The chromium chalcogen halides (CrXH) are an emerging family of 2D semiconductors with a direct bandgap, highly anisotropic structural and electronic properties, and robust magnetic order with ordering temperatures up to room temperature. We seek to understand and engineer interactions between the magnetic order in CrXH compounds and the excitons they host. The unique magnetic and optical properties of CrXH materials present unique opportunities to develop spintronic devices with magnetic and optical controllability.

Superlattices are artificial, periodic potentials which are used to confined particles and quasiparticles such as atoms, electrons, or excitons. If the trapped quasiparticles are allowed to interact, the resulting many-body state can capture essential behaviors of canonical quantum many-body theories (e.g. the Hubbard model). Such artificial quasiparticle lattices are often called analogue quantum simulators. A bit like a quantum wind tunnel, the quantum simulator serves as an engineerable scale model of a real physical system (electrons or other particles in a real crystalline solid) which allows us to probe complicated many-body physics with a high level of control. Such many-body physics are often hard to describe or predict theoretically, so our quantum simulators provide a route to benchmark leading theories and discover new phases of matter beyond current theoretical predictions.

We generate optically active defects in atomically thin 2D materials on a spatial scale of below 10 nm, such that scalable opto/electronics based on individual atomic states seems at reach. We use a helium-ion microscope (HIM) to generate individual defects, e.g. in semiconducting MoS₂. Our single-defect technology is principally applicable to the wide range of 2D materials with more than a thousand different materials and is compatible with standard cleanroom manufacturing steps. This combination makes it possible to realize first devices based on single defects, such as gate-switchable single photon emitters and photodetectors or even solar cells based on individual atomic defects. At the same time, fundamental processes, such as the coherent single electron tunneling dynamics and many-body interactions of localized states in a Fermion boson mixture are experimentally accessible.

Collaborators: Jonathan Finley (TUM), Kai Müller (TUM), Nicolas Leitherer-Stenger (DTU), Sivan Refaely-Abramson (Weizmann), Alex Weber-Bargioni (Berkeley)

Increasing the interaction strength between quasi-particles in solid-state materials can cause strong correlations, collective phenomena and the transition to macroscopic quantum phases, such as a Bose-Einstein condensate. Heterostructures made of semiconducting 2D materials, such as MoSe₂ and WSe₂, are ideal systems to realize interacting exciton ensembles, because they provide large exciton binding energies, long photoluminescence lifetimes, as well as a permanent exciton dipole. The latter allows the manipulation of the exciton ensembles, e.g. via electric fields. Recently, we have reported on several signatures regarding photoluminescence intensity, linewidth, as well as spatial and temporal coherences in accordance with the predicted degeneracy of an exciton ensemble at low temperature. The ongoing studies want to explore further predicted phases, such as an excitonic superfluidity and possible condensation phenomena in momentum space.

Collaborators: Ursula Wurstbauer (University of Münster), Andreas Knorr (TU Berlin).

While 2D semiconductors offer unique electronic and optical properties, realization of their full potential requires scalable growth and controlled integration into functional assemblies. In our research, we aim to create defined 2D/3D heterostructures using chemical vapor deposition (CVD) of transition metal dichalcogenides (TMDs) onto engineered substrates. These heterostructures include thin film junctions and laterally patterned substrates that enable precise spatial control of the 2D material and its electronic environment. In addition, we take advantage of low-temperature atomic layer deposition (ALD) processes to precisely tune interaction lengths and strengths between TMDs and their surroundings. A major emphasis of this work on 2D semiconductor assemblies is devoted to elucidating interfacial mechanisms that govern charge transfer pathways, energetic alignment, and structural and chemical interactions, providing a basis for future applications spanning from advanced optoelectronics to photocatalytic energy conversion.

Superconducting Nanowire Single-Photon Detectors (SNSPDs) are cutting-edge devices that enable the reliable detection of individual photons. They are formed by nano-patterning thin superconducting films to form nanowires wires.  When operated at cryogenic temperatures, they deliver unmatched performance for quantum light detection with high efficiency, low noise, phonon-number resolution and picosecond photon timing precision. Thanks to their speed and sensitivity across a wide range of wavelengths spanning the visible to infrared, SNSPDs are a key technology for quantum communication, secure data transfer, advanced imaging, and fundamental scientific research.  In the WSI we work closely with Munich Quantum Instruments GmbH and the QEC Group led by Kai Müller in the framework of several third-party research projects developing the materials technologies for SNSPDs made from polycrystalline superconducting thin films such as MoSi and NbTiN, as well as emergent van der Waals superconductors like NbSe₂.  We aim to understand the microscopic mechanisms governing the photo-detection process and apply e.g. local He-ion irradiation to enhance detector performance.  Detectors are integrated into quantum photonic circuits with much scope for emergent quantum photonic technologies.   

The Nanoscale Quantum Magnetometer is a scanning probe microscope based on a single nitrogen vacancy (NV) center implanted in a diamond tip. The spin and optical dipole of the NV center can be used to measure stray electric and magnetic fields with nanoscale spatial resolution and exceptional dynamic range. The overarching goal of this instrument is to enable the study of nanoscale quantum materials and hybrid quantum systems using the high-resolution - and crucially - the multi-modal spectroscopy offered by tip-based NV quantum sensing. The unique proposed system moves the boundary of the experimental state-of-the-art by enabling tip-based, time resolved single spin spectroscopy at low temperatures in high magnetic fields. These capabilities are unique but necessary in order to manipulate and investigate the spin-and charge configurations of quantum matter. Moreover, the instrument has a sophisticated optical design to enable advanced optical spectroscopy experiments to correlate in real-time the nanoscale NV probe measurements with global observables. This project will enable completely new insight into the challenging but critical phase space from single particle interactions to a global quantum phase, and therefore overcomes the main experimental roadblocks to understand emergence in quantum matter. 

The overarching goal of the Munich Initiative for Cross-Correlative Spin Microscopy (MICroCoSM) is to facilitate the study of nanoscale quantum materials and hybrid quantum systems using the high-resolution - and crucially - the multi-modal spectroscopy offered by tip-based nitrogen vacancy (NV) quantum sensing.  The unique system installed in the labs of WSI moves the boundary of the experimental state-of-the-art by enabling tip-based, time resolved single spin spectroscopy at low temperatures in high magnetic fields.  This is done using a low-temperature version of the ProteusQ scanning NV microscope from Qnami as a Spin-based Quantum Light Microscope (SQLM).  It provides nanoscale access to magnetic properties of nano-materials subject to electrical / optical perturbations.  Feel free to reach out to Prof. Finley if you are interested!

Besides their spectacular optical, spin and electronic properties 2D materials can also exhibit ferroelectric order that arises as a consequence of the stacking order of van-der-Waals (vdW) layered materials and polarisation fields that emerge within the unit cell of the crystal.  Such ferroelectricity can be controlled by strain & sliding of the component vdW layers of the materials.  We explore high-quality 3R-stacked polytypes of TMDs that exhibit out-of-plane ferroelectricity.  In 3R-TMD bilayers, sliding one monolayer a few Ångströms over the other flips the ferroelectric polarisation, a new ‘slidetronics’ concept in condensed matter physics inspired by twistronics.  Such materials are deal for nanoscale systems in which ferroelectric order can be controlled with light, providing exciting new opportunities for optoelectronics, neuromorphic photonics, and sensing.

Integrated quantum photonics is pivotal for the implementation of quantum technologies and their transition from a laboratory to a real-life context. A photonic circuit may comprise one or more single photon sources and diverse components, such as waveguides, beam splitters, interferometers etc. required for steering and processing the quantum signals produced by the sources. In order to insure performance stability, as well as to minimise energy consumption, photonic circuits should integrate as many components as possible on a same chip. To this respect, a critical hurdle is represented by the necessity to assemble different materials that accomplish different tasks. In the VALERIA project our long term vision is to realise a single-material photonic platform exploiting the nano-structuring of transition metal dichalcogenides. The demonstration of this vision will be the fabrication of all-in-one devices made of a single TMD bulk crystal embedding site-controlled single photon emitters integrated with photonic crystal cavities and coupled to waveguides, beamsplitters amd non linear optical components.