Cambridge 20 February 2007
• New technologies prevent the theft of secret digital keys on fibre networks
Toshiba Research Europe Ltd announced today that it has developed two new
technologies to realize ‘unconditionally secure’ quantum key distribution (QKD).
To achieve this, Toshiba has overcome a potential security loophole in current
commercial QKD systems. Part of these technologies are exhibited in Nano Tech
2007 in Tokyo.
In principle, quantum key distribution provides an absolutely secure means for
transmitting secret keys between two parties on fibre optical networks. However,
the QKD systems developed so far have a vulnerability which leaves them open to
hacking. The weak laser diode used to generate single photon pulses which 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 could
even determine the entire key, by blocking the single-photon pulses and allowing
only the multi-photon pulses to travel through the fibre.
Now two solutions to this problem have been found, the first of which has already
been implemented by Toshiba in their QKD system.
Toshiba has implemented a new method for QKD, in which the photon signal pulses
are interspersed randomly with a number of “decoy pulses”. These decoy pulses
are weaker on average and so very rarely contain two or more photons. If an
eavesdropper attempts a pulse-splitting attack, she will transmit a lower
fraction of these decoy pulses than signal pulses. Thus by monitoring the
transmission of the decoy and signal pulses separately this type of intervention
can be detected.
By introducing decoy pulses, stronger laser pulses may be used securely,
increasing the rate at which keys may be sent. Toshiba have demonstrated a
100-fold increase in the rate that keys could be transmitted securely over a
25km fibre to an average bit rate of 5.5kbits/sec — the highest value to date
for a full QKD system. This work is part of the EU initiative SECOQC to build a
secure communication network based on QKD.
Dr Andrew Shields, Quantum Information Group Leader at Toshiba Research Europe
Ltd comments; “Using these new methods for QKD we can distribute many more
secret keys per second, while at the same time guaranteeing the unconditional
security of each. This enables QKD to be used for a number of important
applications such as encryption of high bandwidth data links.”
The second method, based on nano-technology, will produce even higher bit rates
in the future. Toshiba has created the first semiconductor diode that can be
controlled with electrical signal input to emit only single photons at a
wavelength compatible with optical fibres. This ‘single photon source’ method
eliminates the problem of multi-photon pulses altogether. It was developed as
part of a DTI funded programme involving the University of Cambridge, Imperial
College London and Toshiba.
The single photon diode has a structure similar to an ordinary semiconductor
light emitting diode (LED), like those used in traffic lights and indicator
lamps, except that it contains a tiny semiconductor quantum dot, measuring just
45 nm in diameter and 10 nm in height. The dot can hold only a few electrons and
so can only ever emit one photon at a time at the selected wavelength. The
source operates with only electrical signals, which is essential for practical
applications such as QKD. Initial trials with the new device, reported recently
in the scientific journal Applied Physics Letters, show the multi-photon rate
from the device is fives times lower than that of a laser diode of the same
intensity.
The Managing Director of Cambridge Research Laboratory in Toshiba Research
Europe, Professor Sir Michael Pepper, expressed his delight at the achievements
of his Laboratory and commented; “We are now entering the quantum age and we are
seeing the first few steps in the development of technologies which will have a
profound effect on the development of communications. Some years ago Toshiba
took the decision to invest and build up a team of experts to pursue fundamental
research in an industrial setting, this breakthrough as well as the entire
development of optically based quantum communications is the result of that
decision.”
BACKGROUND
Cryptography, the science of information security, is essential to protect
electronic business communication and e-commerce, enabling, for example,
confidentiality, identification of users and validation of transactions. All of
these applications rely upon digital keys, which are shared between the
legitimate users, but must be kept secret from everyone else. Maintaining the
ability to distribute keys securely is thus one of the most important
battlefields in the cryptography arms race. It is essential to be able to
distribute keys between users securely. Furthermore, in order to protect the
system from crypto-analysis or key theft it is important to change the keys
frequently.
Quantum cryptography allows users on an optical fibre network to refresh
frequently their keys in a completely secret way. It takes advantage of the
particle-like nature of light. Each bit of the key is encoded upon a single
photon (a light particle) sent down the fibre. As a photon is indivisible and
cannot be copied, this ensures that the key cannot be stolen by an eavesdropper
without the sender's and receiver's knowledge.
Toshiba have developed a quantum key distribution system where the photons travel one way from
sender to receiver, the only configuration that has been rigorously proven
secure. This was achieved using an active stabilization system, which manages
and automatically adjusts the hardware to maintain continuous operation. The
result is an efficient, easy-to-use system, which serves keys for crypto
applications and requires no user adjustments.
About Toshiba
Toshiba is one of the world's leading electronics firms. It is a diversified
manufacturer of advanced electronic and electrical products spanning information
and communications equipment and systems, digital consumer products, electronic
devices and components, power systems, industrial and social infrastructure
systems and home appliances.
Toshiba has an unparalleled track record of pioneering innovation and technology
commercialisation for more than 130 years and has delivered a host of ‘world
firsts’ including the world's first laptop PC in 1985, the technologies for the
world's first DVD player in 1996 and world's first HD DVD player in 2006.
Toshiba has 172,000 employees' worldwide and annual sales of over US$54 billion.
(FY2005). Visit Toshiba's web site at
www.toshiba.co.jp/index.htm
About Toshiba Research Europe Ltd
Toshiba Corporation is fully committed to the research and development of future
technologies. This commitment has resulted in Toshiba having a record number of
world firsts, including the first laptop PC (1985), the first single chip MPEG4
videophone LSI (1998) and the first DVD player (Oct. 1998).
Toshiba Corporation established its first overseas research centre in 1991, with
the opening of Toshiba Cambridge Research Centre Ltd in the UK. This was renamed
Toshiba Research Europe Ltd (TREL) in August 1998, when a new telecommunications
laboratory in Bristol was launched. TREL now has two research laboratories in
the UK; the Cambridge Research Laboratory (CRL) in Cambridge, and
Telecommunications Research Laboratory (TRL) in Bristol.
With security of information more critical than ever, Toshiba's Quantum
Information Group is working on techniques for secure communication based on
quantum cryptography. The team has already achieved a number of world firsts
including the realization of the first link exceeding 100km and a unique
hardware management system that allows continuous operation. They have also
pioneered a single photon generation and detection technology based on
semiconductor quantum dots.
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