The Quantum Internet is a complementary tool to the widely spread classical Internet, which has already revolutionized our everyday life. The promise is that the Quantum Internet will unlock new unprecedented capabilities and applications that span from secure communication, to distributed quantum computation and enhanced quantum sensing. The realization of such a powerful tool is the result of a joint effort among several fields, like computer science, physics, engineering, and materials science, which all rely on the fundamentals of quantum mechanics. The introduction of a new computational unit, the qubit, allows for the creation of superposition and entangled states, and the possibility of measuring such states. On a practical level, we can envision the Quantum Internet as a network of interconnected heterogeneous platforms aimed at solving different tasks, such as the processing of quantum information at the end nodes, and the storing and retrieval of quantum information in between end nodes to bridge long distances. The quantum information routing is governed and optimized by a dedicated software architecture that facilitates the user interface, removing the requirement of knowing the hardware’s physical principles for a general user.
In the hardware framework, the Nitrogen-Vacancy center in diamond represents a viable platform as processing end node, thanks to the high quality of its qubits and the capability of generating remote entanglement with other nodes in the network via its optical interface. These properties can be engineered to utilize the NV center as a test-bed for demonstrating crucial steps towards the Quantum Internet final goal.
We first employ a two-node NV quantum network in the laboratory to demonstrate the elementary building-blocks of distributed quantum computation: the generation of a distributed 4-partite Greenberger-Horne-Zeilinger state and the realization of a non-local Controlled-NOT gate between physically separated and non-interacting qubits.
In the long distance scenario, we use the NV center platform to study the photonic interface of solid-state qubits with time-bin qubits compatible with the emission from quantum memory platforms, such as Rubidium gas or Thulium-doped crystals. The interface is benchmarked with a quantum teleportation experiment. Quantum teleportation is the ultimate protocol that enables the transfer of quantum information from one physical point to another. We teleport a photonic time-bin qubit to the communication qubit of the NV center platform, establishing the primary form of communication between heterogeneous platforms in a quantum network.
Finally, the two-node NV network is used as reliable setup to demonstrate the first operating system for quantum network applications, QNodeOS. QNodeOS can schedule and manage quantum network applications in a multitasking fashion. It constitutes a software interface which enables facilitated access for users, boosting the research in quantum network applications and making a first step towards the deployment of such technology into society.

Solid-state quantum registers consisting of optically active electron spins with nearby nuclear spins are promising building blocks for future quantum technologies. For electron spin-1 registers, dynamical decoupling (DD) quantum gates have been developed that enable the precise control of multiple nuclear spin qubits. However, for the important class of electron spin-1/2 systems, this control method suffers from intrinsic selectivity limitations, resulting in reduced nuclear spin gate fidelities. Here, we demonstrate improved control of single nuclear spins by an electron spin-1/2 using dynamically decoupled radio-frequency (DDRF) gates. We make use of the electron spin-1/2 of a diamond tin-vacancy center, showing high-fidelity single-qubit gates, single-shot readout, and spin coherence beyond a millisecond. The DD control is used as a benchmark to observe and control a single 31C nuclear spin. Using the DDRF control method, we demonstrate improved control on that spin. In addition, we find and control an additional nuclear spin that is insensitive to the DD control method. Using these DDRF gates, we show entanglement between the electron and the nuclear spin with 72(3)% state fidelity. Our extensive simulations indicate that DDRF gate fidelities well in excess are feasible. Finally, we employ time-resolved photon detection during readout to quantify the hyperfine coupling for the electron's optically excited state. Our work provides key insights into the challenges and opportunities for nuclear spin control in electron spin-1/2 systems, opening the door to multiqubit experiments on these promising qubit platforms.

The negatively charged tin-vacancy (SnV⁻) center in diamond has emerged as a promising platform for quantum computing and quantum networks. To connect SnV⁻ qubits in large networks, in situ tuning and stabilization of their optical transitions are essential to overcome static and dynamic frequency offsets induced by the local environment. Here, we report on the large-range optical frequency tuning of diamond SnV⁻ centers using micro-electro-mechanically mediated strain control in photonic integrated waveguide devices. We realize a tuning range of >40 GHz, covering a major part of the inhomogeneous distribution. In addition, we employ real-time feedback on the strain environment to stabilize the resonance frequency and mitigate spectral wandering. These results provide a path for on-chip scaling of diamond SnV-based quantum networks.

We demonstrate large-range tuning of the optical transition of Tin-Vacancies (SnV) in diamond using electro-mechanical-induced strain, realizing >40 GHz tuning. We employ real-time feedback on the strain environment to stabilize the resonant frequency.

The ability to sense and control nuclear spins near solid-state defects might enable a range of quantum technologies. Dynamically decoupled radio-frequency (DDrf) control offers a high degree of design flexibility and long electron-spin coherence times. However, previous studies have considered simplified models and little is known about optimal gate design and fundamental limits. Here, we develop a generalized DDrf framework that has important implications for spin sensing and control. Our analytical model, which we corroborate by experiments on a single NV center in diamond, reveals the mechanisms that govern the selectivity of gates and their effective Rabi frequencies, and enables flexible detuned gate designs. We apply these insights to numerically show a 60× sensitivity enhancement for detecting weakly coupled spins and study the optimization of quantum gates in multiqubit registers. These results advance the understanding for a broad class of gates and provide a toolbox for application-specific design, enabling improved quantum control and sensing.

The goal of future quantum networks is to enable new internet applications that are impossible to achieve using only classical communication1, 2–3. Up to now, demonstrations of quantum network applications4, 5–6 and functionalities7, 8, 9, 10, 11–12 on quantum processors have been performed in ad hoc software that was specific to the experimental setup, programmed to perform one single task (the application experiment) directly into low-level control devices using expertise in experimental physics. Here we report on the design and implementation of an architecture capable of executing quantum network applications on quantum processors in platform-independent high-level software. We demonstrate the capability of the architecture to execute applications in high-level software by implementing it as a quantum network operating system—QNodeOS—and executing test programs, including a delegated computation from a client to a server13 on two quantum network nodes based on nitrogen-vacancy (NV) centres in diamond14,15. We show how our architecture allows us to maximize the use of quantum network hardware by multitasking different applications. Our architecture can be used to execute programs on any quantum processor platform corresponding to our system model, which we illustrate by demonstrating an extra driver for QNodeOS for a trapped-ion quantum network node based on a single 40Ca+ atom16. Our architecture lays the groundwork for computer science research in quantum network programming and paves the way for the development of software that can bring quantum network technology to society.

We report on a quantum interface linking a diamond NV center quantum network node and 795nm photonic time-bin qubits compatible with Thulium and Rubidium quantum memories. The interface makes use of two-stage low-noise quantum frequency conversion and waveform shaping to match temporal and spectral photon profiles. Two-photon quantum interference shows high indistinguishability between converted 795nm photons and the native NV center photons. We use the interface to demonstrate quantum teleportation including real-time feedforward from an unbiased set of 795nm photonic qubit input states to the NV center spin qubit, achieving a teleportation fidelity well above the classical bound. This proof-of-concept experiment shows the feasibility of interconnecting different quantum network hardware.