Periodic Reporting for period 2 - QNETWORK (Quantum networks wired by multi-spin entanglement) Reporting period: 2019-11-01 to 2021-04-30 Summary of the context and overall objectives of the project Entanglement is a counterintuitive concept that has a central place in quantum theory. Two or more particlesthat are entangled lose their individual properties while their joint state is perfectly defined. Entanglement isespecially striking when the particles involved are spatially separated, resulting in non-local correlations andrich phenomena like teleportation that go far beyond the realm of classical physics.At the same time, theoretical work is uncovering the exciting and unique possibilities of entanglement that is controllably shared between multiple nodes of a fundamentally new type of network: a quantum network.Realization of a quantum network would provide scientists with a unique new platform allowing both for novel fundamental studies of nature as well as for applications in quantum information processing, communication and metrologyMy QNETWORK project will realize a multi-node entanglement-based quantum network. The network willhave fully controlled multi-spin nodes at individual diamond defects connected by single-photon links. Usingthis quantum network I will demonstrate supremacy of a quantum repeater node over direct photontransmission, generate multi-spin entanglement and study its decoherence, realize quantum teleportationacross multiple nodes and finally exploit the network for new scientific experiments. If successful, QNETWORK will yield a versatile multi-node quantum network that will serve as a novel platform for groundbreaking science and as a test-bed for a future quantum Internet. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far In the first phase of the project, work has concentrated on increasing the rate of entanglement generation between remote nodes and on realizing robust network qubits, as well as solving challenges related to efficient operation of the quantum network. For faster entangling operations, we have designed and implemented a new architecture that enables the required phase stabilization across multiple quantum network nodes. Furthermore, we have increased control and coherence of nuclear spin qubits that can serve as quantum memories, and demonstrated a novel 3-qubit measurement protocol and tests of quantum non-contextuality. We have also unravelled the orbital and spin dynamics of the other charge state of the diamond qubit. Finally, we have achieved a breakthrough in the fabrication of qubits in diamond membrane devices suitable for incorporation into microcavities for further enhancing of entangling rates.Besides the technical work, we have formulated a vision on the future of quantum networks and classified different stages of development and their corresponding applications in an authoritative review. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) The results of the first phase have put us in an excellent position to realize the world’s first multi-node quantum network, which is the main goal of this project. We expect that to demonstrate the first three-node network early in the second phase, with the remainder of the project for exploiting this network for demonstrating both quantum network applications as well fundamental explorations.