# Quantum Information Processing and Communication

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 a broad 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.