Quantum networks promise to be the future architecture for secure communication and distributed quantum computation. This thesis describes experiments on nitrogenvacancy (NV) centres that lead towards a versatile multi-node quantum network consisting ofmulti-qubit nodes. The NV centre in diamond is a spinful optically-active crystal defect. NVs are a prime network-node candidate due to demonstrated coherence times beyond 100ms and longitudinal relaxation times exceeding 1s and their spin-selective optical interface which facilitates the generation of spin-photon entanglement. Entangling links between nodes are therefore readily created by overlapping the emission of two NVs on a beam splitter. Besides NVs, we further address individual 13C nuclear spins in the vicinity and use these spins as a quantum resource. Our goal is to propel these nuclear spins to constitute robust quantummemories which store and manipulate quantum information in an NV-based quantum network. The experiments described in this thesis are thematically separated into three groups. First, we explore the NV-nuclear interplay. We demonstrate nuclear-spin control by observing the Zeno effect on up to two logical qubits within the state space of three nuclear spins (Chapter 3). We further realize that the always-on magnetic hyperfine interaction between NV and nuclear spins will limit the nuclear spin coherence when entangling distant NV centres (Chapter 4). A systematic experimental study probes our theoretical prediction and we additionally demonstrate improved robustness for logical states within decoherence-protected state spaces (Chapter 5) and finally for individual nuclear spins (Chapter 6). Second,we use remoteNV-NV entangled states to demonstrate experimental milestones in quantumnetworks. The realization of a high-fidelity entangled link over a distance of 1.3km permits the loophole-free violation of Bell’s inequality (Chapter 7). We further increase the entangling rate by three orders of magnitude such that it exceeds the decoherence rate of an entangled state on our network. This allows us to convert our probabilistic entanglement generation into a deterministic process which delivers entangled states at prespecifiedmoments in time (Chapter 8). Third, we finally combine the concepts of nuclear-spin quantum memories and remote entanglement generation to demonstrate entanglement distillation in a network setting (Chapter 9). We subsequently generate two raw entangled input states between two remote NV centres. The first state is stored on nuclear spins to liberate both NVs for the second round of state generation. Finally, a higher-fidelity entangled state is distilled via local operations. This constitutes the first quantum-network demonstration that relies on the control of multiple fully-coherent quantum systems per network node.
Quantum networks promise to be the future architecture for secure communication and distributed quantum computation. This thesis describes experiments on nitrogenvacancy (NV) centres that lead towards a versatile multi-node quantum network consisting ofmulti-qubit nodes. The NV centre in diamond is a spinful optically-active crystal defect. NVs are a prime network-node candidate due to demonstrated coherence times beyond 100ms and longitudinal relaxation times exceeding 1s and their spin-selective optical interface which facilitates the generation of spin-photon entanglement. Entangling links between nodes are therefore readily created by overlapping the emission of two NVs on a beam splitter. Besides NVs, we further address individual 13C nuclear spins in the vicinity and use these spins as a quantum resource. Our goal is to propel these nuclear spins to constitute robust quantummemories which store and manipulate quantum information in an NV-based quantum network. The experiments described in this thesis are thematically separated into three groups. First, we explore the NV-nuclear interplay. We demonstrate nuclear-spin control by observing the Zeno effect on up to two logical qubits within the state space of three nuclear spins (Chapter 3). We further realize that the always-on magnetic hyperfine interaction between NV and nuclear spins will limit the nuclear spin coherence when entangling distant NV centres (Chapter 4). A systematic experimental study probes our theoretical prediction and we additionally demonstrate improved robustness for logical states within decoherence-protected state spaces (Chapter 5) and finally for individual nuclear spins (Chapter 6). Second,we use remoteNV-NV entangled states to demonstrate experimental milestones in quantumnetworks. The realization of a high-fidelity entangled link over a distance of 1.3km permits the loophole-free violation of Bell’s inequality (Chapter 7). We further increase the entangling rate by three orders of magnitude such that it exceeds the decoherence rate of an entangled state on our network. This allows us to convert our probabilistic entanglement generation into a deterministic process which delivers entangled states at prespecifiedmoments in time (Chapter 8). Third, we finally combine the concepts of nuclear-spin quantum memories and remote entanglement generation to demonstrate entanglement distillation in a network setting (Chapter 9). We subsequently generate two raw entangled input states between two remote NV centres. The first state is stored on nuclear spins to liberate both NVs for the second round of state generation. Finally, a higher-fidelity entangled state is distilled via local operations. This constitutes the first quantum-network demonstration that relies on the control of multiple fully-coherent quantum systems per network node.
Single defect centers in diamond have emerged as a powerful platform for quantum optics experiments and quantum information processing tasks. Connecting spatially separated nodes via optical photons into a quantum network will enable distributed quantum computing and long-range quantum communication. Initial experiments on trapped atoms and ions as well as defects in diamond have demonstrated entanglement between two nodes over several meters. To realize multi-node networks, additional quantum bit systems that store quantum states while new entanglement links are established are highly desirable. Such memories allow for entanglement distillation, purification and quantum repeater protocols that extend the size, speed and distance of the network. However, to be effective, the memory must be robust against the entanglement generation protocol, which typically must be repeated many times. Here we evaluate the prospects of using carbon nuclear spins in diamond as quantum memories that are compatible with quantum networks based on single nitrogen vacancy (NV) defects in diamond. We present a theoretical framework to describe the dephasing of the nuclear spins under repeated generation of NV spin-photon entanglement and show that quantum states can be stored during hundreds of repetitions using typical experimental coupling parameters. This result demonstrates that nuclear spins with weak hyperfine couplings are promising quantum memories for quantum networks.
A quantumnetwork would allow the distribution of a quantum state over many spatially separated quantum nodes which individually possess the ability to generate, process and store quantum information. Connecting these nodes through quantum communication channels would enable sending quantum information over arbitrarily long distances, secret key distribution with guaranteed secure communication and distributed quantum computing. An appealing platform for implementation of quantum networks is the nitrogen-vacancy (NV) center in diamond. An NV center is an optically active defect in diamond created by the substitution of two adjacent carbon atoms in the diamond crystal matrix by a nitrogen atom and a neighboring vacancy. The NV center fits remarkably well in the described quantum network framework. Its electronic spin can be optically initialized, read out in a single shot, coherently manipulated and coupled to the nearby carbon-13 nuclear spins. These properties represent necessary ingredients of a multi-qubit quantum node. The possibility of the entanglement between the state of the NV center spin and a photon establishes a photonic quantum link which can enable the entanglement of the quantum nodes, forming a quantum network. First building blocks of the proposed quantum networks based on NV centers were demonstrated by performing entanglement between two distant NV centers separated by more than a kilometer. However, the current success rate of the entangling protocols is greatly reduced due to the low emission probability of the NV center photons into the resonant zero phonon line (ZPL) and the inefficient photon extraction from the diamond. This is the central problem which prevents promoting our experiments beyond proof-of-principle demonstration towards practical implementation of the proposed quantumnetworks. The goal of this thesis is tackling these issues by coupling NV centers to optical cavities which would greatly increase the rate of generation and collection of the ZPL photons through themechanism of Purcell enhancement. The design and the fabrication of the components constituting a diamond-basedFabry-Perot microcavity, as used in this thesis, are described in Chapter 4. For large enhancement of the NV center resonant emission, a low cavity mode volume is necessary. This is achieved by inserting a micrometers thin diamond slab into the cavity architecture; we present the fabrication details of such samples. Chapter 5 describes the fabrication of an integrated platform for microwave signal delivery to the NV centers within a diamond membrane in the cavity architecture. Microwave striplines and arrays of unique markers are embedded in the mirror onto which the diamond is bonded. We investigate the mirror optical properties post fabrication and find that the fabrication method preserves the mirror optical performance. We describe the diamond bonding method and demonstrate addressing of the NV center spin. In Chapter 6 we probe the properties of the cavity with the embedded diamond membrane through measurements of the cavity finesse. We investigate the cavity finesse dependence on length, mode structure and temperature. We further explore the operation at cryogenic temperatures by probing the effect of cryostation vibration on the cavity linewidth. Finally, in Chapter 7 we discuss ways of improving the existing experimental capabilities, outline the first steps for demonstrating enhancement of the NV center resonant emission and suggest future experiments that can be performed with this system. We conclude that coupling of the NV centers to the cavities developed in this research could lead to an increase of remote entanglement success rates by more than three orders of magnitude.
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.
A quantum network is the essential resource for distributed quantum computation and the enabling technology for secure quantum communication over large distances. Setting up such a network would require establishing quantum connections between local nodes which are capable of generating, processing and storing quantum information. Intensive research is carried out in laboratories around the world investigating suitable systems for implementing a quantum network. The experiments presented in this thesis explore the possible realisation with nitrogen vacancy (NV) centres in diamond. We study how these centres could serve as nodes in such a network and develop a toolbox that consists of quantum control and measurement acting on these nodes. Furthermore, we demonstrate how to connect nodes and create entanglement between two distant NV centres.
Quantum networks will enable the transmission of quantum information over long distances, allowing us to link quantum computers for applications such as quantum cryptography, astronomical interferometry, and blind quantum computing. The silicon-vacancy center in diamond has emerged as a promising scalable solid-state platform capable of efficiently interfacing its electron spin with photons for long-range communication. This thesis investigates two long-lived nuclear spin qubits as quantum memories to significantly expand the quantum state storage and processing capabilities of this networking node. First, we demonstrate that selective isotope implantation gives a deterministic 29Si high-fidelity nuclear memory with coherence times over two seconds, enabling its integration into a two-node quantum network through 35 km of optical fiber deployed through the Boston metropolitan area. Next, we explore the control of a nearby 13C nuclear spin robust against repeated readouts and resets of the electron networking qubit, and utilize it to show a quadratic rate enhancement for entanglement swapping between two incoming photonic qubits. Together, these results advance our three-qubit node toward realistic operation as a quantum repeater to boost entanglement rates in a long-range network.
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.