Spin-photon interface for color centers in diamond

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.