Understanding the level structure and population dynamics, such as phonon-induced mixing and intersystem crossing, is important to bring color center in diamond into line with applications in quantum sensing and quantum information processing. For example, the intersystem crossing in nitrogen-vacancy centers enables initialization and read-out of electronic spin state, which is utilized in many applications. We are aiming to gain detailed understanding and develop microscopic models for color centers including the nitrogen- vacancy centers, the silicon-vacancy centers and the germanium-vacancy centers. We perform low-temperature, single-site spectroscopy and embed the results into theoretical models. The developed models can be used to enhance the performance of color center, for example, as quantum sensors.

_{Selected references:
S. Häußler, et al., „Photoluminescence excitation spectroscopy of SiV− and GeV− color center in diamond“. New Journal of Physics 19, 063036 (2017)}

_{M. L. Goldman, et al., „Phonon-Induced Population Dynamics and Intersystem Crossing in Nitrogen-Vacancy Centers“. Physical Review Letters 114, 145502 (2015)}

_{M. L. Goldman, et al., „State-selective intersystem crossing in nitrogen-vacancy centers“. Physical Review B 91, 165201 (2015)}

We are aiming to establish color centers in nanodiamonds as a new building block for solid- state based quantum technology. Our vision is to obtain single color center per one, few tens of nanometer sized, nanodiamond with atom-like optical properties. In our research we employ advanced diamond growth and surface treatment in combination with resonant and off-resonant excitation to isolate single color centers.

_{Selected references:}

_{S. Häußler, et al., „Preparing single SiV− center in nanodiamonds for external, opticalcoupling with access to all degrees of freedom“. New Journal of Physics 21, 103047 (2019)}

_{L. J. Rogers, et al., „Single SiV- centers in low-strain nanodiamonds with bulk-like spectral properties and nano-manipulation capabilities“. Phyical Review Applied 11, 024073 (2019)}

_{U. Jantzen, et al., „Nanodiamonds carrying silicon-vacancy quantum emitters with almost lifetime-limited linewidths“. New Journal of Physics 18, 073036 (2016)}

Applications in quantum information processing and, in particular, quantum nanophotonics rely on single photon sources with high-degree of integrateability. Solid-state quantum emitters in low-dimensional hosts enable completely new architectures with novel designs for integrated quantum nanophotonics. Very promising optically active defect center are hosted in layered hexagonal boron nitride (hBN) with emission wavelength distributed over a large spectral range from 580 nm to 800 nm, high brightness, large Debye-Waller factor, high polarization contrast and high photo-stability. Recently, we have demonstrated resonant excitation and observed Fourier-limited linewidth which are the cornerstones for indistinguishable, single photon emission paving the way for remote entanglement distribution and optical coherent control of quantum states. We are aiming to develop novel quantum nanophotonic platforms for applications in quantum information processing and quantum sensing.

_{Selected references:}

_{M. Hoese et al., „Mechanical decoupling of quantum emitters in hexagonal boron nitride from low-energy phonon modes“. Science Advances 6, eaba6038 (2020)}

_{A. Dietrich et al., „Solid-state single photon source with Fourier transform limited lines at room temperature“. Physical Review B 101, 081401(R) (2020)}

_{A. Dietrich et al., „Observation of Fourier transform limited lines in hexagonal boron nitride“. Physical Review B 98, 081414(R) (2018)}

_{T. T. Tran et al., „Resonant Excitation of Quantum Emitters in Hexagonal Boron Nitride“. ACS Photonics 5, 2, 295–300 (2018)}

The controlled interfacing of individually well-isolated atoms is one of the outstanding challenges for bottom-up assemblies of complex quantum systems. In our research, we build on quantum emitters in a nanometer-sized host matrix and utilize AFM-based nano-manipulation to establish controlled evanescent coupling. In this approach classical photonics and plasmonic devices can be post-processed with quantum functionality. Our work is targeted to develop complex quantum systems in a bottom-up approach.

_{Selected references:}

_{K. G. Fehler, et al., „Purcell-enhanced emission from individual SiV⁻ center in nanodiamonds coupled to a Si3N4-based, photonic crystal cavity“. Nanophotonics 20200257 (2020)}

_{H. Siampour et al., „Ultrabright single-photon emission from germanium-vacancy zero-phonon lines: deterministic emitter-waveguide interfacing at plasmonic hot spots“. Nanophotonics 9, 4, 953-962 (2020)}

_{K. G. Fehler, et al., „Efficient Coupling of an Ensemble of Nitrogen Vacancy (NV^-) to the Mode of a High-Q, Si3N4 Photonic Crystal Cavity“. ACS Nano 13, 6, 6891-6898 (2019)}

Efficient spin-photon interfaces are essential for applications like quantum repeaters and quantum networks. We employ quantum interference of indistinguishable single photons as a resource for entanglement generation between remote spin systems. Color center in diamond offer unique capabilities including long-lived, spin-based memory registers and spin-photon interfaces. In an all-diamond approach we utilize macroscopic solid immersion lenses or all-diamond resonators to boost the efficiency realizing scalable platforms with high photon generation rates. Alternatively, we employ optical Fabry-Perot resonators for Purcell- enhanced photon emission offering high connectivity to existing fiber networks.

_{Selected references:}

_{S. Häußler, et al., „Tunable quantum photonics platform based on fiber-cavity enhanced single photon emission from two-dimensional hBN“. arXiv:2006.13048 (2020)}

_{S. Häußler et al., „Diamond-Photonics Platform Based on Silicon-Vacancy Centers in a Single Crystal Diamond Membrane and a Fiber Cavity“. Physical Review B 99, 165310 (2019)}

_{B. J. M. Hausmann, et al., „Integrated Diamond Networks for Quantum Nanophotonics“. Nano Letters 12, 3, 1578-1582 (2012)}

_{A. Sipahigil, et al., „Quantum interference of single photons from remote nitrogen-vacancy centers in diamond“. Physical Review Letters 108, 143601 (2012)}