QuantNet: Spins in Diamond for a Quantum Network

A world without the Internet, a distributed network connecting almost every modern electronic device, is hard to envision. Since its popularization in the 1990s it has generously facilitated our everyday life and has accelerated information retrieval and communication. While already the classical Internet enabled remarkable technological advances, previously unseen opportunities become possible once we delve into the quantum world. The laws of quantum mechanics enable us to dream of a quantum Internet, a network built of quantum devices. At the heart of such a network lies quantum entanglement one of the most surprising concepts of quantum physics. Whenever two particles are entangled they completely lose their individuality and remain strongly correlated no matter how far apart they are. This unique property uncovers possibilities that are intractable with a classical network such as inherently secure communication, perfect clock synchronization, or the combination of modular quantum computers within a powerful quantum computing cluster. With our project QuantNet: Spins in Diamond for a Quantum Network we aim to move one step further in the quest for a quantum Internet. Similar to the idea of a classical Internet, a quantum network has to consist of local quantum processors for quantum memory and a quantum interface to access, send and receive quantum information. We propose to use so-called color centers in diamonds to meet these requirements. Such centers host elementary spins, well-controllable quantum states, that behave quite similarly to atomic systems. Information can be stored within long-lived nuclear spins, and a quantum interface is provided by an easily accessible electron spin. Remarkably, the electron can be used to communicate the internal quantum states via a photon, a particle of light, enabling fast and robust entanglement generation between remote quantum devices. Connecting to the ongoing developments at the host institution at the University of Delft we intend to experimentally realize a small-scale quantum network the first of its kind. By precisely controlling the internal spins and the interaction between various separated diamond quantum devices, we aim to demonstrate distributed entanglement across the network and to manipulate the interlinked quantum states at will. In addition, we want to enable long-range networking by transferring the entanglement links to infrared wavelengths that feature low photon loss in glass fibers. Our research is expected to have a large impact on the field of quantum information and networking and will bring the dream of a global quantum Internet closer to reality.

Within the Erwin-Schrödinger project "QuantNet: Spins in Diamond for a Quantum Network" we have taken major steps towards the development of a future "quantum Internet", a network built of quantum devices. While already the classical Internet enabled remarkable technological advances over the last decades, previously unseen opportunities become possible once we delve into the quantum world. At the heart of such a quantum network lies quantum entanglement - one of the most surprising concepts of quantum physics. Whenever two particles are entangled they completely lose their individuality and remain strongly correlated no matter how far apart they are. This unique property uncovers possibilities that are intractable with a classical network such as inherently secure communication, perfect clock synchronization, or the combination of modular quantum computers within a powerful quantum computing cluster. Similar to the idea of a classical Internet, a quantum network consists of local quantum processors for quantum memory and a quantum interface to access, send and receive quantum information. Within this project we have employed two different experimental platforms to meet these requirements: so-called "color centers" in diamonds as well as electrodynamically trapped ions in ultra-high vacuum. Color centers host elementary spins, well-controllable quantum states, that behave quite similarly to atomic systems. Information can be stored within long-lived nuclear spins, and a quantum interface is provided by an easily accessible electron spin. Remarkably, the electron can be used to communicate the internal quantum states via a photon, a particle of light, enabling fast and robust entanglement generation between remote quantum devices. At the host institution at the University of Delft we have experimentally realized a multinode quantum network - the first of its kind. By precisely controlling the internal spins and the interaction between various separated diamond quantum devices, we demonstrated distributed entanglement across the network and performed quantum teleportation of a quantum state across the network - between non-neighboring quantum network nodes. This key demonstration became possible due to several experimental advances, including major improvements of the quality of the nuclear spin quantum memory. At the host institution at the University of Innsbruck we have realized a two-node quantum network between two trapped ion quantum network nodes distributed across the university campus. In addition, we have investigated the possibility of realizing hybrid quantum networks consisting of the two experimental platforms at hand. Our work paves the way for long-distance networks of entangled quantum processors, with quantum channels extended over hundreds of kilometers, and does bring the dream of a global quantum Internet closer to reality.