4. QKD implementation technologies
Implementation is a way to express an idea. Classically all
ideas along communication are protocols. There are softwares built for
interchange of messages and there exist many protocols which have dif-ferents
functions. To build such software, is what we call implementation. We named
protocols like : http, Smtp, ftp. In quantum, we
described the implementation of BB84 which is privilegie protocol composed by
two sets of non-orthogonal basis and four polarizations. The security of
protocols in quantum cryptography depend on the production of single photon.
There is an art for implementing a new technology with QKD. We will need same
notions like the notions of single photon EPR1 photon source and
photon stabilized interferometer. Those notions are so important for the
implementation of QKD technologies. Quantum technology allows us to organise
and control the components of a complex system governed by the laws of quantum
physics. We have two imperatives driving quantum technology:
1. Technological innovation of miniaturisation,
2. Performance over the classical framework.
Since many years, human did act with matter, they could
explain many phenomenon like periodic table, atoms but they were not be able to
manipulate the behavior of semiconductors. In the second, we see the
implication of human. There are appropriate tools for QKD like sources and
detectors are the mains tools for implementing the QKD technology.
4.1 Sources for QKD
There exist many sources for a QKD. That sources correspond to
the technologies that you want to implement.
1. Single photon : This is a main element for implementation,
the photons have been used in some other fields like quantum optics and quantum
communications. People did experience to perfect a photon applying a single
photon for those subfield of quantum. Photon-gun which is a single-photon, is a
unique photon sources. This elements is given significative progress achieved
in the last decade. In term of emitting information, the number of photons
prove to precise certain number of matters for transmission. There are some
experiments that allows application of laser but indeed, the laser cannot
produce photon number states because the implementation source for a QKD need a
special kind of photon. It is why we are talking about single-photon, that must
have a precise number of photons. This can be understood as follows: the gain
medium of a conventional laser contains a large number of individual atoms or
molecules and each of them has a certain probability to emit a photon during a
fixed time period as said in [QQL10a]. It was found that a single-photon
include atoms or ion gas phase, organic molecules, colour center in crystals,
semiconductor, nanocrystals, quantum dots.
2. EPR photon pair : For this source, the main problem is the
efficent production of photon. The use of entanglement photon pair allow
optical processes and the production of photon pairs need
1EPR : Ekert Protocol
Section 4.1. Sources for QKD Page 22
a spontaneous parametric down-conversion (SPDC). The
conversations of energy and momentum imply that the generated daughter photons
are entangled in spectral and spatial domains. The production of photon pair
here, have two steps using the main toll which is non-linear crystal. Let's see
how its look like the production of photon pair by the figure below.
Figure 4.1: EPR source
(a) Firstly, the first crystal can be selected to phase match
vertical pump photon with horizontal down-converted photon pair
(b) Secondly, the second one will be chosen to phase match
horizontal pump down-converted photon pair
The two crystals will be done with 45 % that to get a high
quality of polarization entangle.
3. Attenuated Laser This source, is where Eavesdrpper use same
probability notion to get the information in the quantum channel. The
difference between the first source is the use of linear relation also, we stay
using single-photon. Laser is used as an attack for Eavesdropper. To produce
single-photon for high quality, this model of source have certains probability
to get non-zero probability to get more or than one photon. This experience did
by people is called photon number Splitting
(PNS) attack. The
functionnality of this attack can be describe well in the article of [QQL10b].
It can be detected by testing quantum channel during QKD process.
Figure 4.2: Laser source
Section 4.2. Detectors Page 23
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