The word "quantum" can conjure images of scientists working on
technology that will one day allow time travel, or, at the very
least, permit travel to alternate realities.
Although such developments may currently be confined to an
episode of Dr. Who,
the
use of quantum information technology in business computing will
become a reality within the next ten years, says Andrew
Shields, leader of the quantum information group at
Toshiba's
Cambridge Research lab.
Shields and his team have made a breakthrough that they say will
allow information to be captured and returned to the user in a
meaningful form after having been processed by a quantum IT
program.
Shields says this would allow future computers to process
complex computations in four months. This may seem like a long
time, but it would take a 1Ghz computer of today's standards longer
than the age of the universe to perform the same calculations.
In traditional computing, a bit is a fundamental unit of
information, represented in binary as 0 or 1 and stored as an "on"
or "off" signal on an electronic circuit. If you search a database
for a particular result, the computer program methodically analyses
each row of data in the database, bit by bit, for a positive match.
Where there is a match, the database returns a value of "true"
(where true is the binary state "1") or "false" (binary state "0")
for no match.
In a quantum computer, the fundamental unit of information is a
qubit. Qubits are stored on particles of light called photons. A
qubit can exist not only in a state corresponding to the logical
values 0 or 1, but also in states corresponding to a blend or
superposition of these values.
When searching a database in quantum computing, the program
would be able to "see" all results at once from a superposition
instantly, rather than having to move through each row.
"The net effect of quantum computing is that data can be
processed much quicker than under a traditional computing model,"
says Shields.
The other benefit of quantum computing is that it can enable
secure communications over optical fibre links. This is where
scientists at Toshiba labs believe they have made progress.
They say they have developed the first
practical semiconductor device that can count the particles or
photons in light signals.
The new device is a significant step towards viable quantum
computers and communication systems, which exploit the
particle-like properties of light.
Counting photons is necessary when sending secret digital keys
over long distances.
"A simple semiconductor device that can count the photons in a
light signal is important for several quantum applications. You can
prevent eavesdropping on optical fibre links by counting if the
photons have been interfered with," says Shields.
Lack of a suitable photon number resolving detector has been a
major obstacle to real-world deployment of quantum technologies,
says Shields.
In principle, quantum key distribution provides a secure means
for transmitting secret keys between two parties on fibre optical
networks. However, the
QKD
systems developed so far are vulnerable to hacking.
The weak laser diode used to generate single photon pulses that
carry the quantum keys, will sometimes generate pulses with
multiple photons.
As a result, an eavesdropper could split off one of these extra
photons and measure it, while leaving the other photons in the
pulse undisturbed, thus determining part of the key while remaining
undetected.
Furthermore, an eavesdropper can determine the entire key, by
blocking the single-photon pulses and allowing only the
multi-photon pulses to travel through the fibre.
"Using these new methods for QKD we can distribute more secret
keys per second, while at the same time guaranteeing their
unconditional security. This enables QKD to be used for a number of
important applications such as encryption of high bandwidth data
links," says Shields.
Until now the most common semiconductor detector, the avalanche
photo-diode, has been able to register only the presence or absence
of one or more photons.
The detector, developed by Toshiba, however, is able to count
the number of individual photons in a pulse. It is the first
practical device with this capability.
The breakthrough is a result of a new technique developed by
Toshiba to detect weak photon-induced avalanches. The electrical
current caused by a single photon in a semiconductor is too weak to
be detected quickly.
Avalanche photo-diodes work by amplifying this current a
million-fold using an avalanche effect. Usually, however, the
strength of the final current does not depend on the number of
photons that initiated it.
The Toshiba device can detect photon-induced avalanches 20 times
weaker than conventionally, and the strength of which scale with
the incident number of photons.
It is a step in the right direction for quantum computing but
there are many challenges that must be overcome before quantum
computing becomes mainstream.
"We need lots of new technology for quantum computing to take
off, such as quantum memory," says Shields.
Shields says that quantum memory can store information in
quantum states. However, this is something the industry does not
currently have. Quantum logic gates and sensors that can accurately
detect quantum states are also pre-requisites.
Nevertheless, Shields remains confident that with the
developments of his lab, it will be only a matter of time before we
are all taking a quantum leap forward in computing.