Quantum Physics May Changs Optical Communication

http://www.physnews.com/showlink.php?id=52039

Quantum Physics Discovery May Bring About Changes in Optical Communication

October 28, 2005

Results from experiments conducted at the University of California, Santa
Barbara may lead to profound changes in optical communications. The
discovery is reported in the October 28th edition of the journal Science.

Physicist Mark Sherwin at UCSB explained that as information technology
advances, scientists are intent on transmitting information much more
quickly. "We are working toward sending information 100 times faster than
it can be sent now," he said. His research group has spent five years on
this project. The experiments were performed using the university's
room-sized, free-electron laser.

"We took an existing semiconductor device that is essentially an
electrically controlled shutter and we have tried to open and close the
shutter at the rate of three trillion times a second," he explained. "We
found that in addition to opening and closing the shutter we are making the
shutter itself vibrate."

Those vibrations of the shutter may enable the shutter to be opened and
closed with weak light beams rather than strong voltages, said Sherwin. In
optical communications there are different channels of communications, so
these light beams could correspond to different channels. "It would be a
way of changing channels really fast," he added. "Right now it is a very
slow process to change channels in optical communications.

Sherwin explained that electronics are much slower than optics and that one
optical fiber could transmit information more than 1,000 times as fast as
the information could be put on it by an electronic device like a computer.

"What we have here at UCSB is a special source of radiation, the
free-electron laser, that can generate electromagnetic oscillations at the
rate of a few trillion per second," said Sherwin. "We found that when you
drive the modulator, or shutter, that fast it acts in a peculiar way.
Rather than absorbing light near a single frequency, it can absorb light
near a second frequency as well. This opens the possibility of a new type
of cross modulation, where a beam of light at one of the absorption
frequencies can turn on or off the light of the other."

Sherwin said that light has been used to send information rapidly over long
distances for more than 3000 years. The ancient Greeks, for example, used
large fires to flash signals from mountain top to mountain top, as
described by Homer in the Iliad. In order to send information, light must
be modulated—that is, one must be able to turn the light beam on and off.
In World War II, ships communicated with one another in code using
searchlights that sailors modulated manually with shutters. Modern
modulators for light are controlled by electrical voltages, explained Sherwin.

"In an electro-absorption modulator, light near a particular frequency, the
carrier frequency, can be blocked or transmitted by tuning a material
oscillation in or out of resonance with the carrier frequency," said
Sherwin. "A common electro-absorption modulator is made of a semiconductor
quantum well, a thin layer of a semiconductor with a relatively small "band
gap" (or a relatively large affinity for negatively charged electrons and
positively charged holes) sandwiched between two layers with a larger band
gap."

Sherwin explained that when light of the correct frequency is incident on a
quantum well, it creates bound electron-hole pairs called excitons and is
absorbed. An electric field applied perpendicular to the plane of the
quantum well shifts the frequency of the excitonic absorption so that light
resonant with the zero-field excitonic resonance is no longer absorbed.
Quantum well electro-absorption modulators are currently used to modulate
light at rates exceeding 10 billion bits per second.

In this article, the scientists report that a quantum well
electro-absorption modulator has been strongly driven at frequencies
exceeding one Terahertz (1 trillion cycles). This is more than 100 times
faster than quantum well modulators are usually operated. At these
extremely high frequencies, internal quantum-mechanical oscillations of the
excitons themselves were excited. When the strong Terahertz drive was
resonant with the excitonic oscillations, the absorption spectrum of weak
light near the excitonic absorption of the quantum well was transformed
from a single peak to a double peak, or doublet. This doublet is a
signature that light with frequency near the excitonic absorption can no
longer simply create an exciton in its lowest-energy state, but must create
a quantum mechanical superposition of an exciton in its ground and excited
states.

A potential application for optical communication is that two arbitrarily
weak light beams separated by the frequency of the Terahertz drive could
modulate one another. "Usually, such cross-modulation occurs only when
light beams have power exceeding a certain threshold," said Sherwin.

On a separate note, Sherwin said, "In atomic gases, the doublet observed
here has been the first step toward creating a system that could greatly
slow or even stop the propagation of light. The ability to slow or stop
light in a semiconductor would also enhance the toolbox for optical
communications and computation. However, in order to achieve slowing or
stopping of light, the mechanisms for energy dissipation in the quantum
well modulator would have to be significantly reduced."

The Science article, "Quantum Coherence in an Optical Modulator," was
co-authored by S. G. Carter, who worked on the experiments at UCSB and then
moved to the University of Colorado; V. Birkedal, from UCSB; C. S. Wang,
from UCSB; L. A. Coldren, from UCSB; A. V. Maslov, from the Center for
Nanotechnology at the NASA Ames Research Center; and, D. S. Citrin from the
Georgia Institute of Technology and Georgia Tech Lorraine in Metz, France.

The research was funded by the National Science Foundation.

Source: University of California, Santa Barbara