NV centers ensemble have proven to be sensitive and versatile sensor platform for many applications such as magnetometry [1, 2], electric field sensing [3], thermometry [4] and rotation sensing [5]. The tradeoff between concentration of NVs and coherence lifetime of the electronic spin limits the maximum sensitivity of such devices. Recently usage of quantum memory associated with nuclear spins nearby NV center was shown to dramatically increase the sensitivity via accumulation of the sensed signal in memory [6]. This way sensing on single NV center can benefit a lot from nearby environment, but extension of this technique on atomic ensemble is still quite challenging. Here we present a versatile platform for quantum manipulation with a highly dense NV ensemble in HPHT diamond plates. In our work, we demonstrate several approaches to initialize quantum memory in regular magnetic fields, manipulation of quantum memory, and reading the quantum information from it. Also, we carefully study decoherence rate in our sample using “Echo” and “FID” techniques. We believe the key sources of decoherence in our samples are natural abundance of13C isotopes and P1 centers. To better understand and clarify contribution of each component we studied number of samples with different defect concentrations. Besides, multifrequency resonances were investigated in NV ensembles both experimentally and theoretically. Previously multifrequency resonances were studied in single NV center system at various combinations of transverse and longitudinal MW and RF field allowing to detect many-photon transitions in electron spin subshell[7]. Here we provide study of additional series of resonances achieved in NV ensemble configuration with a close look to the many-photon transitions both in electron and nuclear eigen states. This resonances could be utilized to hyperpolarize the nuclear spin environment in diamond.
NV centers ensemble have proven to be sensitive and versatile sensor platform for many applications such as magnetometry [1, 2], electric field sensing [3], thermometry [4] and rotation sensing [5]. The tradeoff between concentration of NVs and coherence lifetime of the electronic spin limits the maximum sensitivity of such devices. Recently usage of quantum memory associated with nuclear spins nearby NV center was shown to dramatically increase the sensitivity via accumulation of the sensed signal in memory [6]. This way sensing on single NV center can benefit a lot from nearby environment, but extension of this technique on atomic ensemble is still quite challenging. Here we present a versatile platform for quantum manipulation with a highly dense NV ensemble in HPHT diamond plates. In our work, we demonstrate several approaches to initialize quantum memory in regular magnetic fields, manipulation of quantum memory, and reading the quantum information from it. Also, we carefully study decoherence rate in our sample using “Echo” and “FID” techniques. We believe the key sources of decoherence in our samples are natural abundance of13C isotopes and P1 centers. To better understand and clarify contribution of each component we studied number of samples with different defect concentrations. Besides, multifrequency resonances were investigated in NV ensembles both experimentally and theoretically. Previously multifrequency resonances were studied in single NV center system at various combinations of transverse and longitudinal MW and RF field allowing to detect many-photon transitions in electron spin subshell[7]. Here we provide study of additional series of resonances achieved in NV ensemble configuration with a close look to the many-photon transitions both in electron and nuclear eigen states. This resonances could be utilized to hyperpolarize the nuclear spin environment in diamond.
Quantum sensing has witnessed rapid development and transition from laboratories to practical applications in the past decade. Applications of quantum sensors, ranging from nanotechnologies to biosensing, are expected to benefit from quantum sensors' unprecedented spatial resolution and sensitivity. Solid-state spin systems are particularly attractive platforms for quantum sensing technologies because room temperature operation is viable while reaching the quantum limits of sensitivity. Among various solid-state spin systems, nitrogen-vacancy (NV) centers in diamond demonstrated high-fidelity initialization, coherent control, and high contrast readout of the electron spin state. However, electron spin coherence due to noise from the surrounding spin bath and this environment effect limits the sensitivity of NV centers. Thus, a critical task in NV center-based quantum sensing is sensitivity enhancement through coherence protection. Several strategies, such as dynamical decoupling techniques, feedback control, and nuclear spin-assisted sensing protocols, have been developed and realized for this task. Among these strategies, nuclear spin-assisted protocols have demonstrated greater enhancement of electron spin coherence. In addition, the electron and nuclear spin pair of an NV center in diamond naturally allows the application of the nuclear spin-assisted sensitivity enhancement protocol. Owing to long nuclear coherence times, further enhancement of sensitivity can be achieved by exploiting active nuclear spins (e.g., 14N, 13C) in the proximity of an NV center as memory ancillas when coupled with the NV center. Here, we review the spin properties of NV centers, mechanisms of the nuclear spin-assisted protocol and its gate variation, and the status of quantum sensing applications in high-resolution nuclear spin spectroscopy, atomic imaging, and magnetic field sensing. We discuss the potential for commercialization, current challenges in sensitivity enhancement, and conclude with future research directions for promoting the development of nuclear spin-assisted protocol and its integration into industrial applications.
NV- center has been paid attention as a quantum sensor for highly sensitive detection of magnetic field, electric field, temperature and strain, etc.. Electron beam irradiation followed by subsequent thermal annealing is known as the effective procedures to create NV- center in diamond. The electron irradiation generates vacancies in diamond and the annealing enables vacancies to diffuse in the lattice. Then diffused vacancies are captured by nitrogen atoms, and as a result, NV centers are created. In addition, substitutional nitrogen atom at a lattice site (P1 center) also acts as a donor for NV center, the charge state of NV centers changes from neutral (NV0) to NV-. Creation high concentration of ensemble NV- centers is needed to achieve high sensitively sensing. The magnetic sensitivity is proportional to the reciprocal of the number of NV centers and T2 (spin coherence time), i.e., high concentration NV- centers with long T2 increase the sensitivity. P1 center has an important role for being a component of NV- centers and acting as donor to the NV center. But it also plays a role as decoherence source (decrease in magnetic field sensitivity). Therefore, it is important to create high concentration of NV- centers considering the amount of P1 centers. In current study, we investigate irradiation fluence dependence of the creation behavior of NV centers in type-Ib diamonds by using electron spin resonance (ESR) and photo-luminescence (PL) to understand the creation mechanism of NV- center. The values of T2 are also evaluated by Hahn-echo measurement, since it is one of the key parameters for highly sensitive magnetometry. Then, we discuss the creation mechanism of NV centers based on concentrations of defects.
Abstract Nitrogen-vacancy (NV) center ensembles in diamond are one of the most promising solid-state quantum platforms for various sensing applications. Achieving ultimate sensitivity requires simultaneously long spin dephasing times ( T2∗ T 2 * ) and high NV center concentrations. In this work, we propose a systematic measurement approach to quantify the electron spin dephasing in NV center ensembles and analyze the contributions of various sources to the dephasing time, including NV-NV interactions, strain and electric field distributions, 13 C nuclear spins, and P1 electron spins. Our method is validated using a series of high-performance diamond samples, providing a comprehensive understanding of dephasing mechanisms and revealing correlations between NV concentration and different dephasing sources. Building on these insights, we outline strategies to further enhance the achievable sensitivity for DC magnetic field measurements.
This thesis describes a series of experiments on the control of the optical properties of the nitrogen-vacancy (NV) center in diamond, and on control of the electron and nuclear spin states associated with the NV center. The NV center is a fluorescing atomic defect center in diamond, consisting of a substitutional nitrogen atom adjacent to a vacancy in the diamond carbon lattice. The electron spin of the NV center can be initialized and read out optically, and coherently manipulated with high fidelity even at room temperature. This unique combination of properties has attracted much attention to the NV center in recent years for application in quantum information technologies. The experiments in this work focus on exploiting the intrinsic hybrid nature of the NV center. The NV center forms a hybrid spin register, as its electron spin is always coupled to the nuclear spin of its own nitrogen nucleus, and can furthermore be coupled to additional nuclear carbon-13 spins in the diamond. In addition, the electronic spin state can be mapped onto the state of a photon, in principle allowing long range transmission of quantum information and measurement-based entanglement of distant NV center registers. The long-term goal is to achieve a large scale quantum network where the different constituents each perform the task they are most well suited for: the stable nuclear spins store the quantum information, photons transfer quantum information over long distances, and the fast electron spin interfaces between spin and photon states. For measurement-based entanglement of distant NV centers it is of primary importance to maximize the emission and detection of coherent photons emitted by the NV center. Spontaneous emission can be controlled and enhanced by coupling an emitter to a photonic cavity. Chapters 3, 4, and 5 of this thesis describe experiments aimed at maximizing the coherent photon emission rate of an NV center by coupling the NV center to a photonic crystal cavity. One of the main challenges is to position a single NV center into the nanometer-sized cavity mode. Chapter 3 describes the development of a nanopositioning technique, which allows diamond nanocrystals ~50 nm in size containing single NV centers to be placed at arbitrary locations on a chip, with nanometer precision. By using a sharp etched tungsten tip, individual nanocrystals can be picked up, moved and placed under real-time imaging with an electron microscope. We explicitly demonstrate that the unique optical and spin properties of the NV center are conserved by the nanopositioning process. In chapter 4, we apply the nanopositioning technique to couple single NV centers contained in a diamond nanocrystals to gallium phosphide photonic crystal cavities. These cavities resonate in the visible with high quality factors, and are designed to have the lowest energy mode coincide with the zero-phonon line of the NV center. Efficient coupling is evidenced by a strong enhancement of NV center emission at the cavity wavelength. The high quality factor of a photonic crystal cavity is a result of careful design and fabrication. The introduction of a nanocrystal may therefore be expected to have a detrimental effect on the optical properties of a cavity. In chapter 5 we investigate the effect of a nanocrystal on the optical properties of photonic crystal cavities. Our simulations and measurements show that the effect of a nanocrystal is in fact only minor, a promising result for future work on cavity-QED systems in the solid state and with diamond defect centers in particular. Understanding and mitigating decoherence is a key challenge for quantum science and technology. The main source of decoherence for solid-state spin systems is the uncontrolled spin bath environment. Chapter 6 describes experiments that exploit quantum control of the NV center and recently developed dynamical decoupling techniques to study the properties of the surrounding bath of spins belonging to single substitutional nitrogen atoms. The coherence properties of a single NV center are affected by changes in the state of the bath spins, allowing a single NV center to be used as a probe to detect bath spin resonances and study the quantum dynamics of the spin bath. We use quantum control of the spin bath to extend the dephasing time of the NV center, important for the application of the NV center in DC magnetometry. In chapter 7 we describe the development and implementation of decoherence protected quantum gates for the hybrid spin register formed by the NV center electron spin that is coupled to the nuclear spin of its own nitrogen atom. Electron and nuclear spins evolve and decohere at vastly different rates, making it challenging to use such a hybrid system as a fully functional quantum register. We create a universal set of two-qubit quantum gates by integrating dynamical decoupling techniques into the gate operation. We demonstrate the power of our gate design by implementing for the first time Grover's quantum search algorithm on a solid-state spin register.
By choosing the right system and using the right techniques, it is possible to achieve reliable control of an individual quantum system in a solid. Certain atom-like solid-state systems are especially suited for this goal. The electron spin of the diamond nitrogen-vacancy (NV) impurity center is a leader among such systems and has featured in a great deal of recent experimental work in the context of various quantum technologies. By extending optical control for the NV center we increase the utility of this system, opening it up to fresh applications in quantum optics. \n \nDoing quantum control with a solid-state spin comes with its own challenges. In particular it can be difficult to simultaneously isolate single systems, both for control and from environment-induced decoherence, while also coupling multiple systems together in a controlled way. A goal of the work presented in this dissertation is to develop techniques for answering this problem in the NV center. \n \nOptical control, as opposed to the microwave control usually used for state manipulation in the NV center, would make it easier to address only one spin system at a time. We demonstrate such control using two methods, two-photon optically driven Rabi oscillations and stimulated Raman adiabatic passage. These both have the added advantage that by using Raman-resonant, dipole-detuned optical fields, they protect the spin state from the decoherence normally associated with the optical transitions. Furthermore, we see that this electron spin control is nuclear spin dependent, providing a mechanism for coupling these two spin systems. \n \nWe also investigate a decoherence reduction technique that involves coupling continuous microwave fields to the spin states. The resulting "dressed states" are shielded from spin-bath-induced magnetic field fluctuations. We confirm this using optical coherent population trapping measurements which we have also developed in the NV center. We show that these measurements are sensitive to nuclear spin states as well as to dressed states. \n \nThese results supply the missing piece, optical spin manipulation, to control schemes that are all-optical, and they demonstrate ways to significantly push back the decoherence limit. \n \nThis dissertation includes previously published and unpublished co-authored material.
Abstract Nitrogen-vacancy (NV) centers in diamond have been developed into essential hardware units for a wide range of solid-state-based quantum technology applications. While such applications require the long spin coherence times of the NV centers, they are often limited due to decoherence. In this study, we theoretically investigate the decoherence of NV-spin ensembles induced by nitrogen impurities (P1 centers), which are one of the most dominant and inevitable magnetic field noise sources in diamond. We combined cluster correlation expansion and density functional theory to compute the Hahn-echo spin-coherence time of the NV centers for a broad range of P1 concentrations. Results indicate a clear linear dependence of T2 on P1 concentrations on a log scale with a slope of −1.06, which is in excellent agreement with previous experimental results. The interplay between the Jahn–Teller effect and the hyperfine interaction in the P1 center plays a critical role in determining the bath dynamics and the resulting NV decoherence. Our results provide a theoretical upper bound for the NV-spin T2 over a wide range of P1 densities, serving as a key reference for materials optimization and spin bath characterization to develop highly coherent NV-based devices for quantum information technology.