One of the central goals is to enhance light-matter interactions to realize efficient light-matter interfaces at the single quantum level, and to enable novel schemes for spectroscopy and sensing. We employ and further develop fiber-based Fabry-Perot microcavities, which combine microscopic mode volumes with exceptionally high quality factors, and at the same time offer open access for a variety of samples.

Rare earth ions provide exceptional optical and hyperfine coherence, which makes them promising candidates for quantum information applications, ranging from quantum memories and quantum computing nodes to quantum-nonlinear optics. However, the dipole-forbidden nature of the narrow transitions results in ultra-low emission rates, limiting most experiments to macroscopic ensembles. We want to gain efficient access to individual ions and small ensembles by coupling them to a high finesse optical microcavity. Purcell enhancement can boost the emission rate by several orders of magnitude, thereby making the weak transitions bright and enabling efficient state readout. We use microcavities based on laser-machined optical fibers as mirror substrates, which combine high finesse with small mode cross sections and full tunability.

The fiber-opposing planar mirror incorporates the rare-earth ion doped nanocrystals in the cavity (Fig.1). Addressing of individual ions is achieved in the spectral domain via a stabilized, narrowband laser. This scheme makes use of the large ratio between the broad inhomogeneous crystal linewidth versus a narrow homogeneous single ion linewidth, opening up the possibility to have efficient access to possibly thousands of qubits contained in one cavity mode.

As a first step, we have performed cavity-enhanced fluorescence spectroscopy of individual nanocrystals, allowing the detection of signals from small ensembles of about 10 Europium ions [Casabone et al.]. We were able to demonstrate multi-modal Purcell enhancement and to measure the inhomogeneous and homogeneous linewidth of small ensembles within a single nanoparticle [Eichhorn et al. 2024]. Within a collaboration with Hugues de Riedmatten, this approach could be used to demonstrate strong and dynamically switchable Purcell enhancement of Erbium ions in a nanocrystal [Casabone 2020]. Harnessing emission enhancement by up to hundred-fold, also single Erbium ions with narrow linewidths could be observed [Deshmukh et al. 2023].

Molecular lanthanide complexes

Recently, we started to study molecular complexes based on Europium which show remarkable optical coherence and long-lived nuclear spin states [Serrano 2022, Kuppusamy 2023 Schlittenhardt 2024, Kuppusamy 2024]. Using optical polarization and readout of nuclear spin states together with radio-frequency driving, we were able to demonstrate optically detected nuclear magnetic resonance (ODNMR) and coherent nuclear spin control in an Eu-based molecular complex [Vasilenko 2026].

Synthetic chemistry opens up the possibility to tailor the optical and spin properties, such that optimized photon emission and spin coherence can be specifically targeted. Furthermore, molecules can be arranged in regular arrays such as molecular crystals or supra-molecular assemblies, e.g. via self-assembly. This opens up the perspective for the controlled realization of scalable qubit registers.

Motivation
A long-distance quantum network requires a quantum repeater, and a central ingredient is an efficient, coherent spin–photon interface. Coupling single color centers in diamond to a microcavity is a promising route toward this goal.

The cavity platform
We integrate a diamond membrane into an open-access fiber-based Fabry–Pérot microcavity [Heupel 2020] to enhance emission into a single, well-collectable mode and to provide spectral filtering. Simulations predict a strong enhancement of the zero-phonon-line (ZPL) emission efficiency, up to 80%. We performed spatially resolved characterizations of coupled cavity–membrane devices and observed significant effects of the diamond membrane on the cavity mode properties [Körber et al]. We have also integrated such devices cryogenically, achieving excellent mechanical stability down to the single-pm level for operation under resonance conditions [Pallmann 2023].

Collective emission of NV centers
Coupling ensembles of NV centers to the cavity, we observe under cryogenic conditions Purcell-enhanced emission, a super-linear power dependence, and pronounced bunching in the autocorrelation function [Pallmann 2024]. These features appear only when the zero-phonon line is coupled to the cavity; the phonon sideband shows a flat g⁽²⁾. In a confocal microscope with fewer selected emitters, antibunching becomes visible. This demonstrates the collective emission of few, incoherent, spatially separated emitters coupled through a common cavity mode [Qu 2025].

Coherent control of SnV spins
In collaboration with Wolfgang Wernsdorfer and Christoph Becher, we study the tin-vacancy (SnV) center in diamond as a promising alternative. We observe Fourier-limited, long-term stable optical transitions, a long spin lifetime at sub-Kelvin temperatures, and coherent spin control, achieved by straining the diamond or using a superconducting waveguide. This yields electron spin coherence of up to 10 ms under dynamical decoupling [Karapatzakis 2024]. We can also detect, initialize, and coherently control a neighboring ¹³C nuclear spin, demonstrating coherence times above one second, and achieving single-qubit gate fidelity of 99.92% [Resch 2026].

Cooperative coupling of SnV centers to a cavity
We are coupling SnV centers to a fiber cavity to realize an efficient, coherent spin–photon interface. In a recent experiment [Köster2026], we achieved high-cooperativity coupling in a fully tunable microcavity operating down to 1 K with in-situ magnetic field control. Integrating low-roughness diamond membranes (sub-0.2 nm rms) gave access to the diamond-like regime, where the field is concentrated in the diamond and Purcell enhancement is maximized—more than doubling the effective Purcell factor (C₀ = 4.1 vs. 1.85) while relaxing mechanical stability requirements. Resonant probing revealed coherent coupling with 96% extinction contrast and a cooperativity of C ≈ 4.0. Applying a magnetic field enabled spin-resolved cavity extinction, resolving the two spin-selective transitions with 91% contrast—establishing SnV centers in open microcavities as a promising platform for efficient, scalable spin–photon interfaces.

Motivation: sensing individual nanosystems
Detecting and characterizing individual non-fluorescent nanosystems with high temporal resolution can reveal their intrinsic properties and dynamics. To reach the high sensitivity needed to study single, unlabelled nanosystems in solution, we exploit signal enhancement in a high-finesse (F ~ 10⁵) fiber-based Fabry–Pérot microcavity integrated into a microfluidic channel. A nanoobject passing through the tight focus of the cavity mode (w₀ ~ 1 µm) is detected via the frequency shift and amplitude decrease of the resonant cavity transmission induced by its excess polarizability.

3D Tracking of Brownian motion
Besides determining a nanoobject's size and refractive index, we can track its position by modulating the cavity length at high frequency and recording the resonance frequency and linewidth of several transverse cavity modes. Because these modes have different radial shapes, this yields spatial information that allows us to track the particle's position with high temporal resolution in three dimensions [Kohler et al., Nature Commun. (2021)].

Accurate sizing down to 3 nm
We recently demonstrated quantitative single-particle sizing with high precision and accuracy down to particle diameters of 3 nm [Palkhivala et al., ACS Nano 2025]. We developed an analytical model of the autocorrelation function for particle diffusion in our standing-wave sensing geometry and introduced a method that accounts for the transient nature of single-particle signals. This provides a high-bandwidth, label-free cavity-based sensing tool that captures all relevant time scales of translational diffusion and delivers quantitative information about individual nanoparticles in solution.

Carbon nanotubes (CNTs) offer an intriguing combination of optical, electronic, and mechanical properties. We develop different techniques of cavity-enhanced spectroscopy to characterize individual CNTs. The figure shows exemplary spatially resolved, cavity-enhanced Raman spectroscopy to obtain spatial maps of molecular-fingerprint information (right image). At the same time, cavity-enhanced absorption microscopy allows us to quantitatively determine the extinction cross section (left image). Investigation of the details of Raman modes gives insight into electronic and structural properties. (Hümmer et al., Nature Commun. (2016)). In a different direction, localized excitons in CNTs can be useful as quantum light source. We work on the Purcell enhancement of telecom single photon emission from chemically functionalized CNTs.

Atomically thin crystals of transition metal dichalcogenides (TMDs) host excitons with strong binding energies and sizable light-matter interactions. Coupled to optical cavities, monolayer TMDs routinely reach the regime of strong light-matter coupling, where excitons and photons admix coherently to form quasiparticles known as polaritons up to room temperature. We explore the two-dimensional nature of TMD polaritons with cavity-assisted hyperspectral imaging. We record a spatial map of polariton properties of extended WS₂ monolayers coupled to a tunable micro cavity in the strong coupling regime, and correlate it with maps of exciton extinction and fluorescence taken from the same flake with the cavity. We find a high level of homogeneity, and show that polariton splitting variations are correlated with intrinsic exciton properties such as oscillator strength and linewidth. Moreover, we observe a deviation from thermal equilibrium in the resonant polariton population, which we ascribe to non-Markovian polariton-phonon coupling. Our measurements reveal a promisingly consistent polariton landscape, and highlight the importance of phonons for future polaritonic devices. [Gebhardt et al., Sci. Rep. (2019)]

Analogous concepts for hybrid light-matter systems employing spatially indirect excitons with a permanent electric dipole moment in heterobilayer crystals promise realizations of exciton-polariton gases and condensates with inherent dipolar interactions. Here, we implement cavity-control of interlayer excitons in vertical MoSe₂-WSe₂ heterostructures. Our experiments demonstrate the Purcell effect for heterobilayer emission in cavity-modified photonic environments, and quantify the light-matter coupling strength of interlayer excitons. The results will facilitate further developments of dipolar exciton-polariton gases and condensates in hybrid cavity – van der Waals heterostructure systems. [Förg et al., Nature Commun. (2019)]

Improving the sensitivity of optical microscopy could give access to the optical properties of a large class of interesting nanoscale systems on a single particle level. To boost sensitivity, we are developing a novel scanning microscopy technique that harnesses multiple interactions of probe light with an object. This is realized by placing the sample inside a high-finesse optical microcavity, which provides a signal enhancement on the order of the cavity Finesse (10⁴ – 10⁵). Raster-scanning the sample through the cavity mode enables us to obtain spatial maps of sample absorption, dispersion, and birefringence of individual nanoparticles.

The method should enable e.g. the spectroscopic characterization of individual nanoparticles, dispersive sensing with high spatial resolution, and the detection of non-fluorescent single molecules. [Mader et al., Nature Commun (2015), Hümmer et al., Nature Commun. (2016), Mader et al., ACS Photonics (2022)].