Abstract A key challenge in realizing practical quantum networks for long-distance quantum communication involves robust entanglement between quantum memory nodes connected by fibre optical infrastructure1–3. Here we demonstrate a two-node quantum network composed of multi-qubit registers based on silicon-vacancy (SiV) centres in nanophotonic diamond cavities integrated with a telecommunication fibre network. Remote entanglement is generated by the cavity-enhanced interactions between the electron spin qubits of the SiVs and optical photons. Serial, heralded spin-photon entangling gate operations with time-bin qubits are used for robust entanglement of separated nodes. Long-lived nuclear spin qubits are used to provide second-long entanglement storage and integrated error detection. By integrating efficient bidirectional quantum frequency conversion of photonic communication qubits to telecommunication frequencies (1,350 nm), we demonstrate the entanglement of two nuclear spin memories through 40 km spools of low-loss fibre and a 35-km long fibre loop deployed in the Boston area urban environment, representing an enabling step towards practical quantum repeaters and large-scale quantum networks.
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 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.
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.
Quantum networks will enable the transmission of quantum information over long distances, allowing us to link quantum computers for applications such as quantum cryptography, astronomical interferometry, and blind quantum computing. The silicon-vacancy center in diamond has emerged as a promising scalable solid-state platform capable of efficiently interfacing its electron spin with photons for long-range communication. This thesis investigates two long-lived nuclear spin qubits as quantum memories to significantly expand the quantum state storage and processing capabilities of this networking node. First, we demonstrate that selective isotope implantation gives a deterministic 29Si high-fidelity nuclear memory with coherence times over two seconds, enabling its integration into a two-node quantum network through 35 km of optical fiber deployed through the Boston metropolitan area. Next, we explore the control of a nearby 13C nuclear spin robust against repeated readouts and resets of the electron networking qubit, and utilize it to show a quadratic rate enhancement for entanglement swapping between two incoming photonic qubits. Together, these results advance our three-qubit node toward realistic operation as a quantum repeater to boost entanglement rates in a long-range network.
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.