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Control, measurement and entanglement of remote quantum spin registers in diamond

Hannes Bernien

Published Jan 1, 2014• 7 citations
Publisher

Abstract

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.

DOI
10.4233/uuid:75130c37-edb5-4a34-ac2f-c156d377ca55

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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 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.
Gaining precise control over quantum systems is crucial for applications in quantum information processing and quantum sensing and to perform experimental tests of quantum mechanics. The experiments presented in this thesis implement quantum measurements and real-time feedback protocols that can help to achieve these goals using single electron and nuclear spins associated with the Nitrogen Vacancy (NV) center in diamond. Here we demonstrate that adaptive measurements allow for the manipulation of a quantum system using only measurement backaction and that they can enhance the performance of a single spin magnetometer. Furthermore, by creating entanglement and performing teleportation between two distant NV centers, we implement two elementary operations between two nodes of a quantum network.
Due to its long spin coherence and coherent spin-photon interface the nitrogen vacancy (NV) center in diamond has emerged as a promising platform for quantum science and technology, including quantum networks, quantum computing and quantum sensing. In recent years larger quantum systems have been demonstrated by using optical entanglement links between distant NV centers. These systems were based on high-quality NV centers that exhibit good optical coherence. State-of-the-art experiments with such systems have shown deterministic delivery of entanglement across a two-node quantum network as well as genuine multi-partite entanglement across a three-node quantum network. The additional capability to create larger quantum registers by direct magnetic coupling between high-quality NV centers and to other nearby defects would provide new opportunities for quantum memories in quantum networks but also for enhanced sensing protocols and spin chains for quantum computation architectures. In this thesis, we investigate methods to create larger quantum registers based on magnetic coupling and develop techniques to address and control individual defects in a system consisting of multiple defects. The results provide new insights for extended quantum registers based on magnetically coupled defects...
This thesis presents a set of experiments that explore the possible realisation of a macroscopic quantum network based on solid-state quantum bits. Such a quantum network would allow for studying quantum mechanics on large scales (meters, or even kilometers), and can open new possibilities for applications in quantum information processing. A promising candidate platform for such a network is the nitrogen-vacancy (NV) center in diamond. The NV center is a lattice defect in diamond that has excellent quantum properties and allows optical access to a quantum register of individual electronic and nuclear spins. We use optical spectroscopy and magnetic resonance techniques at liquid helium temperature to manipulate individual spins, and perform projective quantum measurements on them. We first show an experiment in which we create entanglement between two non-interacting nuclear spins by a projective quantum measurement. Second, we demonstrate the creation of an entangled state between two NV electronic spins located in different setups at a distance of three meters. Finally, we demonstrate the deterministic teleportation of the quantum state of a nuclear spin onto an electronic spin over three meters. Our results demonstrate the great potential of the NV center for the realization of a large-scale quantum network.
Quantum mechanics differs deeply from classical intuition and knowledge, sparking fundamental questions and radically new technology. Generating large entangled states between distant nodes of a quantum network will advance both domains. The nitrogen-vacancy (NV) centre in diamond is a promising building block for such a network. However, extending quantum networks to more nodes and larger distances relies upon improving the entangling efficiency of these defect centres. In this thesis we present experimental and theoretical work focused on addressing this challenge through embedding NV centres in an optical cavity, taking care to preserve coherence of the NV optical transition. We further analyze protocols for efficient quantum communication over an NV-based quantum network.