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Physical review. B./Physical review. B

Pure spin squeezing of h -BN spins coupled to superconducting resonator

Yi‐Fan Qiao, Jia‐Qiang Chen, Yuan Zhou, Fuli Li, Weibo Gao

Published May 18, 2023• 5 citations
Publisher

Abstract

The negatively charged boron vacancy (${\text{VB}}^{\ensuremath{-}}$) spin defect in two-dimensional (2D) hexagonal boron nitride ($h$-BN) has attracted much attention for potential applications in quantum photonics recently. Its inherent van der Waals force mechanism guarantees convenient heterostructures for quantum sensing. By virtue of such materials, researchers not only can fabricate enough thin spin film naturally close to the sensing target but also can prepare an almost perfect spin ensemble with a uniform orientation. We here propose a setup with an ensemble of ${\text{VB}}^{\ensuremath{-}}$ spins strongly coupled to the superconducting coplanar waveguide resonator through the magnetic-dipolar interaction. The collective coupling strength is predicted to be $G/2\ensuremath{\pi}\ensuremath{\sim}15$ MHz, which corresponds to the strong coupling region. This collective spin-photon interaction can mimic the one-axis twisting Lipkin-Meshkov-Glick model effectively and therefore guarantee the dynamic generation of the spin-squeezed state. In addition, we show the influence of inhomogeneous coupling caused by sample thickness on squeezing, which proves the validity of the homogeneity assumption in our scheme. This attempt not only explores the possibility and superiority of 2D ${\text{VB}}^{\ensuremath{-}}$ spins but also opens another avenue for quantum hybridization.

DOI
10.1103/physrevb.107.195425

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The negatively charged boron vacancy (VB\ensuremath−{\text{VB}}^{\ensuremath{-}}VB\ensuremath−) spin defect in two-dimensional (2D) hexagonal boron nitride (hhh-BN) has attracted much attention for potential applications in quantum photonics recently. Its inherent van der Waals force mechanism guarantees convenient heterostructures for quantum sensing. By virtue of such materials, researchers not only can fabricate enough thin spin film naturally close to the sensing target but also can prepare an almost perfect spin ensemble with a uniform orientation. We here propose a setup with an ensemble of VB\ensuremath−{\text{VB}}^{\ensuremath{-}}VB\ensuremath− spins strongly coupled to the superconducting coplanar waveguide resonator through the magnetic-dipolar interaction. The collective coupling strength is predicted to be G/2\ensuremathπ\ensuremath∼15G/2\ensuremath{\pi}\ensuremath{\sim}15G/2\ensuremathπ\ensuremath∼15 MHz, which corresponds to the strong coupling region. This collective spin-photon interaction can mimic the one-axis twisting Lipkin-Meshkov-Glick model effectively and therefore guarantee the dynamic generation of the spin-squeezed state. In addition, we show the influence of inhomogeneous coupling caused by sample thickness on squeezing, which proves the validity of the homogeneity assumption in our scheme. This attempt not only explores the possibility and superiority of 2D VB\ensuremath−{\text{VB}}^{\ensuremath{-}}VB\ensuremath− spins but also opens another avenue for quantum hybridization.
Abstract The negatively charged boron vacancy center ( V B − ) in a hexagonal boron nitride ( h -BN) film, represents a promising quantum unit at room temperature due to its exceptional attributes, including remarkable sensitivity to magnetic field or strain, long coherence time, nearly perfect spin homogeneity, and an ultra-thin film structure. We here study a different hybridization proposal of an ensemble of V B − centres magnetically coupled to a superconducting flux qubit (FQ) or a DC superconducting quantum interference device (SQUID). Because the localization degree and density of the magnetic field in the FQ are much greater than that of the superconducting resonator (SR), this type of spins-FQ coupling is much stronger for an identical number of V B − centres, and it can therefore mimic the traditional light–matter interactions, i.e., the Dicke-, Tavis-Cummings- or Rabi-like model. For applications, this hybrid quantum system can potentially guarantee such as the simulation of quantum phase transitions (QPTs) of the anomalous Rabi-like model especially showing the influences to QPTs of the spins’ dephasing and FQ’s decay, criticality-enhanced quantum metrology or quantum sensing, the steady-state preparation of single- and two-mode spin-squeezed state (SSS). This investigation can potentially provide a platform for the subsequent quantum information processing (QIP).
Generating stable entanglement by mimicking a squeezed environment plays an important role in quantum metrology and quantum information. Here, we propose an efficient scheme for generating stable entanglement of two boron-vacancy (VB) defects in a suspended circular hexagonal boron nitride (hhh-BN) membrane via a two-phonon drive. Compared to other reported methods, our scheme only needs a linear resource and thus is more convenient for experimental implementation. In the presence of a gradient magnetic field, the electronic spin degree of freedom of the VB defect can be coupled to the mechanical mode of hhh-BN membrane through spin-phonon interaction. The coupling strength is closely related to the geometry of the magnetic field and the parameters of the membrane, which allows us to realize different coupling regimes by choosing the parameter appropriately. After applying a time-dependent pump to the hhh-BN membrane, we can modulate its spring constant and obtain a tunable two-phonon drive to the mechanical mode. We then investigate the dissipative coupling between two VB spins in the condition of a large dissipation limit, showing that the stable entanglement can be generated under a two-phonon drive. In addition, we also take the spin dephasing into consideration to explore the robustness of our scheme. We then extend this scheme to multiparticle systems, which shows that the entanglement degree deepens with the increase of VB spin number. This work offers a fascinating platform for generating stable entanglement with VB spins in two-dimensional hhh-BN.
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.
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.
Negatively charged boron vacancy (VB−\mathrm{V_B^-}VB−​) in hexagonal boron nitride (hBN) is the most extensively studied room-temperature quantum spin system in two-dimensional (2D) materials. Nevertheless, the current effective readout of VB−\mathrm{V_B^-}VB−​ spin states is carried out by systematically optical methods. This limits their exploitation in compact and miniaturized quantum devices, which would otherwise hold substantial promise to address quantum sensing and quantum information tasks. In this study, we demonstrated a photoelectric spin readout technique for VB−\mathrm{V_B^-}VB−​ spins in hBN. The observed photocurrent signals stem from the spin-dependent ionization dynamics of boron vacancies, mediated by spin-dependent non-radiative transitions to a metastable state. We further extend this electrical detection technique to enable the readout of dynamical decoupling sequences, including the Carr-Purcell-Meiboom-Gill (CPMG) protocols, and of nuclear spins via electron-nuclear double resonance. These results provide a pathway toward on-chip integration and real-field exploitation of quantum functionalities based on 2D material platforms.
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.