[Infowarrior] - Scientists make quantum encryption breakthrough

[Richard Forno]

Scientists make quantum encryption breakthrough

http://www.itpro.co.uk/news/105452/scientists-make-quantum-encryption-breakt
hrough.html

Researchers have managed to close a loophole in quantum cryptography that
could allow a hacker to determine a secret key transmitted using the
technology.

Working at Toshiba Research Europe in Cambridge, scientists found that laser
diodes used to transmit keys used to encrypt data, known as Quantum Key
Distribution (QKD), sometimes transmitted more than one photon at a time.
Quantum encryption works by transmitting key data as a stream of single
photons.

Should an eavesdropper try to intercept the transmission, monitoring a
single photon would change the state of that photon, and this would make
both ends of the transmission aware that the data had been eavesdropped.
However, the laser diodes can sometimes transmit more than one photon and so
a hacker could monitor the second photon, leaving the first photon unchanged
and this would not alert anyone that the key transmission had been
compromised.

But scientists have now added decoy photons to the key data. When an
eavesdropper now tries to monitor extra photons, they will also monitor the
decoy photons. Scientists said these decoy photons or "decoy pulses" are
weaker on average and so very rarely contain two or more photons.

If an eavesdropper attempts a pulse-splitting attack, they will transmit a
lower fraction of these decoy pulses than signal pulses. By monitoring the
transmission of the decoy and signal pulses separately this type of
intervention can be detected, according to scientists.

By introducing decoy pulses, the researcher found that stronger laser pulses
could be used securely, increasing the rate at which keys may be sent. By
using this method keys could be transmitted securely over a 25km fibre to an
average bit rate of 5.5kbits/sec, a hundred-fold increase on previous
efforts.

"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," said Dr Andrew Shields, Quantum Information group leader at Toshiba
Research Europe. "This enables QKD to be used for a number of important
applications such as encryption of high bandwidth data links."

The researchers also discovered a second method to push bit-rates even
higher for QKD. The scientists have 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,
claimed the research.

The single photon diode has a structure similar to an ordinary semiconductor
light emitting diode (LED), but measures 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, showed the multi-photon rate from the
device to be fives times lower than that of a laser diode of the same
intensity.