In ultracold quantum gases, the interactions between the individual atoms can be controlled by applying magnetic bias fields. As magnetic field fluctuations limit the precision here, typically a feedback loop needs to be employed to regulate the current through a pair of Helmholtz coils. No commercially available magnetic field sensor allows to measure large fields directly with high enough precision, leading to many unsatisfactory solutions being used in experiments. Here, we demonstrate a direct magnetic field stabilization in a regime previously not accessible, using NV centers as the magnetic field sensor. This allows us to measure and stabilize fields of 4.66 mT down to 12 nT RMS noise over the course of 24 h, measured on a 1 Hz bandwidth. We achieve a control of better than 1 ppm after 20 min of integration time, ensuring high long-term stability for experiments. This approach extends direct magnetic field control to strong magnetic fields, which could enable new precise quantum simulations in this regime.

We observe cavity-enhanced ensemble fluorescence of nitrogen-vacancy centers in diamond, showing Purcell-enhancement of the Zero Phonon Line (ZPL). The emission features temporal bunching of ZPL photons, indicating collective emission that can be attributed to superfluorescence.

We demonstrate photoassisted cold field emission (PFE) from a tungsten tip induced by tunable-wavelength low-power femtosecond laser excitation. The emission current from the apex of the (310)-oriented single-crystalline emitter is shown to linearly depend on the incident laser power, while the effective work function is reduced by the respective photon energy. Our results promote the implementation of the linear regime in laser-triggered cold field emission for ultrafast transmission electron microscopy.

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