Session T52: Quantum Sensing with Defect Spin Sensors II

Quantum Diamond Microscope for Real-space NMR Imaging

Nitrogen vacancy (NV) centers in diamond enable high-resolution nuclear magnetic resonance (NMR) spectroscopy of small samples. We are developing a quantum diamond microscope (QDM) to provide real-space NMR imaging with micron-scale spatial resolution and millimeter-scale field-of-view. The QDM-NMR will provide spatio-temporal information of diverse molecules and nuclear species in bulk matter, with braod applications in the physical and life sciences.   

Quantum Sensing of Magnetic Materials by Spin Defects in Hexagonal Boron Nitride

Optically active quantum spin defects contained in wide band-gap semiconductors have been demonstrated as a sensitive probe to investigate local electromagnetic property of condensed matter systems. Novel spin defects in van der Waals two-dimensional (2D) crystals are naturally relevant in this context due to their significantly improved compatibility with nanodevice integration and atomic length scale proximity readily established between the spin sensors and objects of interest. Taking advantage of boron vacancy spin defects in hexagonal boron nitride (hBN), we report nanoscale quantum sensing and imaging of static and dynamic magnetic field environment of low-dimensional magnetic materials under a broad range of experimental conditions. Our results highlight the appreciable capability of 2D spin defects of evaluating local magnetic properties of solid-state materials in an accessible and precise way, which can be extended readily to a broad family of quantum materials and devices.

    

All-optical, Microwave-free NV-Diamond Magnetometer for Weak Magnetic Field Environments

Sensitive nitrogen-vacancy (NV) magnetometry typically requires a bias field to lift the spin level degeneracy and has commonly utilized microwave sources to selectively manipulate the sensor's spin state. However, this external bias field and microwave sources may perturb the sample of interest or be incompatible with operational conditions. Here, we demonstrate an all-optical, microwave-free NV-diamond magnetometer in the presence of weak or no bias magnetic field (<20 G). We experimentally measure photoluminescence spectra using diamonds with varying NV concentrations; and numerically simulate matching features due to cross-relaxation between NVs. Our results pave the way towards a sensitive probe to study magnetic systems that can be disrupted by microwave signals and can only tolerate weak bias magnetic field.   

Ultrahigh Vacuum Surface Chemistry for Nanoscale Sensors in Diamond

Shallow nitrogen vacancy (NV) centers in diamond are actively explored for quantum sensing applications due to their high sensitivity and nanoscale resolution. However, the diamond surface can host contaminants and defects that give rise to magnetic noise and NV charge state instability. To prepare pristine diamond surfaces and systematically study the surface noise, we have constructed a novel cluster tool combining surface preparation and spectroscopy with cryogenic, confocal microscopy of single NV centers inside the same ultrahigh vacuum (UHV) environment. The UHV confocal microscope chamber allows for high NA imaging, high Rabi frequency microwave driving, and precise magnetic field alignment. Using in situ annealing, we isolate the contribution of adventitious carbon to magnetic noise. We are also able to study surface terminations that are unstable in ambient conditions, allowing us to design new surfaces to enable state-of-the-art nanoscale quantum sensors.   

Diamond defect-based sensing of programmably patterned molecular spin arrays with single-spin sensitivity

The assembly of solid-state spins with controlled nanoscale spatial precision is an outstanding challenge in quantum technologies. Here we combine a DNA-based patterning technique with nitrogen-vacancy (NV) quantum sensors in diamond to sense 2D arrays of molecular spins programmably patterned via a monolayer of DNA origami on a diamond surface. We control the spacing of chelated Gd³⁺ spins down to 6 nm precision and verify this control by observing a linear relationship between proximal NVs’ T₁ relaxation rate, and the designated number of Gd³⁺ spins per origami unit. We confirm the preservation of the charge state and spin coherence of the proximal, shallow NV centers and discuss ongoing work towards probing ordered, strongly interacting 2D spin networks on the diamond surface.   

Probing Critical Dynamics in 2D CrSBr via NV Spin Decoherence

Recently, it has been proposed that critical scaling laws at magnetic phase transitions in materials can be analyzed on the microscopic scale using the decoherence dynamics of a nearby NV spin qubit as a probe [1]. Here, we use coherent NV spin resonance techniques to study critical fluctuations in a 2D layered antiferromagnetic material, CrSBr, interfaced with shallow NV probe spin ensembles. We observe temperature dependent changes in T2* of the NV probe spins consistent with macroscopic scaling of critical fluctuations near the Neel temperature in 2D flakes of CrSBr. Furthermore, we study microscopic variations in critical behavior by imaging NV probe spin decoherence with confocal scanning. Our work demonstrates that NV spin decoherence can be used as a powerful probe of critical behavior in 2D magnetic materials on the microscopic scale.

[1] Machado, F., Demler, E. A., Yao, N. Y. & Chatterjee, S. Quantum Noise Spectroscopy of Dynamical Critical Phenomena. Phys Rev Lett 131, 070801 (2023)   

Nanotube spin defects for omnidirectional magnetic field sensing

Optically addressable spin defects in solids are revolutionizing nanoscale quantum sensing. Spin defects in one-dimensional (1D) vdW nanotubes will provide unique opportunities due to their small sizes in two dimensions and absence of dangling bonds on side walls. However, optically detected magnetic resonance of localized spin defects in a nanotube has not been reported. Here, we report the observation of single optically addressable spin defects in boron nitride nanotubes (BNNTs) at room temperature. Our findings suggest that these BNNT spin defects possess a spin S=1/2 ground state without an intrinsic quantization axis, leading to orientation-independent magnetic field sensing. We harness this unique feature to observe anisotropic magnetization of a 2D magnet in magnetic fields along orthogonal directions. Additionally, we develop a method to deterministically transfer a BNNT onto a cantilever and use it to demonstrate scanning probe magnetometry. Further refinement of our approach will enable nanoscale quantum sensing of magnetic fields in any direction.   

Spin defects in aqueous environments and confined geometries

Nano nuclear magnetic resonance (nano-NMR) spectroscopy aims at applying NMR techniques to nanoscale samples, and eventually at enabling high-resolution imaging of single molecules. The nitrogen-vacancy (NV) center-based spectrometer is a promising platform for nano-NMR: the defect characteristic decoherence time (T2) can be used, for example, to observe the time evolution of proton spins from simple molecules located in proximity of a diamond surface. In this work, we analyzed the magnetic field induced by protons of water in confined geometries and how this affects the coherence times of spin defects in two-dimensional materials, including graphene and h-BN. We considered graphene and hexagonal boron nitride (hBN) layers as test systems and generated a structural model of the layer/water interface by performing classical molecular dynamics (MD) simulations using the LAMMPS code. We then used the MD trajectories to generate spin bath configurations and compute hyperfine interactions using the PyCCE code [1]. An NV-like spin defect in graphene and a negatively charged boron-vacancy in hBN were considered. In this talk, we will discuss how both the parallel and perpendicular components of the magnetic noise induced by protons affects the lifetime of the spin qubit. 

[1] Onizhuk, M. et al. Adv. Theory Simul. 2021   

Towards Geometric Phase Magnetometry in Nitrogen-Vacancy Center Ensembles

Nitrogen vacancy (NV) centers in diamonds have emerged as compelling quantum sensors, par-

ticularly as sensitive magnetometers at ambient conditions providing sub-micrometer resolution.

Conventionally, interferometry-based protocols (e.g., Ramsey) for broadband magnetometry encode

magnetic field information into the dynamic phase accumulated by the NV spin state. However, for

the detection of large magnetic fields, the sensor’s dynamic range and sensitivity are constrained by

phase ambiguities upon accumulating phase ≥ 2π. To circumvent these limitations, Arai et al. (2018)

introduced the use of geometric phases for magnetometry with a single NV center. Extending these

methods to an ensemble of NV centers, we examine the impact of magnetic disorder and control

field inhomogeneities through numerical simulations. Furthermore, we present preliminary exper-

imental results including polarization of the NV nitrogen nuclear spin and the implementation of

fabricated transmission lines for homogeneous delivery of microwave control fields. These findings

offer promising insights into enhancing the capabilities of NV based magnetometers.   

Nuclear magnetic resonance with a single NV scanning probe

The single nitrogen vacancy (NV) probe has found extensive applications in performing scanning magnetometry to investigate various magnetic and electric phenomena. On the other hand, static diamond membranes or pillar arrays are typically used to conduct nanoscale nuclear magnetic resonance (NMR). While prior efforts have explored scanning the studied sample over a static diamond membrane, this configuration has limited practical uses. 

Our study showcases the possibility to utilize a scanning NV probe to perform nanoscale NMR measurements. Using dynamic decoupling, different nuclear signals on sample surfaces have been detected at a high signal to noise ratio. An NV-to-sample distance of 8 nm is determined, by fitting the magnetic fluctuations from ¹⁹F nuclear. In a proof-of-concept demonstration, we show the feasibility of studying nanoscale hydrophilicity and hydrophobicity by using a single NV probe. ¹H from water films on various sample surfaces have been measured. Notably, an increase in ¹H signals upon retracting the probe from a superhydrophobic surface is observed, which behaves distinctively compared to hydrophilic surfaces. 

Our results affirm the viability of using scanning NV probes for NMR type examinations of surfaces characteristics, which opens the door for the nanoscale NMR imaging in a wide variety of scientific domains via scanning NV.   

AC sensing wtih Spin Locked 13C Nuclear Spins

Quantum sensors are one of the most prominent candidates to achieve submicronscale NMR spectroscopy. However, such sensors predominantly operate in low-field regime, which exhibits low chemical resolution. Here, we demonstrate an AC sensing scheme in high field using hyperpolarized 13C nuclear spins in diamonds that can be exploited to achieve submicronscale NMR microscopy. The 13C nuclear spins are first hyperpolarized and initialized by transferring polarization from a central NV center. After placing the 13C nuclear spins in an initial transverse state, we spin lock them to extend their lifetimes (in order of minutes). As a proof of concept, we apply and measure an AC field with varying amplitude and frequency with high resolution, using the phases of the 13C nuclear spins in the rotating frame.   

Decoherence of the central spin in dissipative spin baths

Mykyta Onizhuk, Yuxin Wang, Jonah Nagura, Aashish Clerk, and Giulia Galli

 

To harness the potential of solid-state spin qubits for quantum sensing applications, it is critical to obtain a deep understanding of their intricate interactions with surface-bound spins.

This work presents a new methodology based on cluster correlation expansion to study how a central spin interacts with other spins in a dissipative bath. We tested the method on several model systems and found it faithfully reproduces reference numerical results.

Using the framework developed here, we identify a subtle interplay between dissipation and coherent spin exchange. Specifically, we find that fast dissipation in the bath can increase the coherence time of the central spin. Finally, we use our method to simulate nitrogen-vacancy (NV) centers close to a diamond surface, and we find that bath-induced dissipation is the main factor determining the NV center's decoherence in experimentally relevant regimes.

Our method holds promises to study the spin dynamics of defects in complex systems, thus providing a useful tool to understand the quantum behavior of the central spin in dissipative environments.