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A Quantum Network Node Based on the Silicon-Vacancy Defect in Diamond

David Levonian

Published Jul 12, 2021• 0 citations
OpenAlex

Abstract

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 $^{29}$Si 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.

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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 29^{29}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.
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.
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
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%1\%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>T_2 >T2​> \SI{13}{\milli\second} and spin relaxation time T1>T_1 >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>30C > 30C>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.
Long-distance quantum communication and networking require quantum memory nodes with efficient optical interfaces and long memory times. We report the realization of an integrated two-qubit network node based on silicon-vacancy centers (SiVs) in diamond nanophotonic cavities. Our qubit register consists of the SiV electron spin acting as a communication qubit and the strongly coupled silicon-29 nuclear spin acting as a memory qubit with a quantum memory time exceeding 2 seconds. By using a highly strained SiV, we realize electron-photon entangling gates at temperatures up to 1.5 kelvin and nucleus-photon entangling gates up to 4.3 kelvin. We also demonstrate efficient error detection in nuclear spin-photon gates by using the electron spin as a flag qubit, making this platform a promising candidate for scalable quantum repeaters.
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.
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.