Developing technologies for distributing entanglement over long distances, and employing this technology to create quantum communication networks, has been a standing goal of the quantum optics community for almost two decades. Such networks could be used for secure communication enabled by quantum key distribution (QKD), enhancement of quantum sensors, and even the creation of networked quantum computers. We present a platform, based on the negatively charged Silicon Vacancy (SiV) in diamond, capable of correcting for the photon loss that currently limits the size of such networks. Furthermore, we lay the groundwork for its use for scalable implementation of the full quantum repeater protocol, the missing ingredient for the deployment of large quantum networks. First, we demonstrate improved diamond nanofabrication techniques that enable the creation of an integrated, cooperativity 100 spin-photon interface. We further discuss the techniques used to integrate this platform with microwave coplanar waveguides (CPWs) that enable coherent control of the SiV spin state. Next, we demonstrate the integration of diamond nanophotonic technology with nanomechanical strain control, enabling the reduction of variations between, and fluctuations of, diamond color center optical and spin properties. These techniques are used to enable quantum interference between two initially distinguishable quantum emitters. To further demonstrate the utility of this platform, we employ coherent spin control and our platform's high cooperativity spin-photon interface to create an integrated quantum network node capable of high fidelity, coherent photon storage and the formation of multiqubit quantum registers. We discuss the engineering and physics principles relevant to the implementation of such a system, and provide a framework for future improvements to the underlying technology. Finally, we employ the developed techniques to perform Bell state measurements between asynchronously arriving photons. By utilizing this technique we demonstrate enhancement of the quantum communication rate between two parties, thus implementing the key functionality of a single quantum repeater node. This demonstration, combined with the other technical advances presented here, positions the diamond photonics-SiV platform as a leading candidate for the implementation quantum networking technology and provides a path towards large scale deployment of such systems.
Developing technologies for distributing entanglement over long distances, and employing this technology to create quantum communication networks, has been a standing goal of the quantum optics community for almost two decades. Such networks could be used for secure communication enabled by quantum key distribution (QKD), enhancement of quantum sensors, and even the creation of networked quantum computers. We present a platform, based on the negatively charged Silicon Vacancy (SiV) in diamond, capable of correcting for the photon loss that currently limits the size of such networks. Furthermore, we lay the groundwork for its use for scalable implementation of the full quantum repeater protocol, the missing ingredient for the deployment of large quantum networks. First, we demonstrate improved diamond nanofabrication techniques that enable the creation of an integrated, cooperativity 100 spin-photon interface. We further discuss the techniques used to integrate this platform with microwave coplanar waveguides (CPWs) that enable coherent control of the SiV spin state. Next, we demonstrate the integration of diamond nanophotonic technology with nanomechanical strain control, enabling the reduction of variations between, and fluctuations of, diamond color center optical and spin properties. These techniques are used to enable quantum interference between two initially distinguishable quantum emitters. To further demonstrate the utility of this platform, we employ coherent spin control and our platform's high cooperativity spin-photon interface to create an integrated quantum network node capable of high fidelity, coherent photon storage and the formation of multiqubit quantum registers. We discuss the engineering and physics principles relevant to the implementation of such a system, and provide a framework for future improvements to the underlying technology. Finally, we employ the developed techniques to perform Bell state measurements between asynchronously arriving photons. By utilizing this technique we demonstrate enhancement of the quantum communication rate between two parties, thus implementing the key functionality of a single quantum repeater node. This demonstration, combined with the other technical advances presented here, positions the diamond photonics-SiV platform as a leading candidate for the implementation quantum networking technology and provides a path towards large scale deployment of such systems.
Quantum networks require functional nodes consisting of stationary registers with the capability of high-fidelity quantum processing and storage, which efficiently interface with photons propagating in an optical fiber. We report a significant step towards realization of such nodes using a diamond nanocavity with an embedded silicon-vacancy (SiV) color center and a proximal nuclear spin. Specifically, we show that efficient SiV-cavity coupling (with cooperativity C>30) provides a nearly deterministic interface between photons and the electron spin memory, featuring coherence times exceeding 1 ms. Employing coherent microwave control, we demonstrate heralded single photon storage in the long-lived spin memory as well as a universal control over a cavity-coupled two-qubit register consisting of a SiV and a proximal ^{13}C nuclear spin with nearly second-long coherence time, laying the groundwork for implementing quantum repeaters.
The realization of quantum networks capable of distributing quantum information over kilometer scale distances is key to maximizing the potential of quantum information technology. Implementing this technology requires both the ability to generate remote entanglement between quantum memory nodes, requiring an efficient spin-photon interface, and the ability to efficiently scale the classical optical control and multiplexing optics within a given node. In this work, we develop integrated photonics platforms in diamond and thin film lithium niobate to address both of these issues. Together, these platforms provide the necessary ingredients to create large scale quantum networking technology. For the quantum network nodes, we extend previous results combining diamond nanophotonic resonators with the silicon-vacancy defect as a quantum networking platform. We demonstrate, for the first time, quantum entanglement between SiV quantum memories across 35km in a deployed fiber network. We also develop additional functionalities within the SiV platform including the incorporation of deterministic strain to increase defect yield and deterministic single photon generation with the SiV. In order to enable the scalable control and multiplexing of individual SiV centers necessary for realizing practical quantum network nodes, we develop a state-of-the-art electro-optics platform operating at visible wavelengths using thin film lithium niobate. We develop an understanding of fabrication induced losses at visible wavelengths in the platform and a variety of key devices in the platform including high-bandwidth, low-drive voltage amplitude and phase modulators, as well as high-efficiency off chip couplers. Utilizing this platform we perform ultra-high efficiency electro-optic frequency shifting, achieving a 15 GHz frequency shift with over 50% efficiency. Finally, using our measured device performance, we discuss and model how these devices can effectively multiplex SiV-based quantum nodes. Thus through these two platforms we have demonstrated the necessary ingredients to realize large-scale quantum network nodes.
The construction of a quantum internet, an interconnected network which can distribute and store quantum states, is an essential step towards the development of next generation quantum technologies, including unconditionally secure communication, enhanced metrology, and distributed quantum computing. Development of these networks require functional nodes consisting of stationary registers with the capability of high-fidelity quantum information processing and storage, which efficiently interface with photonic qubits. This thesis outlines one emerging platform for building a quantum network node using the silicon-vacancy color center in diamond (SiV). Although previous work has established the SiV as a superior optical emitter capable of strong light-matter coupling, its feasibility as a quantum memory has remained elusive, primarily due to coupling to thermal phonons. This suggests two branching research directions: one where this coupling is investigated and characterized as a nanoscale sensor, and one where this coupling is supressed in order to realize the SiV as a quantum information processor. This thesis begins by highlighting the extreme phonon sensitivity as a metrological tool, where an ensemble of SiVs are used as an all-optical thermometer, capable of \SI{70}{\milli\kelvin} precision at room temperature. When incorporated into nanodiamonds, the sensing properties deviate by less than 1% between nanodiamonds, enabling calibration-free thermometry for sensing and control of complex nanoscale systems. For quantum information tasks, coherent control of the SiV electronic spin qubit is enabled by suppressing phonon-induced dephasing by five orders of magnitude at temperatures below \SI{500}{\milli\kelvin}. By aligning the magnetic field along the silicon-vacancy symmetry axis, optical transitions are highly spin-conserving, allowing for single-shot readout of the qubit with high fidelity. This coherent control is used to demonstrate a spin coherence time T2> \SI{13}{\milli\second} and spin relaxation time T1> \SI{1}{\second} at \SI{100}{\milli\kelvin}. Finally, SiV quantum memories are packaged into a fully integrated quantum network node by incorporating SiV qubits into nanophotonic cavities. The efficient SiV-cavity coupling (with cooperativity C>30) provides a nearly-deterministic interface between photons and the spin qubit, which enables heralded single-photon storage. Coherent coupling to nearby \cnuc\ nuclear spins (with nearly second-long coherence times) enables multi-qubit registers and paves the way towards large-scale quantum networks.
Efficient interfaces between optical photons and quantum bits are fundamental building blocks for quantum networks and large-scale quantum computers. We demonstrate an integrated platform for scalable quantum optics based on color centers coupled to diamond nanodevices. First, we incorporate nitrogen-vacancy (NV) centers in diamond into hybrid nanophotonic crystal cavities. Despite our progress towards coupling the NV to cavity photons, we find that the NV-cavity interface is limited by the imperfections of the color center's properties and its local environment. We approach this challenge first by characterizing and modifying the diamond surface to reduce the environmental noise affecting the NV optical transition. Subsequently, we develop a new approach that makes use of a quantum emitter with intrinsic protection from this noise: the silicon-vacancy (SiV) center in diamond. We demonstrate that SiV centers introduced through ion implantation feature highly coherent optical transitions, even inside nanostructures. By placing SiV centers inside all-diamond photonic crystal cavities, we achieve strong SiV-photon coupling with a cooperativity of greater than 20. Using this platform, we realize a quantum-optical switch controlled by the spin degree of freedom of a single SiV. By measuring intensity correlations of indistinguishable Raman photons emitted into a single waveguide, we observe a quantum interference effect resulting from the superradiant emission of two probabilistically entangled SiV centers. Finally, we demonstrate controllable photon-mediated interactions between the spin degrees of freedom of two cavity-coupled SiV centers. When the optical transitions of the two SiV centers are tuned into resonance, the coupling of each SiV to the common cavity mode results in a coherent interaction between the two SiVs, leading to the observation of super- and sub-radiant two-SiV states.
Quantum networks hold the potential to enable quantum-secured communication, distributed quantum computing, and non-local quantum sensing. To realize these applications, a versatile quantum network infrastructure must support entanglement between network nodes capable of storing, processing, and distributing quantum information, alongside high-fidelity photonic qubits that efficiently interface with these nodes. The silicon-vacancy (SiV) center in diamond, coupled to nanophotonic cavities, has recently emerged as a promising platform to meet these requirements. This system provides access to an electron spin as an optically active communication qubit and a 29Si nuclear spin as a memory qubit, capable of storing quantum information for extended periods. We first demonstrate that the SiV center's electron spin can efficiently generate single photons with complex spatiotemporal waveforms, facilitated by a novel asymmetric nanophotonic cavity design. We then establish a two-node quantum network between two SiV centers housed in separate laboratories and connected via optical fiber. To do so, we use photonic qubits to mediate entanglement between two spatially separated electron spins, as well as between two spatially separated nuclear spins. Finally, using bidirectional quantum frequency conversion, we convert the photonic qubits to telecommunication frequencies, enabling entanglement generation between the two nuclear spins via a 35 km fiber deployed in the Boston metropolitan area. These advancements mark progress toward large-scale, deployable quantum networks using SiV centers coupled to nanophotonic cavities.
A Quantum Repeater Platform based on Single SiV− Centers in Diamond with Cavity-Assisted, All-Optical Spin Access and Fast Coherent Driving
Quantum key distribution enables secure communication based on the principles of quantum mechanics. The distance in fiber-based quantum communication is limited to about a hundred kilometers due to signal attenuation. Thus, quantum repeaters are required to establish large-scale quantum networks. Ideal quantum repeater nodes possess a quantum memory which is efficiently connected to photons, the carrier of quantum information. Color centers in diamond and, in particular, the negatively-charged silicon-vacancy centers are promising candidates to establish such nodes. The major obstacle is an inefficient connection between the color centers spin to the Gaussian optics of fiber networks. Here, we present an efficient spin-photon interface. Individual silicon-vacancy centers coupled to the mode of a hemispherical Fabry-Pérot microcavity show Purcell-factors larger than 1 when operated in a bath of liquid Helium. We demonstrate coherent optical driving with a Rabi frequency of 290MHz and all-optical access to the electron spin in strong magnetic fields of up to 3.2T. Spin initialization within 67ns with a fidelity of 80% and a lifetime of 350ns are reached inside the cavity. The spin-photon interface is passively stable, enabled by placing a color center containing nanodiamond in the hemispherical Fabry-Pérot mirror structure and by choosing short cavity lengths. Therefore, our demonstration opens the way to realize quantum repeater applications.