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Quantum Computing, Communication, and Simulation

Quantum networks based on diamond NV centers

Matteo Pompili, Sophie Hermans, Simon Baier, Hans K. C. Beukers, Peter C Humphreys, Raymond N. Schouten, R. F. L. Vermeulen, M. J. Tiggelman, Laura dos Santos Martins, Bas Dirkse, Stephanie Wehner, Ronald Hanson

Published Mar 4, 2021• 0 citations
Publisher

Abstract

Quantum networks are poised to enable large-scale secure communication, distributed quantum computing and simulation by means of shared entangled states over the nodes of the network. The Nitrogen-Vacancy (NV) centre in diamond is a promising candidate to act as node of such a quantum network, with long-lived multiple qubits and an optical interface for entanglement. We present updates on our effort towards the realisation of such a network.

DOI
10.1117/12.2583075

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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.

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Candidate platforms in the global endeavour to build long distance quantum networks need to satisfy a variety of properties to act as a network node. These include - interfacing between stationary spins and photonic flying qubits, long quantum memory times, indistinguishable photons and a large photon flux. Cavities operating in the weak coupling regime offer the advantages that the spontaneous emission rate of the emitter is enhanced while the loss rate from the cavity is also high. With optimized output couplers, this results in an increased photon extraction efficiency and the proportion of coherent photons is also improved. Cavity-coupled Nitrogen-vacancy centers in diamond (NV) fulfill most of these criteria with one weakness remaining - sensitivity to electronic noise [1]–[3].

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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.

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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.

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Faraday Discussions•Jan 1, 2015•39 citations
Quantum networks are poised to enable large-scale secure communication, distributed quantum computing and simulation by means of shared entangled states over the nodes of the network. The Nitrogen-Vacancy (NV) centre in diamond is a promising candidate to act as node of such a quantum network, with long-lived multiple qubits and an optical interface for entanglement. We present updates on our effort towards the realisation of such a 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.
Candidate platforms in the global endeavour to build long distance quantum networks need to satisfy a variety of properties to act as a network node. These include - interfacing between stationary spins and photonic flying qubits, long quantum memory times, indistinguishable photons and a large photon flux. Cavities operating in the weak coupling regime offer the advantages that the spontaneous emission rate of the emitter is enhanced while the loss rate from the cavity is also high. With optimized output couplers, this results in an increased photon extraction efficiency and the proportion of coherent photons is also improved. Cavity-coupled Nitrogen-vacancy centers in diamond (NV) fulfill most of these criteria with one weakness remaining - sensitivity to electronic noise [1]–[3].
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.
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.
Nitrogen-vacancy (NV) centres in diamond show great promise for quantum computing, where photonic entanglement can be generated using long-lived optically-active spins [1]. A quantum processor would require a large number of interconnected NV centres, and an integrated platform becomes necessary for control and routing. Benefits of integration include nanophotonic cavities with ultra-small mode volumes [2], multilayer electronics, and use of existing integrated quantum photonic components and architectures. Pick-and-place methods have been adopted to combine diamond microchiplets with aluminium nitride [3]. Photonic and spin properties has also been measured in nanodiamond encapsulated in optimised silicon nitride (SiNx) [4]. However, challenges remain to interface diamond with photonics in a scalable and manufacturable process. In particular, the need to identify and manipulate stochastically located emitters requires precise and time-intensive confocal microscopy.
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.