Diamond micro-chip for quantum microscopy

The solid spin of ensembles of the nitrogen vacancy (NV) centers in diamond offers unique features to be employed as atomic-scale sensors for detection of weak magnetic field. Here, we mainly demonstrate two types of DC magnetometer based on ensembles of the nitrogen vacancy (NV) centers in diamond with the method of analyzing optically detected magnetic resonance (ODMR) spectra and microwave frequency modulation technology, respectively and experimentally. Firstly, we demonstrate ODMR spectra and resonance frequency of ODMR with the various external magnetic field magnitudes applied to the NV centers in diamond. The minimum detectable magnetic field of the magnetometer is 0.0498 Gauss, and a magnetic sensitivity of 5.22 μT/Hz obtained with the analysis of ODMR spectra. Further, we utilize microwave frequency modulation technology and analyze the power spectra of noise of demodulated signal of the Lock in Amplifier. The sensitivity of the magnetometer can reach as low as 14.3 nT/Hz with microwave frequency modulation. This study lays a foundation for the advancement of magnetic detection research with higher sensitivity based on ensembles of NV centers.

Nitrogen-vacancy (NV) centers in diamond have been used as platforms for quantum information, magnetometry and imaging of microwave (MW) fields. The spatial distribution of the MW fields used to drive the electron spin of NV centers plays a key role for these applications. Here, we report a system for the control and characterization of MW magnetic fields used for the NV spin manipulation. The control of the MW field in the vicinity of a diamond surface is mediated by an exchangeable lumped resonator, coupled inductively to a MW planar ring antenna. The characterization of the MW fields in the near-field is performed by an FFT imaging of Rabi oscillations, by using an ensemble of NV centers. We have found that the Rabi frequency over a lumped resonator is enhanced 22 times compared to the Rabi frequency without the presence of the lumped resonator. Our system may find applications in quantum information and magnetometry where a precise and controlled spin manipulation is required, showing NV centers as good candidates for imaging MW fields and characterization of MW devices.

This thesis is concerned with studies involving quantum sensing based on Nitrogen-Vacancy (NV) center in diamond. Essentially, it attempts to address two important problems. First, enhancing the sensitivity of a single NV center to external static magnetic fields, and second, probing the phase transitions in a soft condensed matter system using a near surface NV center. NV center’s sensitivity to time-varying or AC external magnetic fields is in the range of a few nanotesla and is limited by coherence time of NV. For various applications, including the bio-magnetic measurements, it is necessary to sense nearly static or DC fields rather than the AC fields. However, in diamond samples with natural abundance 13C concentration (1.1%), the best reported sensitivity to DC fields is in microtesla range and is limited by short lived spin dephasing time of NV. In the present work, a novel hybrid magnetometer consisting of a ferromagnetic material and a single NV center is utilized to achieve DC field sensitivity down to tens of nanotesla. The ferromagnetic material exhibits a property known Giant magneto-impedance (GMI) and is sensitive to DC fields under certain conditions. The achievable sensitivity of GMI sensors is in the range of picotesla. By positioning a GMI microwire in close vicinity of a NV center, magnetic interaction between the microwire and NV center can be realized. In the presence of minute changes in the DC fields in the surroundings, the GMI wire responds and encodes information to the NV center. By employing a standard magnetometry sequence on NV this information can be readout. As the sequence relies on long lived NV coherence time, its overall sensitivity could be enhanced by over two orders of magnitude. Second study deals with detecting the temperature driven phase transitions in a soft matter system, namely a liquid crystal (LC) material. The chosen LC material shows distinct ordered phases close to room temperature. By varying the temperature, transitions from solid-like phases to liquid-like phases in a thin layer of LC can be induced. The NV sensors located at a few nm depths detect these transitions in terms of changes in the spin noise signal emanating from nanoscopic volumes containing LC molecules above the diamond surface. Temperature plays a key role in determining the soft matter properties. Since NV centers are also known as nanoscale temperature sensors, it is possible to tune the temperature precisely to the transition points. This way, NV based method is demonstrated as a dual mode sensing for studying soft matter systems at nanoscale, with a control over temperature. The work aims at extending NV centers as novel probes for exploring soft matter systems and address some important questions in that area.

On-chip magnetic field sensing with nitrogen-vacancy (NV) centers in diamond requires scalable integration of 3D waveguides into diamond substrates. Here, we develop a sensing array device with an ensemble of shallow implanted NV centers integrated with arrays of laser-written waveguides for excitation and readout of NV signals. Our approach enables an easy-to-operate on-chip magnetometer with a pixel size proportional to the Gaussian mode area of each waveguide. The performed continuous wave optically detected magnetic resonance on each waveguide gives an average dc-sensitivity value of (195±3) nT/Hz, which can be improved with lock-in-detection or pulsed-microwave sequences. We apply a magnetic field to separate the four NV crystallographic orientations of the magnetic resonance and then utilize a DC current through a straight wire antenna close to the waveguide to prove the sensor capabilities of our device. We reconstruct the complete vector magnetic field in the NV crystal frame using three different NV crystallographic orientations. By knowing the polarization axis of the waveguide mode, we project the magnetic field vector into the lab frame.

We present a density-functional theory analysis of nitrogen-vacancy (NV) centers in diamond, which are located in the vicinity of extended defects, namely, intrinsic stacking faults, extrinsic stacking faults, and coherent twin boundaries on {111} planes in diamond crystals. Several sites for NV centers close to the extended defects are energetically preferred with respect to the bulk crystal. This indicates that NV centers may be enriched at extended defects. We report the hyperfine structure and zero-field splitting parameters of the NV centers at the extended defects, which typically deviate by about 10% but in some cases up to 90% from their bulk values. Furthermore, we find that the influence of the extended defects on the NV centers is of short range: NV centers that are about three double layers (corresponding to $\ensuremath{\approx}6\phantom{\rule{4pt}{0ex}}\text{\AA{}}$) away from defect planes already show bulklike behavior.

Currently, thermally excited magnons are being intensively investigated owing\nto their potential in computing devices and thermoelectric conversion\ntechnologies. We report the detection of thermal magnon current propagating in\na magnetic insulator yttrium iron garnet under a temperature gradient using a\nquantum sensor: electron spins associated with nitrogen-vacancy (NV) centers in\ndiamond. Thermal magnon current was observed as modified Rabi oscillation\nfrequencies of NV spins hosted in a beam-shaped bulk diamond that resonantly\ncoupled with coherent magnon propagating over a long distance. Additionally,\nusing a nanodiamond, alteration in NV spin relaxation rates depending on the\napplied temperature gradient were observed under a non-resonant NV excitation\ncondition. The demonstration of probing thermal magnon current mediated by\ncoherent magnon via NV spin states serves as a basis for creating a device\nplatform hybridizing spin caloritronics and spin qubits.\n

Nitrogen vacancy (NV) color centers in diamond have shown great potential for various applications in quantum technology due to their long coherence times, high sensitivity to magnetic fields and atomic scale resolution. However, one major challenge in utilizing near surface NV centers is the decoherence caused by spins and charges fluctuating on the surface, which affects the spin properties of the sensors. To reduce the induced noise, various oxygen surface treatments such as low power oxygen plasma treatment and annealing under oxygen atmosphere have been explored to terminate the diamond surface and reduce its impact on NV coherence. We showed that the NV center’s coherence time can be enhanced up to a factor of 3 over a large spectral range of noise. Double electron–electron resonance measurements revealed an extra source of decoherence, scaling similarly as the P1 spin bath. The improvement in coherence times is accompanied with an increase in measured ketone/ether content and reduction of sp 2 signal in x-ray photoelectron spectroscopy measurements. Finally we compared the performance of different NV ensembles and surface treatments for sensing external proton spins. The oxygen annealing is an effective procedure of enhancing the spin coherence times and reducing broad band spin noise experienced by shallow implanted ensemble NV centers in diamond.

The nitrogen-vacancy (NV) center in diamond is very sensitive to magnetic and electric fields, strain, and temperature. In addition, it is possible to optically interrogate individual defects, making it an ideal quantum-limited sensor with nanoscale resolution. A key limitation for the application of NV sensing is the optical brightness and collection efficiency of these defects. Plasmonic resonances of metal nanoparticles have been used in a variety of applications to increase the brightness and efficiency of quantum emitters, and therefore are a promising tool to improve NV sensing. However, the interaction between NV centers and plasmonic structures is largely unexplored. In particular, the back-action between NV and plasmonic nanoparticles is nonlinear and depends on optical wavelength, nanoparticle position, and metal type. Here we present the general theory of NV-plasmonic nanoparticle interactions. We detail how the interplay between NV response, including optical and vibrational signatures, and the plasmonic response of the metal nanoparticle results in modifications to the emission spectra. Our model is able to explain quantitatively the existing experimental measurements of NV centers near metal nanoparticles. In addition, it provides a pathway to developing new plasmonic structures to improve readout efficiencies in a range of applications for the NV center. This will enable higher precision sensors, with greater bandwidth as well as new readout modalities for quantum computing and communication.

Photonic spin texture (PST), the spatial distribution of the spin angular momentum (SAM) of light, is connected to unique properties of light, such as optical skyrmions and topological optical N-invariants. There has been recent progress on the generation and manipulation of PST using various methodologies. However, a challenge remains for the sub-wavelength characterization of PST. Here, we demonstrate nitrogen-vacancy (NV) centers in diamond as nanoscale quantum sensors for imaging the PST of a beam with orbital angular momentum (OAM). Leveraging the coherent interaction between photon spin and NV center electron spin at cryogenic temperature (77 K), and using the Hahn-Echo magnetometry technique, we experimentally demonstrate the imprinting of the PST on the quantum phase shift of NV centers. Our work can lead to the development of a quantum imaging platform capable of characterization of the spin texture of light at sub-wavelength scales.

Non-invasive imaging of microwave (MW) magnetic fields with microscale lateral resolution is pivotal for various applications, such as MW technologies and integrated circuit failure analysis. Diamond nitrogen-vacancy (NV) center magnetometry has emerged as an ideal tool, offering micrometer-scale resolution, millimeter-scale field of view, high sensitivity, and non-invasive imaging compatible with diverse samples. However, up until now, it has been predominantly used for imaging of static or low-frequency magnetic fields or, concerning MW field imaging, to directly characterize the same microwave device used to drive the NV spin transitions. In this work, we leverage an NV center ensemble in diamond for wide-field imaging of MW magnetic fields generated by a test device employing a differential measurement protocol. The microscope is equipped with a MW loop to induce Rabi oscillations between NV spin states, and the MW field from the device-under-test is measured through local deviations in the Rabi frequency. This differential protocol yields magnetic field maps of a 2.57 GHz MW field with a sensitivity of ∼9 μT Hz−1/2 for a total measurement duration of T=357 s, covering a 340×340μm2 field of view with a micrometer-scale spatial resolution and a device-under-test input power dynamic range of 30 dB. This work demonstrates a novel NV magnetometry protocol, based on differential Rabi frequency measurement, that extends NV wide-field imaging capabilities to imaging of weak MW magnetic fields that would be difficult to measure directly through standard NV Rabi magnetometry.

Nitrogen Vacancy (NV) centers in Diamond exhibits the advantages of high resolution, high sensitivity, and error-free sensing in the field of vector field detection, which bolstering the potential of NV centers magnetometers for practical applications. However, most of the NV centers magnetometers is still limited by the constraints imposed by bulky laboratory instruments, and the method of sweeping frequency points to detect resonance frequencies can also lead to slower detection of vector magnetic field (MF). To address these limitations, enhancing the portability of integrated vector magnetometers, we made an integrated vector magnetometer based on NV centers and proposed a microwave (MW) frequency-hopping method to realize vector MF triaxial real-time (400 ms) detection. The experimental results indicate that the magnetometer dynamic range is ± 130 μT along the x-axis, ± 110 μT along the y-axis, and ± 260 μT along the z-axis, with the minimum standard deviation of vector MF measurements across all three axes being 1.84 nT. The frequency-hopping method is more than 87 times faster than the sweeping method and the size of the integrated NV magnetometer fit in the palm of the hand to portable usage. This proposal, which accomplishes vector MF rapidly measure using a self-made NV centers magnetometer, has potential applications in fields such as medical diagnosis, new energy battery testing and geomagnetic detection.

We propose a scheme to achieve switchable coupling between nitrogen-vacancy (N-V) center in diamond and superconducting charge qubit by a quantized nanomechanical resonator (NAMR) as a data bus. This procotol can be used to transfer the quantum state from charge qubit to N-V center. Owing to the N-V center has relatively long decoherence time, the information can be stable stored in it. Finally, we discuss the experimental feasibility of our scheme.

In the paper, we present schemes for implementing multi-qubit quantum gates in quantum networks based on the nitrogen-vacancy (NV) centers in diamond. In our schemes, NV centers coupled to the whispering-gallery modes of microtoroidal resonators serve as quantum registers to store quantum information. By coding the qubits of NV centers into decoherence-free subspaces, quantum information encoded in logical qubits is protected from collective dephasing, and quantum controlled-NOT and Toffoli gates between logical qubits from the same or different quantum nodes can be implemented. Compared with the previous works, our schemes are simpler and reduce the consumption of resources. Furthermore, the modularized design of multi-qubit quantum gates can be extended to other quantum platforms, which may lead to more efficient construction of quantum networks for distributed quantum computation and quantum communication.

Generation of a local magnetic field at the nanoscale is desirable for many applications such as spin-qubit-based quantum memories. However, this is a challenge due to the slow decay of static magnetic fields. Here, we demonstrate a photonic spin density (PSD)-induced effective static magnetic field for an ensemble of nitrogen-vacancy (NV) centers in bulk diamond. This locally induced magnetic field is a result of coherent interaction between the optical excitation and the NV centers. We demonstrate an optically induced spin rotation on the Bloch sphere exceeding 10 degrees which has potential applications in all-optical coherent control of spin qubits.

Solid-state spin systems with nitrogen-vacancy (NV) centers in diamonds constitute an increasingly popular platform for quantum sensing. However, most existing platforms designed with ensemble NV centers exhibit a sensitivity that is significantly less than the theoretical maximum. This low sensitivity limits the expansion of the experimental results and application areas. In this study, the sensitivity is improved by increasing the pumping depth of the excitation beam to increase the number of particles involved in spin polarization at a given laser intensity. Compared with the proposed Airy beam with a long focal depth (25.46 λ) and the widely utilized Gauss beam pumping ensemble NV centers, the spin resonance factor fSR can be improved by 10.02%. This sensitivity-optimized approach enhances the functionality of sensors with NV centers.