Single
particles of light, also known as photons, have been produced and
implemented into a quantum key distribution (QKD) link, paving the way
for unbreakable communication networks.
The
results of the experiment, undertaken by a close collaboration of
researchers based in Wuerzburg, Munich and Stuttgart, have been
published today, Thursday 2 August, in the Institute of Physics and
German Physical Society’s New Journal of Physics.
The
single photons were produced using two devices made of semiconductor
nanostructures that emitted a photon each time they were excited by an
electrical pulse. The two devices were made up of different
semiconductor materials so they emitted photons with different colours.
QKD
is not a new phenomenon and has been in commercial use for several
years; one of its first uses was to encode the national election ballot
results in Switzerland in 2007. The techniques currently being used on a
commercial scale rely on lasers to create the source of photons;
however, researchers hope to further increase the efficiency of QKD by
returning to the original concept of using single photons for generating
a secure key.
One
of the project coordinators, Dr Sven Hoefling, said: “The nature of
light emitted by lasers is very different from light emitted by single
photon sources. Whereas the emission events in lasers occur completely
random in time, an ideal single photon source emits exactly one photon
upon a trigger event, which in our case is an electrical pulse.
“The
random nature of emission events from strongly attenuated lasers
sometimes results in the emission of two photons very close to each
other. Such multiple photon events can be utilized by an eavesdropper to
extract information.
“Single
photon sources, such as the ones used in our study, are predestined for
use in the secure communication systems using quantum communication
protocols.”
QKD
is a process that enables two parties, ‘Alice’ and ‘Bob’, to share a
secret key that can then be used to protect data they want to send to
each other. The secret key is made up of a stream of photons that ‘spin’
in different directions – vertically, horizontally or diagonally –
according to the sender’s preferences.
The
laws of physics state that it is not possible to measure the state, or
‘spin’, of a particle like a photon without altering it, so if ‘Eve’
attempted to intercept the key that was sent between ‘Alice’ and ‘Bob’,
it would become instantly noticeable.
In
their experiment, the single photons were produced with high
efficiency, then made into a key and successfully transmitted from the
sender to the receiver across 40 cm of free space in the laboratory.
The
researchers are aware that to make this experiment more practical and
commercially viable, it needs to be scaled up so that quantum keys can
be sent over larger distances. To do this, quantum repeater stations
need to be incorporated into the network to ‘amplify’ the message.
“Meanwhile,
quantum keys have been sent over 500 m of free space on top of the
roofs in the centre of Munich, Germany. Several projects have also been
funded to develop this technology further,” continued Hoefling.
Source: Institute of Physics
