Quantum networks enable applications ranging from quantum secure communication to distributed quantum computing and quantum-enhanced non-local sensing. Silicon-vacancy (SiV) centers in diamond nano-photonic cavities are a strong candidate platform for quantum networking due to their access to multiple qubits, high operation fidelities, and an efficient matter qubit-photon interface. In this thesis, I provide a background overview of both the field of quantum networking in general and the experimental platform of SiVs in diamond nano-photonic cavities. The thesis shows experimental demonstrations of SiVs from the last six years, including the development of the 29SiV as a quantum node with a two-qubit register and the subsequent remote entanglement of two such nodes over a metropolitan fiber. With this two-node network of SiVs, we implement two quantum networking applications. The first application, blind quantum computing, allows a user to remotely run an arbitrary algorithm on a quantum computer by controlling the measurement basis of photons they receive from the quantum computer, while the quantum computer receives no information about the implemented algorithm. We demonstrate a full blind quantum gate set required to implement an arbitrary quantum algorithm with information-theoretic security. The second application is entanglement-assisted remote optical interferometry. This experiment shows how remotely entangled SiVs can assist the phase sensitivity of long-baseline telescope arrays to better observe weak thermal photonic signals. I finally provide some remarks and propose future directions for the SiV as a quantum networking platform
Quantum networks enable applications ranging from quantum secure communication to distributed quantum computing and quantum-enhanced non-local sensing. Silicon-vacancy (SiV) centers in diamond nano-photonic cavities are a strong candidate platform for quantum networking due to their access to multiple qubits, high operation fidelities, and an efficient matter qubit-photon interface. In this thesis, I provide a background overview of both the field of quantum networking in general and the experimental platform of SiVs in diamond nano-photonic cavities. The thesis shows experimental demonstrations of SiVs from the last six years, including the development of the 29SiV as a quantum node with a two-qubit register and the subsequent remote entanglement of two such nodes over a metropolitan fiber. With this two-node network of SiVs, we implement two quantum networking applications. The first application, blind quantum computing, allows a user to remotely run an arbitrary algorithm on a quantum computer by controlling the measurement basis of photons they receive from the quantum computer, while the quantum computer receives no information about the implemented algorithm. We demonstrate a full blind quantum gate set required to implement an arbitrary quantum algorithm with information-theoretic security. The second application is entanglement-assisted remote optical interferometry. This experiment shows how remotely entangled SiVs can assist the phase sensitivity of long-baseline telescope arrays to better observe weak thermal photonic signals. I finally provide some remarks and propose future directions for the SiV as a quantum networking platform
Blind quantum computing is a promising application of distributed quantum systems, in which a client can perform computations on a remote server without revealing any details of the applied circuit. Although the most promising realizations of quantum computers are based on various matter-qubit platforms, implementing blind quantum computing on matter qubits remains a challenge. Using silicon-vacancy (SiV) centers in nanophotonic diamond cavities with an efficient optical interface, we demonstrated a universal quantum gate set consisting of single- and two-qubit blind gates over a distributed two-node network. Using these ingredients, we performed a distributed algorithm with blind operations across our two-node network, proving a route to develop blind quantum computation with matter qubits in distributed, modular architectures.
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.
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.
A key challenge in realizing practical quantum networks for long-distance quantum communication involves robust entanglement between quantum memory nodes connected via fiber optical infrastructure. I will talk about our recent work on utilizing silicon vacancy (SiV) centers in nanophotonic diamond cavities integrated with a telecommunication (telecom) metropolitan quantum network. Specifically, remote entanglement generation is generated by the cavity-enhanced interactions, and long-lived auxiliary memory qubits are used to provide second-long entanglement storage with integrated error detection. By interfacing with efficient bi-directional frequency conversion to telecommunication frequencies, we demonstrate entanglement of two nuclear spin memories through a 35 km long fiber deployed in the Boston urban area, an enabling step towards practical quantum repeaters and large-scale quantum network. I will also talk about an in-progress work on exploring blind and distributed quantum computation through this two-node quantum network.
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.
Entangling spatially separated quantum bits is an important tool for quantum key distribution, quantum sensing, and scalable quantum computing. The negatively charged silicon-vacancy center in diamond (SiV) is unique in its potential for efficient, high-fidelity gates with optical photons, which is the prerequisite for entangling networks of separated quantum bits. We have recently implemented a scheme to entangle SiVs with different optical transition frequencies, overcoming the inhomogeneous distribution common in solid-state optical qubits. Additionally, we have engineered high fidelity gates between the SiV spin and the spin of the 29Si nucleus, which can serve as a long-lasting quantum memory. These two advances present the prospects for entanglement distillation, device-independent quantum key distribution and distributed quantum sensing.