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.
Quantum emitter under electron microsope
Colleagues 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
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.
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
, 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 electron.
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
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.
Lots of dots: quantum dots as synthesized.
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, -263 Celsius).
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 are noise.
Viterbi School electrical engineer Alan Willner 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.