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arXiv:2512.24951

Laser intracavity absorption magnetometry for optical quantum sensing

J. M. Wollenberg, F. Perona, A. Palaci, H. Wenzel, H. Christopher, A. Knigge, W. Knolle, J. M. Bopp, T. Schröder

quant-phSubmitted Dec 31, 2025• Announced Jan 1, 2026
v1 · Dec 31, 2025v2 · Aug 28, 2026
arXivHTMLPDF

Abstract

Intracavity absorption spectroscopy (ICAS) is a well-established technique for detecting weak absorption signals with ultrahigh sensitivity. Here, we extend this concept to magnetometry using nitrogen-vacancy (NV) centers in diamond. We introduce laser intracavity absorption magnetometry (LICAM), a concept that is in principle applicable to a broader class of optical quantum sensors, including optically pumped magnetometers. Using an electrically driven, edge-emitting diode laser that operates self-sustainably, we show that LICAM enables highly sensitive magnetometers operating under ambient conditions. Near the lasing threshold, we achieve a 475-fold enhancement in optical contrast and a 180-fold improvement in magnetic sensitivity compared with a conventional single-pass geometry. The experimental results are accurately described by a rate-equation model for single-mode diode lasers. From our measurements, we determine a projected shot-noise-limited sensitivity in the $\mathrm{pT}\,\mathrm{Hz}^{-1/2}$ range and show that, with realistic device improvements, sensitivities down to the $\mathrm{fT}\,\mathrm{Hz}^{-1/2}$ scale are attainable.

Comments
8 pages, 5 figures

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Nitrogen-vacancy centers (NV) in diamond are promising quantum systems for magnetic field sensing. The sensitivity and the linearity of such a quantum sensor can be greatly improved by using stimulated emission of the NV centers in the concept of laser threshold magnetometry (LTM), which is projected to reach the fT/sqrt(Hz) regime. Previous implementations of NV centers in optical cavities relied on external seed lasers, pulsed operation, or sensing via NVabsorption. In our work, we combine the NV centers with a second gain medium, a laser diode, within the same cavity, achieving self-sustainable continuous-wave lasing. This approach compensates the intrinsic losses of the cavity and the diamond, through a fixed additional gain below the threshold of the laser system. A continuous-wave laser threshold and a linewidth narrowing is observed with increasing pump power on the NV centers of the combined laser system. An improved laser system shows a magnetic field-dependent laser threshold, which is the basis for improved-sensitivity NV magnetometry via LTM.
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Nitrogen-vacancy (NV) centers in diamond are a promising platform for high-precision magnetometry. In contrast to the use of spontaneous emission in a number of NV-magnetometers, laser threshold magnetometry exploits stimulated emission of NV centers by placing an NV-doped diamond inside an optical cavity. The NV laser system is predicted to reach a high magnetic-field-dependent contrast and coherent signal strength, leading to an improved magnetic field sensitivity combined with high linearity. Here, we consider a two-media setup where the cavity additionally includes a vertical external cavity surface emitting a laser. This optically active material compensates cavity losses at 750 nm while still allowing for magnetic-field-dependent effects from the NV-diamond. We demonstrate a magnetic-field-dependent laser threshold and investigate the effects of pump laser induced absorption of the diamond. The experimental data are supported by an analytical simulation based on a rate model. Furthermore, we derive a generalized formula to compute the shot-noise-limited magnetic field sensitivity in the regime of high contrast, yielding 49.07(0.33)pT/Hz for the present setup. Simulations with an optimized NV-diamond suggest that values down to 4.9fT/Hz are possible.