Group theory is used to identify analyze and predict the properties of Nitrogen Vacancy (NV) centres.This could be
extended to other defects in solids.Analysis of the properties of defects through group theory
helps us to explain the spin-photon entanglement in NV centres.A theoretical demonstration
of entanglement between two single electron spins in separate Nitrogen Vacancy Centre is
done.Future work include the possible demonstration of entanglement in NV centres inferring
other coherent interactions in them.
Group theory is used to identify analyze and predict the properties of Nitrogen Vacancy (NV) centres.This could be
extended to other defects in solids.Analysis of the properties of defects through group theory
helps us to explain the spin-photon entanglement in NV centres.A theoretical demonstration
of entanglement between two single electron spins in separate Nitrogen Vacancy Centre is
done.Future work include the possible demonstration of entanglement in NV centres inferring
other coherent interactions in them.
We present a procedure that makes use of group theory to analyze and predict the main properties of the negatively charged nitrogen-vacancy (NV) center in diamond. We focus on the relatively low temperature limit where both the spin–spin and spin–orbit effects are important to consider. We demonstrate that group theory may be used to clarify several aspects of the NV structure, such as ordering of the singlets in the ( e 2 ) electronic configuration and the spin–spin and spin–orbit interactions in the ( ae ) electronic configuration. We also discuss how the optical selection rules and the response of the center to electric field can be used for spin–photon entanglement schemes. Our general formalism is applicable to a broad class of local defects in solids. The present results have important implications for applications in quantum information science and nanomagnetometry.
We use a group theoretical approach to model the nitrogen-vacancy defect in diamond. In our analysis we clarify several properties of this defect that have been source of controversy such as the ordering of the singlets and the mechanism that leads to spin mixing in the excited state of this defect. In particular, we demonstrate that the ordering of the ground state configuration (e2) is {3A2, 1E, 1A1} and that the spin-spin interaction causes the mixing in the excited state. In addition, we analyze the angular momentum and spin properties of the excited state structure that enables a spin photon entanglement scheme that has been recently demonstrate experimentally. Our description is general and it can be easily applied to other defects in solid-state systems.
Individual nitrogen vacancy (NV) centers in diamond have recently emerged as a leading candidate for a building block for quantum information processing systems. Individual NV centers have many properties that are desirable for qubits. For example their spin states can be manipulated, initialized and measured even at room temperature. In order to build a quantum information processing system that consists of more than a single NV center, these individual NV centers have to be connected via quantum channels that distribute entanglement and allow quantum state transfer between NV centers. This thesis explores how the optical properties of the NV center can be used and manipulated to form these quantum channels. Likewise we show how the same properties can be used to better control the nuclear spin environment surrounding the NV center. We first introduce, review and experimentally study the relevant optical properties of the NV center. These quantum optical techniques are then used to experimentally demonstrate spin-photon entanglement between the electronic spin of an individual NV center and a single photon. We next demonstrate quantum interference of two photons produced by NV centers in distinct diamond samples separated by two meters. These two demonstrations pave the way for entanglement generation, leading to quantum channels, between remote NV centers. Finally we demonstrate optical cooling, real-time measurement and conditional preparation of the nuclear spin environment around an NV center using all-optical manipulation of its electronic spin. This technique can lead to better control of individual NV centers and thus more robust quantum information processing systems.
Abstract The nitrogen‐vacancy (NV) center in diamond is a solid‐state defect qubit with favorable coherence time up to room temperature, which could be harnessed in several quantum‐enhanced sensor and quantum communication applications, and has a potential in quantum simulation and computing. The quantum control largely depends on the intricate details about the electronic structure and states of the NV center, the radiative and nonradiative rates between these states, and the coupling of these states to external spins, electric, magnetic, and strain fields, and temperature. This review shows how first‐principles calculations contributed to understanding the properties of the NV center and briefly discusses the issues to be solved toward the full ab initio description of solid‐state defect qubits.
In this thesis we experimentally investigate quantum nonlocality: entangled states of spatially separated objects. Entanglement is one of the most striking consequences of the quantum formalism developed in the 1920's; the predicted outcomes of independent measurements on entangled objects reveal strong correlations that cannot be explained by classical physics. Early on, such predictions led physicists to doubt the validity and completeness of quantum theory. At the same time, entanglement is a key resource for applications in quantum information processing and a pre-requisite for many tasks in quantum communication and computation. This thesis attempts to answer two application driven questions: Firstly, can we generate useful entangled states of solid state spins for applications in quantum information processing? Secondly, can we use such entangled states as a resource to teleport an unknown quantum state? Finally, we ask a foundational question: Are our entangled states indeed inconsistent with the classical notions of free choice, locality and realism? Can we prove this experimentally, under the minimal assumptions of a loophole-free Bell test? To answer these questions we use single spins in ultra-pure diamonds. In particular, we use the electronic and nuclear spins associated with single nitrogen-vacancy (NV) defects. The NV centre is a point defect in diamond, consisting of a substitutional nitrogen (N) atom and a neighbouring missing carbon atom (vacancy, V). The NV centre possesses bound electronic states, whose energy levels lie well within the bandgap of the diamond host and whose spin degree of freedom can be used as a quantum bit (qubit). Because of the large diamond bandgap and the 99% spin free carbon-12 environment, the electronic spin qubit has exceptional coherence properties even at room temperature. Optical and microwave fields allow control of the electronic spin, which in turn allows control of nearby nuclear spins (the host nitrogen nuclear spin, and nearby carbon-13 spins). At liquid helium temperatures, spin-preserving optical transitions provide a powerful optical interface to the electronic spin, allowing, for example, projective readout of the spin state. By employing a protocol where entanglement is heralded by the detection of a single photon from each of two NV centres in diamonds separated by three metres, we find we can answer the first question in the affirmative. We show for the first time heralded entanglement between solid state quantum systems separated by a human-scale distance. Then, by combining the heralded entanglement with a deterministic local Bell state measurement and fast feed-forward, we show for the first time unconditional quantum teleportation over human-scale distances. We teleport an unknown quantum state from a nuclear spin in one diamond to an electronic spin in a diamond three meters away. Finally, by employing techniques from the previous experiments, we implement the first loophole-free Bell test. We separate two diamonds by 1.3 kilometres and optimize all operational fidelities, collection efficiencies and rates. This allows us to generate heralded entanglement between them approximately once an hour. The distance provides us with time to read out the electronic spin state in each diamond, faster than any lightlike signal could travel between them. The high-fidelity entangled state preparation and spin readout are sufficient to violate the Clauser-Horne-Shimony-Holt Bell-inequality. Combined with fast random number generators and a robust statistical analysis, we find a significant rejection of the local-realist hypothesis, without requiring additional experimental assumptions. The results in this thesis open the door to various applications in quantum information processing. In particular, a remote photonic entangling operation may enable future quantum networks. In such a network the nodes would be formed by the NV centre's combined electronic and nuclear spin register. The nodes would be linked by photonic entanglement operations. Such a network could be used for long distance secure communication, provide a connection between separated quantum computers, or form the basis of a fault tolerant quantum computer by itself. Furthermore, a loophole-free Bell test demonstrates the possibility to do device independent randomness generation and key distribution, that could form the basis for future secure communication channels.