Temporal filtered quantum sensing with the nitrogen-vacancy center in diamond

TL;DR

The paper tackles the challenge of NV-based sensing in high-background environments by combining pulsed excitation with time-gating to suppress fast, background fluorescence while preserving spin-dependent NV signals. It develops a theory of temporal filtering, defines the SNR metric $SNR = \frac{N_0 - N_1}{\sqrt{N_0 + N_1}}$ and the enhancement factor $EF_{SNR}$, and demonstrates how the gate window $\tau_c$ and repetition rate $f_L$ dictate performance. Experimentally, it shows substantial SNR improvements: up to $EF_{SNR} \approx 2$ in bulk NV/SiV ensembles (with a $\sim4$× speedup) and up to $EF_{SNR} \approx 4$ for fluorescent nanodiamonds on nitrocellulose (with a $\sim16$× speedup), using both TCSPC-based and hardware time-gating approaches. The work provides a practical, scalable route to faster, more robust NV-based biosensing in complex environments, including real-time hardware gating that circumvents the need for TCSPC and enables broader application in in vitro diagnostics and wide-field imaging.

Abstract

Nitrogen vacancy centers in diamond are among the leading solid state quantum platforms, offering exceptional spatial resolution and sensitivity for applications such as magnetic field sensing, thermometry, and bioimaging. However, in high background environments,such as those encountered in in vitro diagnostics, the performance of NV based sensors can be compromised by strong background fluorescence, particularly from substrates such as nitrocellulose. In this work, we analytically and experimentally investigate the use of pulsed laser excitation combined with time gating techniques to suppress background fluorescence and enhance the signal to noise ratio in NV based quantum sensing, with an emphasis on spin enhanced biosensing. Through experimental studies using mixed ensembles of silicon vacancy and NV centers in bulk diamond, as well as fluorescent nanodiamonds on NC substrates, we demonstrate significant improvements in NV spin resonance visibility, demonstrated by an increase of the SNR by up to 4x, and a resulting measurement time reduction by 16x. The presented technique and results here can help significantly increase the readout efficiency and speed in future applications of NV centers in high background environments, such as in IVD, where the NV centers are used as a fluorescent label for biomolecules.

Figures (8)

• Figure 1: (a) Simplified level structure of the NV center, showing the effective lifetime $\tau$ of the spin sublevels $m_S = 0$ and $m_S = \pm 1$, due to the excited state decay rate being the sum of the radiative decay rate and inter-system crossing rate (solid lines show radiative transitions, and broken lines non-radiative transitions). (b) Analytic representation of decay curves with different fluorescence lifetimes $\tau_{m_S = 0}=12\,$ns (red solid line), and $\tau_{m_S = \pm1}=8\,$ns (blue dashed line) of the $m_S=0$ and $m_S=\pm1$ spin sublevels, as well as the fast decay curve of a background source with a shorter lifetime of $\tau_\text{BG}=3\,$ns (grey dotted line and the shaded area). The temporal differences allow for high contrast detection of spin resonance with proper time-gating. (c) Principle of pulsed laser CW-ODMR and temporal filtering. The detected TCSPC curve shows the experimentally detected signal from a sample which exhibits NV fluorescence, as well as a strong short-lifetime background.
• Figure 2: Calculated ODMR contrast and sensitivity with different gating window and laser repetition rate, with fluorescence lifetimes $\tau_{m_S = 0}=12\,$ns, and $\tau_{m_S = \pm1}=8\,$ns of the $m_S=0$ and $m_S=\pm1$ spin sublevels, and the fast decay curve of a background source with a shorter lifetime of $\tau_\text{BG}=1.7\,$ns. (a) ODMR contrast and the shot noise component $\sqrt{N_{0} + N_{1}}$ when varying the onset time of the gating window $\tau_\text{c}$, and (b) SNR enhancement calculated with \ref{['eq:SNR']} and \ref{['eq:SNR_enhancement']}, and the magnetic field sensitivity calculated with \ref{['eq:sens']} when varying $\tau_\text{c}$. The noise levels are depicted as the amplitude ratios between the fast decay noise and NV fluorescence decay. Simulated ODMR contrast and shot noise component (c), and the SNR enhancement as well as the magnetic field sensitivity (d) as a function of the laser repetition rate $f_{\text{L}}$ with $\tau_\text{c}=10\,$ns.
• Figure 3: (a) TSPC signal with contribution from SiV and NV centers. The strong short lifetime component is mainly due to SiV centers ($\tau_\text{SiV}\approx1.7\,$ns), and the long-lifetime component from NV centers ($\tau_\text{NV}\approx(11-12)\,$ns). (b) Fluorescence recorded from the sample, showing the contribution of both NV and SiV color centers. (c) Resulting ODMR contrast $C = (N_{0} - N_{1})/N_{0}$, and shot noise component $\sqrt{N_{0} + N_{1}}$ when varying the onset time of the gating window $\tau_\text{c}$. The dashed line shows the ideal value of $\tau_\text{c} = 6\,$ns. (d) The resulting SNR enhancement $EF_\text{SNR}=\frac{\text{SNR}_\text{gated}}{\text{SNR}_\text{ungated}}$ and magnetic field sensitivity (calculated from \ref{['eq:SNR']} and \ref{['eq:sens']}, respectively) when varying $\tau_\text{c}$, which both show an enhancement of $\approx 2$ when the time gate $\tau_\text{c}$ is chosen optimally. (e) CW-ODMR spectra resulting from the ungated and gated detection (data points) shown in (d), as well as double Lorentzian fits to the data (solid lines), showing the strong increase of ODMR contrast ($C_\text{ungated} \lesssim 2.5\% \rightarrow C_\text{gated} \sim 15\%$). (f) SNR without and with time gating for different laser repetition rates (datapoints), and simplified model (solid lines). More details can be found in \ref{['app:rep_rate']}.
• Figure 4: (a) Fluorescence spectra of a pristine NC strip (offset by 1), and from a FND, showing mainly NV fluorescence. (b) TCSPC curves from the pristine NC strip, the FND measured on glass substrate with the MW either on or off resonance, and FND on the NC strip with MW off resonance. The NC shows a lifetime of about $\tau_\text{NC}\approx4\,$ns and the FNDs show fluorescence lifetimes of about $\tau_\text{FND}\approx(20-30)\,$ns. (c) ODMR contrast and shot noise component when varying the onset time of the gating window $\tau_\text{c}$. The dashed line shows the ideal value of $\tau_\text{c} = 16.6\,$ns. (d) SNR (calculated from \ref{['eq:SNR']}) when varying $\tau_\text{c}$, which shows an enhancement of up to $\approx 4$ when the time gate is chosen optimally.
• Figure 5: (a) Schematic of experimental setup for real-time gating without TCSPC. A pulse generator (Pulser) is generating the trigger for the pulsed Laser, as well as a related pulse of defined length and phase to trigger a fast Switch. The switch routes the TTL signals from the APD either into a $50\Omega$ termination or to the photon counting electronics. This method shows a similar real-time SNR enhancement as obtained via time-gating using TCSPC and post-processing. (b,c) Ungated and gated confocal scans of a $80\,$µ m$\times80\,$µ m region on the NC strip covered with FNDs, respectively. (d,e) The resulting SNR calculated from two subsequent confocal scans, one with the MW frequency set on resonance and one with the MW off.