Quantum Key Distribution and Quantum Optics

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.

Quantum Optics

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.

Quantum Networks

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