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Applied Physics Express

Multi-frequency composite pulse sequences for sensitivity enhancement in hexagonal boron nitride quantum sensor

Hao Gu, Yuki Nakamura, Kento Sasaki, Kensuke Kobayashi

Published May 1, 2023• 7 citations
Publisher

Abstract

Abstract A negatively charged boron vacancy ( V B − ) in hexagonal boron nitride (hBN) has recently started to be used as a magnetic sensor. However, V B − has magnetic resonance spectra broadened by hyperfine interactions, which decreases the signal contrast and thus degrades the magnetic field sensitivity. Here we propose multi-frequency composite pulse sequences to enhance the contrast of the measurement signal in the Rabi oscillation. We demonstrate that the sequence using five microwave frequencies enhances the signal contrast, improving the sensitivity by a factor of 2.3 compared to the conventional sequence.

DOI
10.35848/1882-0786/acd1d1

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Juan M. Florez, Eric Suárez Morell

The negatively charged boron vacancy ($\VB$) in hexagonal boron nitride (\hBN{}) was the first optically addressable spin qubit identified inside a van der Waals crystal, allowing atomically defined placement relative to a target. Its coherence in bulk \hBN{} is limited by the boron nuclei of the layers flanking the defect plane, which a thin flake removes. Here, we use a generalized cluster-correlation expansion with an extended central-spin block to calculate the Hahn-echo coherence time of $\VB$ in h$^{11}$B$^{15}$N as a function of layer number. Our results show that $T_2$ rises from $199\ns$ in the bulk limit to $653\ns$ in a monolayer, a factor of $3.3$ that is already saturated at three layers and that a sublattice decomposition attributes entirely to boron. The enhancement is confined to low field, is insensitive to stacking registry and twist angle, and requires alignment to within about two degrees, an onset we reproduce with no free parameters. Under dynamical decoupling, a $1/e$ threshold returns $206\ns$ for every thickness, but this is a zero of the first-shell modulation rather than a decay: measured without a threshold, the layer contrast survives and reaches a factor of $170$. Layer number thus emerges as a design parameter for $\VB$-based sensing.

cond-mat.mes-hallCross-listedarXiv:2609.01812•Sep 3, 2026
Abstract A negatively charged boron vacancy ( V B − ) in hexagonal boron nitride (hBN) has recently started to be used as a magnetic sensor. However, V B − has magnetic resonance spectra broadened by hyperfine interactions, which decreases the signal contrast and thus degrades the magnetic field sensitivity. Here we propose multi-frequency composite pulse sequences to enhance the contrast of the measurement signal in the Rabi oscillation. We demonstrate that the sequence using five microwave frequencies enhances the signal contrast, improving the sensitivity by a factor of 2.3 compared to the conventional sequence.
Negatively charged boron vacancies (VB−_{\text{B}}^{-}B−​) in hexagonal boron nitride (hBN) comprise a promising quantum sensing platform, optically addressable at room temperature and transferrable onto samples. However, broad hyperfine-split spin transitions of the ensemble pose challenges for quantum sensing with conventional resonant excitation due to limited spectral coverage. While isotopically enriched hBN using 10^{10}10B and 15^{15}15N isotopes (h10^{10}10B15^{15}15N) exhibits sharper spectral features, significant inhomogeneous broadening persists. We demonstrate that, implemented via frequency modulation on an FPGA, a frequency-ramped microwave pulse achieves around 4-fold greater ∣0⟩→∣−1⟩|0\rangle\rightarrow|-1\rangle∣0⟩→∣−1⟩ spin-state population transfer and thus contrast than resonant microwave excitation and thus 16-fold shorter measurement time for spin relaxation based quantum sensing. Quantum dynamics simulations reveal that an effective two-state Landau-Zener model captures the complex relationship between population inversion and pulse length with relaxations incorporated. Our approach is robust and valuable for quantum relaxometry with spin defects in hBN in noisy environments.
Due to its localized nature and controllability, the negatively charged boron vacancy centers (VB−_\text{B}^-B−​) in hexagonal boron nitride (hBN) are a promising spin platform for accessing its neighboring nuclei with potential for performing quantum computational tasks. However, the methods of utilizing and manipulating the nuclear spins are still lacking. In this work, we propose a protocol for the preparation of single- and multi-qubit gates on the nuclear spins, utilizing the electron spin as an auxiliary qubit. By applying a background magnetic field and a multi-tone continuous drive, we show that the electron spin coupling to the nuclei can be efficiently engineered. This allows for suppressing the undesired electron-nuclear interactions through the Hahn echo pulse sequence. The target gates are then implemented by employing proper RF drives. Our numerical results for realistic parameters show gate fidelities as high as 99%99\%99% for single-qubit and 95%95\%95% for multi-qubit gates. With the gate execution durations being less than 300300300 ns, our protocol evades electron spin decoherence effects. Therefore, our scheme sets the stage for the practical application of VB−_\text{B}^-B−​ in hBN for quantum computation purposes.
Abstract By sequentially recording the phase of an AC signal relative to an external clock, quantum heterodyne schemes have recorded MHz and GHz signals with Fourier-limited precision. However, in systems with large inhomogeneous broadening, existing heterodyne protocols provide limited protection of the spin coherence, impacting amplitude sensitivity. Here, we use a continuous microwave scheme that extends spin coherence towards the effective T2≈12T1{T}_{2}\approx \frac{1}{2}{T}_{1}T2​≈21​T1​ T 2 ≈ 1 2 T 1 limit and resolves the frequency, amplitude and phase of MHz to GHz magnetic fields. In an ensemble of boron vacancies in hexagonal boron nitride the scheme achieves an amplitude sensitivity of η≈3−5 μT/Hz\eta \approx 3-5\,\mu {{{\rm{T}}}}/\sqrt{{{{\rm{Hz}}}}}η≈3−5μT/Hz​ η ≈ 3 − 5 μ T / Hz and phase sensitivity of ηϕ≈0.076 rads/Hz{\eta }_{\phi }\approx 0.076\,{{{\rm{rads}}}}/\sqrt{{{{\rm{Hz}}}}}ηϕ​≈0.076rads/Hz​ η ϕ ≈ 0.076 rads / Hz . We demonstrate that the scheme is compatible with quantum heterodyne detection, recording a GHz signal with a resolution < 1 Hz and SNR of 235 over a 10 s measurement. Achieving this performance in a two-dimensional material platform could have broad applications in probing nanoscale condensed matter systems.
Spin defects in two-dimensional materials hold significant potential for quantum information technologies and sensing applications. The negatively charged boron vacancy (VB-) in hexagonal boron nitride (hBN) has attracted considerable attention as a quantum sensor due to its demonstrated sensitivity to temperature, magnetic fields, and pressure.1 However, its applications have thus far been limited by inherently dim photoluminescence (PL). By fabricating a van der Waals heterostructure with a sensitizing donor layer, lead iodide (PbI2), we effectively enhance the PL intensity from the VB- by 5-45x, while maintaining compatibility with other heterostructures and vdW optoelectronic platforms. The type-I band alignment at the heterojunction enables efficient exciton migration while suppressing back-electron transfer, and the strong spectral overlap between the PbI2 emission and defect absorption supports efficient fluorescence resonance energy transfer. Ab initio density functional theory (DFT) predicts a photon-ratcheting mechanism that boosts absorption and emission while maintaining magnetic resonance (ODMR) contrast through minimal hybridization. Experimentally, the heterostructure exhibits enhanced continuous-wave ODMR sensitivity and functions as a precise probe of external magnetic fields. This work establishes a proof-of-concept for amplifying weak defect signals in nanomaterials, highlighting a new strategy for engineering their optical and magnetic responses.
High fidelity quantum operation of qubits plays an important role in realistic quantum sensing. It becomes more challenging when there are inevitable interactions between qubits in the solid system. We employ a composite pulse sequence to deal with the problems. The electron spin state of the nitrogen-vacancy center in diamond is flipped with high fidelity by the composite pulse at room temperature. In contrast with traditional rectangular pulses, the composite pulse has a wider excitation profile at the same Rabi frequency. Hence, the three sublevels of host nitrogen nuclear spin can be flipped efficiently and simultaneously, which enhances the signal contrast and detection sensitivity of quantum sensing universally. The enhancement effect works over a wide range of bias magnetic fields. Our scheme can be used for sensing temperature, strain, and electric field and can be applied to other spin–spin coupling systems.
The negatively charged boron vacancy ($\VB$) in hexagonal boron nitride (\hBN{}) was the first optically addressable spin qubit identified inside a van der Waals crystal, allowing atomically defined placement relative to a target. Its coherence in bulk \hBN{} is limited by the boron nuclei of the layers flanking the defect plane, which a thin flake removes. Here, we use a generalized cluster-correlation expansion with an extended central-spin block to calculate the Hahn-echo coherence time of $\VB$ in h11^{11}11B15^{15}15N as a function of layer number. Our results show that T2T_2T2​ rises from $199\ns$ in the bulk limit to $653\ns$ in a monolayer, a factor of 3.33.33.3 that is already saturated at three layers and that a sublattice decomposition attributes entirely to boron. The enhancement is confined to low field, is insensitive to stacking registry and twist angle, and requires alignment to within about two degrees, an onset we reproduce with no free parameters. Under dynamical decoupling, a 1/e1/e1/e threshold returns $206\ns$ for every thickness, but this is a zero of the first-shell modulation rather than a decay: measured without a threshold, the layer contrast survives and reaches a factor of 170170170. Layer number thus emerges as a design parameter for $\VB$-based sensing.