Laser-enhanced quantum sensing boosts sensitivity and dynamic range.

While magnetometers based on nitrogen-vacancy (NV) centers in diamond have emerged as the most important solid-state quantum sensors, ensembles are typically limited to a few percent optical contrast. Here, we demonstrate a laser threshold magnetometry-based NV system that achieves 100% optical contrast in combination with strong output signals up to 50 mW. By integrating the NV centers into a laser cavity, we show magnetic-field-dependent shifts of the laser threshold, allowing the laser system to be entirely switched off with the NV centers' magnetic resonance. We determine the photon-shot-noise fundamental sensitivity limit to be  < 400 fT/ Hz for all vector components and demonstrate that it improves super-linearly with contrast. This sensitivity is not achieved at the usual cost of smaller linewidth, i.e., maintains good intrinsic dynamic range. This provides a perspective for highly sensitive magnetometry operated outside magnetically-shielded rooms for applications including magnetoencephalography, magnetic navigation, and magnetic anomaly detection.