The calculation of the branching ratio for the inclusive decay $\bar{B} \rightarrow X_s \gamma$ has been an active field of research for multiple decades, yielding results that work very well as a standard candle of the Standard Model of Particle Physics (SM). In this work, we calculate the remaining pieces for the branching ratio of the four-body decay of a b quark into an s quark, a photon $\gamma$ and two additional quarks $q\bar{q}$ at NLO in the strong coupling $\alpha_s$. This calculation involves the one-loop process $b \rightarrow s q \bar{q} \gamma$, with a virtual gluon, as well as the tree-level contribution to $b \rightarrow s q \bar{q} g \gamma$. In this talk, I will highlight the challenges that arise in the computation of the contribution and present a result which allows for a reduction of the theory uncertainty in the $\bar{B} \rightarrow X_s \gamma$ decay rate.

tba

Quantum sensing based on nitrogen-vacancy (NV) centers in diamond is a disruptive technology with immense scientific and technological potential. These sensors can detect faint magnetic fields from nuclear spins for NMR applications [1] and monitor ion interactions in battery cells [2]. To date, however, the most outstanding scientific results have been achieved in complex laboratory setups, whose size and complexity limit the deployment of quantum sensing in industry-relevant fields such as medical technology, automotive, or aerospace. Recognizing this challenge, the scientific community has begun developing portable quantum sensors [3]. A major hurdle is that while lab-based systems allow for full control over all parameters affecting sensitivity, enabling remarkable magnetic field sensitivities of < pT/√Hz, portable sensors inherently lack this level of control, leading to inferior performance. Our research focuses on a specific class of portable devices: endoscopic based quantum sensors with a diamond crystal integrated onto the tip of an optical fiber. This design faces the particular challenge of limited photon collection efficiency through the fiber. To overcome this limitation, we have investigated a novel approach utilizing a multi-core optical fiber design. In our setup, one core is dedicated to the 532 nm laser excitation of the NV centers, while the remaining cores are used exclusively for collecting the emitted fluorescence of the phonen side band. This separation of excitation and collection paths has enabled us to achieve a magnetic field sensitivity of ~150 pT/√Hz. To the best of our knowledge, this result represents a significant improvement over the state-of-the-art for endoscope-based quantum sensors (typically in the nT/√Hz range) and marks a critical step towards practical, high-performance quantum sensing applications. Furthermore, we were able to integrate all required electronic in optical components in a small box, which makes the quantum sensor truly portable. [1] Jonas Meinel et al., High-resolution nanoscale NMR for arbitrary magnetic fields, Communications Physics 6, 302 (2023) [2] Maximilian Hollendonner et al., Quantum Sensing of electric field distributions of liquid electrolytes with NV-centers in nanodiamonds, New J. Phys. 25, 093008 (2023) [3] A. J. Newman et al., Endoscopic diamond magnetometer for cancer surgery, Phys. Rev. Applied 24, 024029 (2025)