## 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.