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arXiv:2310.01316 · Nature

Entanglement of nanophotonic quantum memory nodes in a telecom network

Can M. Knaut, Aziza Suleymanzade, Yan-Cheng Wei, Daniel Assumpção, Pieter-Jan Stas, Yan Qi Huan, Bartholomeus Machielse, Erik Knall, Madison Sutula, Gefen Baranes, Neil E. Sinclair, Chawina De-Eknamkul, David Levonian, Mihir K. Bhaskar, Hongkun Park, Marko Lončar, Mikhail D. Lukin

Published May 15, 2024• 310 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1038/s41586-024-07252-z

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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.
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.
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.
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.