Quantum information processing is a rapidly growing field that  exploits the rules of quantum  mechanics as applied to physical systems on a microscopic scale.  Quantum information  is represented  by the state  vector of a micro- scopic system.  The state vector is a mathematical representation describing the characteristics of the system.  This division deals with the evolution  of the  state  vector  through  time.   It  is abroad  field  and  includes amongst others quantum  cryptography and quantum  computing

Evolution of Entanglement  in the Presence of Noise

Entanglement plays a key role in quantum  computation as well as in quantum cryptography. However, it is a fragile resource when distributed through noisy quantum  channels.   Most Quantum  Key Distribution (QKD)  proto- cols using single photons  can  be  transcribed  into protocols using pairs of entangled  photons.   The  transcribed  versions  can then  be investigated  in order to proof or disproof the security against  evesdropping of the original protocols which depends crucially on the amount of entanglement which can be  distributed via the optical  fibres used to transport the single photons. Moreover entanglement is an essential ingredient in quantum  computing.  In the context  of quantum computing  with colour centres in diamonds  one of our aims is to investigate  the sensitivity of entanglement present in cluster states  which are required for one-way quantum  computing.

We found an evolution  equation  for entanglement of qubit  pairs where one qubit  is  sent  through  a noisy quantum  channel.  This equation  shows for pure states  and general qubit  channels that  the entanglement after the channel is proportional to initial entanglement. The proportionality factor completely  characterizes  the  channel  as far as  entanglement  reduction  is concerned.

We are currently  working on a generalization  of the formula for entanglement decay to higher dimensional bipartite systems.  A further  general- ization to multipartite system is planned.

Heralding photons from imperfect single-photon sources

We investigated  a set-up to herald single photons using two-photon absorp- tion.   For  this  purpose  we consider  single-photon  sources which produce arbitrarily polarized  single-photon  states  with  an  admixture   of vacuum. We envisage to  use two  such  sources  in order  to  create  an  admixture  of photonic  states  of the same polarization  in which one part  is a two-photon Fock state.   A two-photon  absorbing  medium can be used  to enhance the amplitude  for the  resulting  two-photon  part  in which one of the  photons is horizontally  polarized while the  other  one is vertically  polarized.   This strategy enables us to deflect the vertically  polarized photon  and to detect only the horizontally polarized one. The detection  can only happen if there were two  photons  in the  loop  set-up.    Thus  a detection  event  heralds  a vertically  polarized photon  without destroying it.

We are currently optimizing the parameters using a more realistic model of two-photon  absorption.     Our  strategy  of maximising  the  efficiency of heralding single photons  is closely related  to the Quantum  Zeno effect.

Spin entanglement  with NV centers in diamonds

Our research of Nitrogen-Vacancy  centers  in diamonds  has shown advan- tages  and  limitations  of these  systems  which we used to  develop a draft proposal for experiments to prepare  and dectect  entanglement in multiple- party states.  These are important on the way to implement simple quantum  algorithms.  Combining EPR and NMR techniques we designed a method to create  so-called GHZ states  (containing  four-partite entanglement)  which involve spins of remote  NV centers  as well as nuclear  spins of neighboring carbon atoms.  GHZ states  show highly non-classical behaviour,  especially nonlocal correlations and can be utilized for fundamental  tests  of quantum  theory.  They serve in our context as a testing ground on the way to prepare cluster states  which can be used to preform quantum  computing.

Improving the Efficiency of Single-Photon Sources

Present single-photon sources do not generate single photons with certainty. Instead  they  produce  statistical mixtures  of single photons  and  vacuum. The probability  that  a particular single-photon source emits a single photon is called the efficiency of the source. The central aim of this research division is the  development  of methods  to process the  output of imperfect  single- photon  sources in order to detect  and announce  with high probability  the production  of single photons  and thus improve the efficiency of the source.

Protecting  Quantum  Correlations

Quantum information  technology  largely  relies on a precious  and  fragile resource,  quantum  entanglement, a form of super correlation  between mi- croscopic systems, which exceeds all classical relations.  However, entangle- ment,  when employed for information processing purposes, is often dimin- ished by uncontrolled  degrees of freedom in the processing equipment.  The Centre  has developed a method  to analyze the reduction  of  entanglement and investigate  tools to protect  information  carriers against  it.

Measurement and Control of Quantum  Systems

In order  to study  quantum  effects and  control  quantum  systems  individ- ually,  CQT  is  developing systems  to  monitor  the  dynamics  of individual quantum  systems  in  real-time.   Further research  is being conducted  into control techniques for dynamics based on a feedback system for continuous estimation  of system properties. These methods may contribute to coherent control and error correction  required in QIPC.