September 2nd, 2005 — One single photon.
One solitary quantum pulse of electromagnetic
radiation, no more, no less, produced
by one single electron, will be the product
of a new device under construction by
nanotechnologists at the USC Viterbi
School of Engineering.
at the University of Texas/Austin will
build the USC device's counterpart, a
detector for that single pulse, as their
part of a joint $1.3 million study just
funded by the National Science Foundation.
The interdisciplinary team includes three
members of the National Academy of Engineering.
John D. O'Brien of the Viterbi School's
electrical engineering department, principal
investigator in the project, says the
ultimate goal is
to use such singleton photons in cryptographic devices and, ultimately, general
purpose computers, as part of the continuing
search for smaller, faster, and more
efficient information processing devices.
The award is part a new NSF progam encouraging Nanoscale
Interdisciplinary Research Teams (NIRTs),
which is part of of NSF's Nanoscale Science
and Engineering programs. "This is an ambitious project that requires
an exceptionally broad range of expertise in numerous electrical
engineering disciplines," says Viterbi School Dean Yannis Yortsos. "Swift
success in a project this bold is never guaranteed, but I am extremely
proud we have been able to assemble an in-house team that has the
background to even attempt it."
O'Brien says that theory, and particularly a
classic paper by mathematician Peter Shor , indicate that a computational
device using quantum phenomena to represent information should be able to
perform certain tasks, particularly securely encrypting and decrypting messages,
far faster than traditional chips. A 2001 paper by Emanuel Knill, Raymond
Laflamme and Gerard Milburn suggested that such a machine could be made using
devices that created (and detected) single photons.
But realizing the real-world photon machine has
proved a forbiddingly difficult task. As O'Brien's
detailed paper describing the project notes, "to
work, these systems must be isolated from noise
to an almost unheard of degree."
Fittingly, the USC/UT effort begins in the centenary
year of Albert Einstein's classic 1905 paper explaining
the photoelectric effect, the paper that laid the
foundations for quantum understandings of mass and
The “quantum dots” that the USC team will use to generate single photons,
one at a time, are ultra-small (“nanoscale”) devices that perform the photoelectric
process Einstein explained in reverse. The dots are minute particles of
a highly engineered semiconductor material. Classic photoelectric materials
produce electric current — electrons — when struck by sufficiently energetic
photons, in a mechanism Einstein explained. The same mechanism, working
in reverse, sends out a single photon when energized by an electron.
While single photon emitters have been built before,
the USC model is designed as a model of Einsteinian
economy. The excitation will come from one single
The USC group will use expertise accumulated over
decades in the Compound Semiconductor Laboratory
of USC National Academy of Engineering member P.
Daniel Dapkus. Dapkus, who holds W.M. Keck chair
in the Viterbi School department of electrical
engineering, decades ago pioneered the creation
of the quantum well laser devices, considered
tiny at the time
He subsequently moved on to nanotechnology and with
collaborators including O'Brien learned to grow extremely
regular arrays of quantum dots, looking in electron
microscope photos like a field of seedling trees,
using a variation of the lithographic processes now
used to create chips.
To turn a mesa containing an array of such dots into a single photon signal
device, an array of microscopic photonic crystal resonant cavities is built
in the mesa. Each resonance cavity will contain a single quantum dot.
Creating the crystal is only the first step. To activate it
in a useful way, an elaborate electronic control system is
needed, which will feed a single electron of precisely the
correct electric potential into the system at precisely the
right time. This
potential is so minute that, to avoid introduction of potentially
stray electrons into the system, the electronics will function
at extremely low temperature -- 10 Kelvin, (-441 Fahrenheit,
Using resonance effects, the group hopes to speed
up the rate of production of single photons, so that
the process happens in 100 picoseconds -- ten times
faster than existing devices. (100 picoseconds are
to one second what one second is to 317 years).
The interface to classical electronics will be
designed by Anthony F.J. Levi, who has joint appointments
in the Viterbi School and the USC College of Letters,
Arts, and Science. He specializes in
Adaptive Quantum Design -- that is, creating systems that can
work at the quantum, nanoscale level, as well as
in nanoscale manufacturing. Levi's systems will
process the single photon signals using beam splitters
and wave guides that will be able to verify which
of the photons detected are signals, and which
Viterbi School electrical engineer Alan Wilner
is an expert on photonic transfer of information.
He will be studying how far single photon quantum
information can be transmitted, how it happens,
and what can be done to protect it: "I want to
enable the information to be transmitted over longer
distances in as pristine a state as possible."
National Academy of Engineering member William
Lindsey will also contribute his communications
expertise to the project, investigating how the
classic insights of Claude Shannon apply when information
is coded not as electronic bits but rather as "qubits" — quantum
bits. Specifically, "I am specifying the Shannon-equivalent communications
capacity that defines limits on the number of classical information
bit per qubit that can be sent error free through a communications
channel disturbed by thermal noise." Lindsey and his
students are also developing single photon synchronization
requirements and achievable performance from a systems perspective.
Another Viterbi EE department faculty member, Todd
Brun, provides the theoretical support for the
project. An expert in quantum
information processing, he will develop theoretical models of the
single photon sources and detectors, assess their properties, and
develop designs for quantum gates, circuits, and communication channels,
in collaboration with the experimenters. Brun was the
first of several theorists in quantum information processing
to be hired recently by USC.
The detectors themselves will be created in the University
of Texas/Austin Microelectronics Research Laboratory
by a group led by National Academy of Engineering
member Joseph Campbell, who holds a Cockrell Family
Regents chair in Engineering.