Quantum cryptography, or more correctly named, Quantum Key Distribu- tion (QKD), uses the theory of quantum mechanics to secure the informa- tion, by generating a secret, random key between the two parties over an insecure communication channel. QKD is based on two fundamental princi- ples of quantum mechanics, namely the Heisenbergs Uncertainty Principle and the Superposition Principle. The securely distributed key is used to implement a one-time pad encryption scheme. Since the key is used only once and is completely random, it cannot be predicted beforehand by the sender, receiver or eavesdropper. The major advantage of QKD is that the one-time pad encryption scheme provides unconditional security over an insecure communication channel. The CQT is currently developing a QKD system for low-cost applications as well as investigating new QKD proto- cols. The cryptographic system is designed for the transfer of quantum information through the use of phase-encoded single photons over optical fibre.
Single-Photon Sources and Detectors
An essential obstacle and key concern in quantum cryptography and opti- cal quantum computing is the generation of single photons. CQT has been investigating various implementations for solutions to the above. Defects in diamonds have recently been shown to provide an effective means of gener- ating single photons on demand, with an efficiency of over 90%. Diamonds have further been shown to be promising candidates for a solid state quan- tum computer. The principle of operation is associated with the electron and nuclei spin states within a particular implanted defect. Another method of achieving pseudo-single photons is to attenuate a laser pulse, such that on average there is less than one photon per pulse. Although suitable for Quantum Key Distribution, weak laser pulses are not truly single photon sources, but rather a Poisson distribution of photon number states. Such a photon source has been realized and a prototype developed by the CQT. Detecting single photons requires specialised optics and electronics; optimi- sation of the signal from an avalanche photodiode to detect single photons is a key aspect in this research.
The critical nature of secure communication in our society has prompted CQT to explore beyond a two-node QKD setup. Quantum networks facili- tate QKD ’on-demand’ between two arbitrary network users. They consist of a number of hybrid quantum channels integrated to form a complete net- work. This is essential for the optimization of network throughput as some quantum channels are better suited to particular terrains. There is also greater robustness against Denial of Service attacks due to redundant light paths within a network. This also assists in the reduction of key relation knowledge by any adversary. A vast number of topologies are currently being envisaged, many based on present day network topologies. This will allow for transparent integration into current networks. These networks will, at present, be valuable to the finance and communication sectors; however with the continuous improvements of computational power and the inadvertent realization of the Quantum Computer, QKD will become necessary for any form of secure data transfer, thus paving the way for the development of Quantum Cities and national and international Quantum Grids