## Experimental realization of quantum key distribution.

##### Abstract

Nowadays, the information society that presides the everyday life is dependent on the
communication industry to facilitate unintelligible data transfers between authenticated
parties. Human desire to communicate secretly since the beginnings of the civilisation.
Methods of secret communication were developed by many ancient societies, including
those of Mesopotamia, Egypt, India, China and Japan, but details regarding the origins
of cryptology, i.e. the science and art of secure communication, remain unknown. Secure
communication as well as the protection of sensitive data against unauthorised eavesdropping
are inevitably important. For example, the device, used for communication between
military commanders, consisted of a tapered baton around which was wrapped a spiral
strip of parchment or leather containing the message.
The key is a random sequence of 0’s and 1’s, and therefore the resulting cryptogram, i.e.
the plaintext plus the key, is also random and completely scrambled unless one knows the
key. Indeed, Shannon proved that if the key is secret, the same length as the message,
truly random, and never reused, then the one-time pad is unbreakable. All one-time pads
suffer from serious practical drawback, known as the key distribution problem. The key
itself must be established between the sender and the receiver by means of a very secure
channel for example a very secure telephone line, a private meeting or hand-delivery by a
trusted courrier.
Even if a secure channel is available, this security can never be truly guaranteed, a fondamental
problem remains because any classical private channel can be monitored passively
without the sender or receiver knowing that the eavesdropping has taken place. Since all
information, including cryptographic keys, is encoded in measurable physical properties of
some object or signal, classical theory leaves open the possibility of passive eavesdropping,
because in principle it allows the eavesdropper to measure physical properties without disturbing
them. This is not the case in quantum theory, which forms the basis for quantum
cryptography.
Modern cryptographic practice rests on the use of one-way functions which are easy to
evaluate in the forward direction but infeasible to compute in the reverse direction without
additional information. For example, multiplying large prime numbers can be done in
a time that is a polynomial function of their size, but finding the prime factors of the
product is believed to require exponential time. Factoring the product of two large prime
numbers can be accomplished in polynomial time on a quantum computer. However, the
advancement of computing power and the advent of the quantum computer together with
the vulnerability of this scheme to mathematical progress have prompted the introduction
of quantum cryptography which process through the laws of quantum mechanics, ensures
provably secure data transfers.
The use of physical mechanisms for cryptography is well known in quantum cryptography,
based on the combinations of concept from quantum mechanics and information theory,
i.e. the impossibility of cloning quantum information. The Heisenberg’s uncertainty principle
is exploited to designe an unconditionally secure quantum communications schemes.
Quantum cryptography mades enormous progress in the technology of quantum optics,
optical fibers and free space optical communication. It can be used over a classical communications
channel providing a physical protection to individual bits of information as
well as a hardware implemented solution. The implementation of this theoretical concept
requires much practical innovation for transparent deployment into current cryptographic
solutions.
The theory of quantum cryptography as well as its potential relevance and the application of
prototype system at the University of KwaZulu-Natal are described and the phenomenon
of single-photon interference is used to perform quantum cryptography over an optical
communications link. The method of BB84 (a quantum key distribution protocol that
works with qubits which are two-dimensional) is presented to solve the problem of key
distribution between two parties. Theoretically, BB84 is secured under certain conditions.
The practical of id 3000 Clavis (quantum key distribution system) over installed terrestrial
cables of distances 13,08 km at Cato Manor in Durban between Central Application Office
and Minicipal original Office buildings and 15.6 km in Pinetown between Pinetown Civic
Center and Pinetown Clinic buildings is the proof that the solution to the key distribution
problem is given by quantum cryptography. The experiments in this work are the practical
real quantum key distribution that produces the key which can be shared between two
parties at the distances enunciated above.