Switchable coupling between nitrogen-vacancy center in diamond and charge qubit

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.

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.

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.

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.

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

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.

We propose a method to achieve coherent coupling between nitrogen-vacancy (NV) centers in diamond and superconducting (SC) flux qubits. The resulting coupling can be used to create a coherent interaction between the spin states of distant NV centers mediated by the flux qubit. Furthermore, the magnetic coupling can be used to achieve a coherent transfer of quantum information between the flux qubit and an ensemble of NV centers. This enables a long-term memory for a SC quantum processor and possibly an interface between SC qubits and light.

The application of magnetic resonance spectroscopy at progressively smaller length scales may eventually permit 'chemical imaging' of spins at the surfaces of materials and biological complexes. In particular, the negatively charged nitrogen-vacancy (NV(-)) centre in diamond has been exploited as an optical transducer for nanoscale nuclear magnetic resonance. However, the spectra of detected spins are generally broadened by their interaction with proximate paramagnetic NV(-) centres through coherent and incoherent mechanisms. Here we demonstrate a detection technique that can resolve the spectra of electron spins coupled to NV(-) centres, in this case, substitutional nitrogen and neutral nitrogen-vacancy centres in diamond, through optically detected cross-relaxation. The hyperfine spectra of these spins are a unique chemical identifier, suggesting the possibility, in combination with recent results in diamonds harbouring shallow NV(-) implants, that the spectra of spins external to the diamond can be similarly detected.

It is desirable to implement an efficient quantum information process demanding fewer quantum resources. We designed two compact quantum circuits for determinately implementing four-qubit Toffoli and Fredkin gates on single-photon systems in both the polarization and spatial degrees of freedom (DoFs) via diamond nitrogen-vacancy (NV) centers in resonators. The gates are heralded by the electron spins associated with the diamond NV centers. In contrast to the ones with one DoF, our implementations reduce the quantum resource and are robust against the decoherence. Evaluations of fidelities and efficiencies of our gates show that our schemes may be implemented with current technology.

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.

A negatively charged nitrogen vacancy (NV) center in diamond has been recognized as a good solid-state qubit. A system consisting of the electronic spin of the NV center and hyperfine-coupled nitrogen and additionally nearby carbon nuclear spins can form a quantum register of several qubits for quantum information processing or as a node in a quantum repeater. Several impressive experiments on the hybrid electron and nuclear spin register have been reported, but fidelities achieved so far are not yet at or below the thresholds required for fault-tolerant quantum computation (FTQC). Using quantum optimal control theory based on the Krotov method, we show here that fast and high-fidelity single-qubit and two-qubit gates in the universal quantum gate set for FTQC, taking into account the effects of the leakage state, nearby noise qubits and distant bath spins, can be achieved with errors less than those required by the threshold theorem of FTQC.

The optical coupling of guided modes in a GaP waveguide to nitrogen-vacancy (NV) centers in diamond is demonstrated. The electric field penetration into diamond and the loss of the guided mode are measured. The results indicate that the GaP-diamond system could be useful in realizing coupled microcavity-NV devices for quantum information processing in diamond.