been demonstrated to be violated
anywhere.
... ... L.J. Wang et al (3 authors at NEC Research Institute, US)
present a report of experiments on superluminal light
propagation, the authors making the following points:
1) The authors report the use of gain-assisted linear
anomalous dispersion to demonstrate superluminal light
propagation in atomic cesium gas, with the group velocity of a
laser pulse of wavelength near the absorption line of the medium
exceeding (c) and even becoming negative, while the shape of the
pulse is preserved. The authors report they measured a group
velocity index of -310(+-5), which in practice "means that a
light pulse propagating through the atomic vapor cell appears at
the exit side so much earlier than if it had propagated the same
distance in a vacuum, that the peak of the pulse appears to leave
the cell before entering it." The authors suggest the observed
superluminal light pulse propagation is not at odds with
causality, since it is "a direct consequence of classical
interference between its different frequency components in an
anomalous dispersion region [of the curve relating refractive
index to wavelength]."
2) In a commentary on this work in the same journal, Jon
Marangos (Imperial College London, UK) states: "Traditionally,
the signal velocity of a light pulse is defined as the speed at
which the half peak-intensity point on the rising edge of the
waveform travels; in [the Wang et al] experiment, this is clearly
superluminal. In contrast, some researchers argue that the true
speed at which information is carried by a light pulse is not the
group velocity of a smooth pulse, but rather the speed at which a
sudden step-like feature in the waveform travels, which so far
has not been shown to exceed (c)."
-----------
Nature 20 Jul 00 406:243,277
SCIENCE-WEEK 2000 4 Aug
-----------
SCIENCE-WEEK 21 Dec 2001 http://scienceweek.com