Narrow-band Multi-photon Generation for Quantum Information Applications

The harness of quantum mechanical phenomena allows for new quantum technology that holds the promise to advance a variety of information processing tasks. In particular, single photons are a promising quantum system due to their robustness and mobility. This project aims to develop a new single-photon source that emits multiple photons whose frequency is very well defined the photons have an extreme narrow-bandwidth. This opens the path for using the frequency-spectrum and the associated coherence length of photons, i.e. the length of which photons can interfere, as additional and novel feature for complex quantum information tasks. Within this project we will generate multiphoton states in a controlled manner, where each photons coherence length is in the order of meters for performing quantum computations that rely on complex interferometric networks. In addition we will perform so-called loophole- free quantum computations that are based on the superposition of the order of quantum gates. These experiments will not only provide new insights in complex quantum computations but also break the ground for boosting the applicability of optical quantum computing when using narrow-bandwidth photons. The narrow linewidth of the generated multiphoton states is also suitable for resonant coupling to atomic transitions, which is important for tailored light matter interactions. We will build our source in a compact and portable arrangement for investigating two-photon quantum computer gates that use atoms as medium. This will be done in collaboration with the research group of Arno Rauschenbeutel at the Technische Universität.

The project "Narrow-band Multi-photon Generation for Quantum Information Applications" has made a significant contribution to the development of photon sources for special applications in the field of quantum communication and quantum computers. The main focus of this project was the development of specially tailored single photons, which could effectively couple with atoms. Such couplings are of great interest for future optical quantum computers, since the atoms can be used as a medium to enable efficient photon-photon quantum computer gates. Therefore, within the scope of this research project a compact and portable photon source was built that could also be used for other experiments. The developed design of this photon source showed a number of advantages over existing concepts and was therefore successfully patented. Unfortunately, due to the travel restrictions caused by COVID-19, the planned scientific collaborations did not take place until the end of this project. The experience gained in the quantum control of photon sources based on spontaneous parametric fluorescence was direct use for other experiments, contributing to a total of six peer-reviewed publications. Of particular international importance was the demonstration of counterfactual communication, implemented through the usage of an adjustable nanophotonic processor. In addition, this FWF-Project supported the development and demonstration of an integrated, all-optical photon-photon quantum computer gate. With regard to quantum communication, this project was able to support the demonstration of 2-way-communication, although only one light particle was used. The results of this project have led to a new application of tailor-made photons and thus to the further development of optical quantum technology.