Speedy photon detector debuts
Technology Research News
human eye is sensitive enough to detect a few or even single photons of
light. Many groups of researchers have been working on devices designed
to automatically detect individual photons quickly and efficiently.
Improving single photon detection would improve medical imaging
and diagnosis, chemical analysis, laser ranging and timing, sensing applications,
fiber optic testing, and quantum cryptography. Quantum cryptography is a
method of sending secure encryption codes using the properties of photons
to represent the ones and zeros of digital information.
Researchers from Toshiba Research Europe and the University of Cambridge
have devised a fast, efficient photon detector that senses individual photons.
"We have invented a new type of detector which responds to individual photons
-- the particles of light," said Andrew Shields, a group leader at Toshiba
A candle produces about one billion billion photons per second.
The researchers' device is a semiconductor resonant tunneling diode
that contains a layer of quantum dots. A resonant tunneling diode limits
electrical current to one direction and to discrete amounts. Quantum dots
are nanoscale bits of semiconductor that act like artificial atoms, trapping
one or a few electrons. The quantum dots used by the device measure 30 nanometers
wide and eight nanometers tall, or more than two orders of magnitude smaller
than a red blood cell. A nanometer is one millionth of a millimeter.
When a single hole, or positive charge left by the absence of a
negatively-charged electron, is excited by a single photon, the hole combines
with and so neutralizes an electron trapped in one of the quantum dots.
"The capture of a hole excited by the photon by one of the dots can switch
the magnitude of the current flowing through device," said Shields. This
increase in the amount of current flowing through the device can be sensed,
allowing the photon to be detected.
The photon detector is faster and more efficient than existing single
photon detectors that use transistors, including a combination transistor-quantum
dot device previously demonstrated by the researchers, said Shields. The
prototype has an efficiency of 12.5 percent and can detect a new photon
every 150 nanoseconds, or billionths of a second, giving the device a theoretical
data rate of 5 megahertz.
This is faster and an order of magnitude more efficient than the
researchers' transistor-based quantum dot device, said Shields. Existing
detectors, which make single photons trigger avalanches of electrons, have
higher efficiencies but are slower and more prone to noise.
Making the resonant tunneling diode detector's layers thinner and
applying an antireflection coating could boost the device's efficiency to
65 percent and its speed as high as 100 megahertz, according to the researchers.
"We are particularly interested in using the device for quantum
cryptography," said Shields. The researchers recently demonstrated a quantum
key distribution system using conventional single photon detectors that
secured a video conferencing transmission. "We plan to incorporate the quantum
dot resonant tunneling diode detector, which will further enhance the key
exchange rate," he said.
Quantum cryptography techniques that use single photons to represent
each bit of information are theoretically perfectly secure because information
can be stored in a photon in one of two ways, and it is only possible to
read the information stored in one way before the photon is destroyed. If
an eavesdropper intercepted a key and tried to send duplicate photons on
to the recipient, the eavesdropper would have to guess at half of the bits
and it would be apparent to the sender and receiver that the key was compromised
because a large percentage would be different from those sent.
The device is especially appropriate for quantum cryptography applications
because it has a very low dark count, or false positive rate, said Shields.
The photon detector could be ready for practical use in two to three
years, said Shields.
Shields' research colleagues were James Blakesley of Toshiba Research
Europe and the University of Cambridge, Patrick See and B. E. Kardynal of
Toshiba Research Europe, and P. Atkinson, I. Farrer and D. A. Ritchie of
the University of Cambridge. The work appeared in the February 18, 2005
issue of Physical Review Letters. The research was funded by the
Engineering and Physical Sciences Research Council (EPSRC) and the European
Timeline: 1-3 years
TRN Categories: Optical Computing, Optoelectronics and Photonics;
Quantum Computing and Communications
Story Type: News
Related Elements: Technical paper, "Efficient Single Photon
Detection by Quantum Dot Resonant Tunneling Diodes," Physical Review Letters,
February 16, 2005
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