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Journal of Applied Physics

Optimization and characterization of laser excitation for quantum sensing with single nitrogen-vacancy centers

Alejandro Martínez-Méndez, Jesús Moreno-Meseguer, Mariusz Mrózek, Adam M. Wojciechowski, Raúl González Brouwer, Priya Balasubramanian, Fedor Jelezko, Javier Prior

Published Feb 5, 2026• 0 citations
Publisher

Abstract

In this work, we present a comprehensive method of characterization and optimization of laser irradiation within a confocal microscope tailored to quantum sensing experiments using nitrogen-vacancy (NV) centers. While confocal microscopy is well-suited for such experiments, precise control and understanding of several optical parameters are essential for reliable single-emitter studies. We investigate the laser beam intensity profile, single-photon emission statistics, fluorescence response under varying polarization and saturation conditions, spectral characteristics, and the temporal profiles of readout and reinitialization pulses. The beam quality is assessed using the beam propagation factor M2, determined via the razorblade technique. An optical fluorescence spectrum is recorded to confirm NV center emission. To confirm single-emitter operation, we measure second-order autocorrelation function g(2)(τ). Saturation behavior is analyzed by varying laser power and recording the corresponding fluorescence, while polarization dependence is studied using a half-wave (λ/2) plate. A temporal laser pulse profile is examined by modulating the power of an acousto-optic modulator. After optimizing all relevant parameters, we demonstrate the microscope’s capabilities in driving spin transitions of a single NV center. This work establishes a straightforward and effective protocol for laser excitation optimization, enhancing the performance and reliability of NV-based quantum sensors.

DOI
10.1063/5.0293773

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In this work, we present a comprehensive method of characterization and optimization of laser irradiation within a confocal microscope tailored to quantum sensing experiments using nitrogen-vacancy (NV) centers. While confocal microscopy is well-suited for such experiments, precise control and understanding of several optical parameters are essential for reliable single-emitter studies. We investigate the laser beam intensity profile, single-photon emission statistics, fluorescence response under varying polarization and saturation conditions, spectral characteristics, and the temporal profiles of readout and reinitialization pulses. The beam quality is assessed using the beam propagation factor M2, determined via the razorblade technique. An optical fluorescence spectrum is recorded to confirm NV center emission. To confirm single-emitter operation, we measure second-order autocorrelation function g(2)(τ). Saturation behavior is analyzed by varying laser power and recording the corresponding fluorescence, while polarization dependence is studied using a half-wave (λ/2) plate. A temporal laser pulse profile is examined by modulating the power of an acousto-optic modulator. After optimizing all relevant parameters, we demonstrate the microscope’s capabilities in driving spin transitions of a single NV center. This work establishes a straightforward and effective protocol for laser excitation optimization, enhancing the performance and reliability of NV-based quantum sensors.
Single nitrogen vacancy (NV) centers in diamond serve as a photonic platform for quantum sensing and computing applications. They can be created deterministically using femtosecond laser pulses. Such writing process requires high-resolution fluorescence detection with a high signal-to-noise ratio, which is accomplished by setting up a homebuilt confocal fluorescence microscope. The microscope shall monitor the fluorescence of newly formed NV centers during the writing process. In addition, it will be used to characterize the NV centers’ spin coherence times, orientation in the diamond lattice and single-emitter characteristics via the intensity correlation function g(2)(tau). In the future, this setup is also planned to be employed for STED-microscopy of twin NV centers.
Nitrogen-vacancy (NV) centers in single-crystal CVD diamond are promising candidates for quantum sensing due to their remarkable optical and spin properties [1]. These defects exhibit stable photoluminescence (PL), long spin coherence times, and high sensitivity to external fields [2]. In this study, we present a room-temperature PL characterization of NV centers excited with a 532 nm laser. The setup was optimized for high signal-to-noise ratio and spectral resolution. Key parameters analyzed include emission intensity, zero-phonon line (ZPL) at ~637 nm, phonon sidebands, and the charge state distribution between NV⁰ and NV⁻. The NV⁻ state, crucial for spin-based sensing, is evaluated to understand its stability under different nitrogen doping conditions [3]. The relative intensities of NV⁰ and NV⁻ provide insights into nitrogen incorporation and vacancy formation. These results contribute to optimizing synthesis conditions for enhanced NV⁻ concentration, supporting the advancement of diamond-based quantum sensing technologies.
So far most well-established quantum sensing techniques based on the negatively charged nitrogen-vacancy (NV) center in diamond utilize the recorded fluorescence intensity to detect the electronic spin states. However, the fluorescence intensity of a NV center is not only dependent on its spin state, but also affected by measurement issues, such as fluctuations of the excitation laser power and charge state transformation of the NV center. Instabilities in terms of output power or polarization changes in the laser source as well as sample drifts during a measurement are common factors that weaken the precision. Here, we demonstrate proof-of-principle of a sensing method making use of the excited-state lifetimes of a NV center for magnetometry experiments.
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.
Nitrogen-vacancy (NV) color centers in diamond serve as promising atomic spin systems for measurement applications requiring high accuracy and sensitivity. A key challenge in NV-based quantum sensing is minimizing spin readout noise to approach the standard quantum limit (SQL). Based on a six-level model, this work analyze the dependence of NV-based quantum sensing performance, including spin state readout noise and signal-to-noise ratio (SNR), on controllable parameters such as the Purcell factor and excitation laser pulse characteristics. This study demonstrates that a shorter excitation pulse duration results in a higher saturation value of ground-state spin polarization, while the total time required for the polarization process remains constant. Additionally, the spin readout noise does not improve monotonically with decreasing excitation pulse duration; instead, it initially decreases and subsequently increases as the pulse duration varies. The spin readout noise reaches its optimal level when the pulse duration is 3 ns. Furthermore, no positive correlation exists between the signal-to-noise ratio (SNR) and the Purcell factor, and there is also an optimal value for SNR. When the pulse duration ranges from 1 ns to 40 ns, the variation in SNR is relatively insignificant. This research offers a novel perspective for enhancing the performance of quantum sensing based on diamond defects, such as nitrogen-vacancy (NV) centers.
Among quantum sensors, the single nitrogen-vacancy (NV) defect in diamond has the highest sensitivity-to-size factor. For instance, a single-NV spin when used as a magnetometer could achieve sensitivities of the order of nT/Hz, while the dimensions of the sensor are merely atomic in size. The sensitivity is limited only by the photon shot noise. One method to boost the magnetometer sensitivity to pico-tesla scales is to use many NV defect centers as an ensemble sensor [Phys. Rev. X5, 041001 (2015)PRXHAE2160-330810.1103/PhysRevX.5.041001]. However, during the absorption-emission (fluorescence) optical cycles, the NV centers transfer a portion of the irradiation laser energy into phonons and heat the diamond matrix. This results in unintended fluorescence decrease, spin resonance lines shifts, and fluctuations. Hence, the advantages gained by packing a high density of NV centers are significantly reduced. Here we investigate the heat generation of ensemble NV centers in micrometer-sized diamond under 532 nm laser irradiation and its effects pertaining to sensing applications. These investigations help us to find strategies that mitigate the detrimental effects of heating and yet permits the use of ensemble NV defects for improved metrology applications.