Cavity-Driven Multispectral Gain for High-Sensitivity NV Center Magnetometers

TL;DR

This work tackles the challenge of achieving high-sensitivity magnetometry withNV centers at ambient conditions by coupling a dense NV ensemble to a dielectric cavity and driving it strongly to realize Autler-Townes splitting and Mollow triplets. The server multispectral approach yields a nine-peak landscape of doubly dressed states, from which coherence enhances magnetic-field sensitivity by about a factor of three, achieving $12~\mathrm{pT}/\sqrt{\mathrm{Hz}}$ at room temperature. A cascaded Tavis–Cummings model accurately describes the observed spectra and predicts near-term sensitivities as low as $100~\mathrm{fT}/\sqrt{\mathrm{Hz}}$, approaching the Johnson–Nyquist limit of roughly $97~\mathrm{fT}/\sqrt{\mathrm{Hz}}$. This frequency-multiplexed, coherence-based paradigm offers a scalable path to robust quantum metrology in ambient environments and can be extended to multimode or frequency-comb cavities for further gains.

Abstract

We report a cavity-enabled solid-state magnetometer based on an NV ensemble coupled with a dielectric cavity, achieving 12 pT/$\sqrt{\rm{Hz}}$ sensitivity and a nearly threefold gain from multispectral features. The features originate from cavity-induced splitting of the NV hyperfine levels and leverages robust quantum coherence in the doubly dressed states of the system to achieve high sensitivity. We project simulated near-term sensitivities approaching 100 fT/$\sqrt{\rm{Hz}}$, close to the Johnson-Nyquist limit. Our results establish frequency multiplexing as a new operational paradigm, offering a robust and scalable quantum resource for metrology under ambient conditions.

Figures (4)

• Figure 1: (a) Schematic of the experimental setup showing a diamond inside a dielectric cavity, housed inside an Aluminum shell and placed between magnets. Other components include low-noise amplifier (LNA), local oscillator (LO), low-pass filter (LPF), lock-in amplifier (LIA) and National Instruments data acquisition (NI DAQ) system. (b) Energy level diagram of an NV spin state, with frequency $\omega_s = \omega_j$ and nuclear spin $m_I$, dressed with a strong classical field, with frequency $\omega_d$. The dressed states are $\ket{\lambda_{\pm,n}}$ and the transitions clearly show the Mollow triplets. Here, $\Delta_j = \omega_j - \omega_d$ and $\Omega$ is the driving strength or the Rabi frequency. The double dressing occurs when the spin states hybridize with a cavity photon of frequency $\omega_c$, leading to states $\ket{\lambda_{-,n},0} \pm \ket{\lambda_{+,n-1},1}$. This leads to additional splitting and new transition lines, governed by the spin-cavity coupling $g_\text{ens}$.
• Figure 2: Amplitude modulated ODMR spectrum. Measurements at 1.5kHz modulation frequency, 100m lock-in time constant, and -45dBm power. The plots in (a,c) show normalized experimental and simulated ODMR spectra, respectively, for sweeping spin and drive frequency detunings, $\Delta_s$ and $\Delta_c$. Plot (b) shows the spectra for values of $\Delta_s$ along the two horizontal lines in (a), represented by red-cross and blue-plus symbols in the scatter plot. The solid lines correspond to the simulated spectra for the same values. The scatter plot in (d) shows the same, but for three vertical lines in (a), represented by red-cross, yellow-circle and blue-plus symbols.
• Figure 3: Homodyne measurements. Plots (a) and (b) show experimentally measured averaged reflected quadrature spectrum and simulated values, respectively. Experimental measurements were done for 0dBm LO power, -40dBm excitation power, and 700kHz LPF bandwidth. The spin and drive detuning $\Delta_d$ and $\Delta_s$, with respect to cavity frequency, are adjusted in steps of 10kHz and 5.2kHz. Plot (c) shows the quadrature spectrum along the horizontal cut lines for experimental (orange) and simulation (blue) data. The green dotted straight lines are used to calculate spectral slope. Plot (d) shows magnetically equivalent noise density spectrum showing an experimental sensitivity of 12pT√Hz.
• Figure 4: Theoretically projected sensitivity. The plot shows variation of sensitivity with $g_{\rm{ens}}$, for $\Gamma=250$ kHz (solid-blue) and $\Gamma=167$ kHz (orange-dash-dot). The blue-dashed and orange-dotted lines show sensitivity with a multispectral 3 fold enhancement due to coherent, doubly dressed states. Experimentally measured sensitivities 28 pT/$\sqrt{\rm{Hz}}$ (red) and 12 pT/$\sqrt{\rm{Hz}}$ (green) are for single and multiple peaks measurements. Near term projection show sensitivity as high as 100 fT/$\sqrt{\rm{Hz}}$ for $g_{\rm{ens}}\approx$ 0.26 MHz and $\Gamma=167$ kHz.