# Secure quantum computing in quantum networks

My research focuses on quantum cloud computing and addresses the question how distributed
quantum computing can be realized in quantum networks. Cloud computing - the storage and processing of data on remote servers - has become a cornerstone of the present information age. As an increasing amount of commercial, public and private data is processed at remote servers, preserving the privacy of the data and the computation becomes ever more important. In the face of the challenges inherent to constructing a quantum computer, it is conceivable that quantum-computing capabilities may for some time be limited to a few specialized facilities around the world. Similar to classical cloud computing, central remote quantum servers might be used to store and process data, and clients will outsource their quantum computations to these servers. In both the classical and quantum setting, data security is a key issue for distributed computing. We have shown experimentally that quantum computers not only offer speed-ups in data processing, but also allow one to preserve the privacy of a computation, thus solving a key challenge of distributed computation. We have demonstrated for the first time how a nearly-classical client can access the resources of a more computationally-powerful quantum server without divulging the content of the requested computation. This work thus addresses one of the most important challenges of our society - how to make distributed computing secure. These results have been published in Science 335, 303-308 (2012). |

# Foundations and verification of quantum computing

The promise of quantum computers is that they can solve problems intractable for classical computers such
as the simulation of complex materials. This superiority of quantum computers opens up new questions,
namely how the results of quantum computations can be verified if they cannot be computed by classical
computers. We have developed a method for the verification of quantum computations and have for the first time demonstrated such a verification in an experiment. In this work, We have shown how an entity that has access only to classical computers can verify a complex quantum computation. This forward-looking results is highly relevant in the context of large-scale quantum computations and quantum simulations. The results of this study have been published in Nature Physics 9, 727 (2013). |

# Implementation of new quantum algorithms

The implementation of quantum algorithms is another focus of my research. For example, quantum
computers can solve systems of linear equations exponentially faster than classical computers. As large
systems of linear equations appear in almost every scientific discipline, this is one of the most promising applications of a quantum computer. We have implemented this quantum algorithm for the first time in an experiment and have shown how various instances of this algorithm can be computed using a photonic quantum computing architecture. These results are important since they show an important application of quantum computers in the future. This work has been published in Scientific Reports 4, 6115 (2014). |

# Quantum error correction

In realistic scenarios for quantum information processing, noise causes errors and might cause the quantum
computation to fail. Thus, these errors have to be taken into account and correct for any application of
quantum information processing. Using a photonic system as a model platform, We have shown how quantum information processing can be implemented in the presence of noise, which constitutes an important building block for for realistic scenarios of quantum computing [Phys. Rev. A 90, 042302 (2014)]. |

# Photonic quantum simulation

One of the most important applications of quantum computers will be the simulation of other complex
quantum systems. These simulations will be pf particular importance for the analysis and development
of new materials. We have implemented such quantum simulations experimentally and have studied the simulation of the properties of two spins in an magnetic field. This work is first simulation of this model using linear optics and opens the door to future simulation of complex materials [arXiv:1410.1099]. |

# Photonic quantum technology

Another focus of my research is technical aspects of quantum information processing as my aim is to
bring quantum technology to a level where it can be used for everyday applications. We have performed two
experiments in this context, which demonstrate steps towards scalable quantum computing with photons. The first experiment deals with the generation of photon pairs for quantum information processing experiments. These photonic experiments are usually limited by the process of photon generation which is probabilistic. Our experiment has overcome this limitation and has shown that photon pairs can be generated in an heralded manner. These photon pairs are "polarization-entangled" and form the basis for many quantum information processing tasks. This work has been published in Nature Photonics 4, 553 (2010) . The second experiment in the field of quantum technology has aimed for the realization of universal photonic quantum computers. These universal quantum computers are designed to be able to implement arbitrary quantum algorithms - instead of being designed to solve only a particular problem. We have been able to show for the first time how two consecutive quantum gates can be applied to the same pair of quantum bits. This experiment is an important technological advance, which brings photonic quantum information closer to a comprehensive control [Scientific Reports 4, 6115 (2014)]. |