Towards a quantum internet
A future quantum network [1-3] will consist of quantum processors that are connected by quantum channels, just like conventional computers are wired up to form the Internet. In contrast to classical devices, however, the information that can be encoded in a quantum network grows exponentially with the number of nodes, and entanglement of remote particles gives rise to non-local correlations. Exploring these effects facilitates fundamental tests of quantum theory and the quantum-to-classical transition. In addition, quantum networks will enable applications in precision sensing and in distributed quantum information processing, which will fundamentally enhance computational power and ensure unbreakable encryption over global distances.
Pioneering experiments with atomic ensembles, single trapped atoms  and solid-state spins have demonstrated the connection and entanglement of two quantum nodes separated by up to 1.3 km . However, accessing the full potential of quantum networks requires scaling of these prototypes to more network nodes and even larger distances. To this end, a new technology that overcomes the bottlenecks of existing physical systems has to be developed.
The “Quantum Networks Group” investigates novel quantum systems in this context, with a focus on individual erbium dopants in optical resonators, a solid-state platform with exceptional coherence properties [5, 6] that has unique potential towards this end.
Funding is provided by the Max-Planck Society, by the ERC via a Starting Grant, by the Cluster of Excellence MCQST and the Munich Quantum Valley , and also by the DFG and the BMBF.
 Kimble: The quantum internet. Nature 453, 1023–1030 (2008).
 Reiserer and Rempe: Cavity-based quantum networks with single atoms and optical photons. Rev. Mod. Phys. 87, 1379 (2015).
 Wehner, Elkouss, and Hanson: Quantum internet: A vision for the road ahead. Science 362, 9288 (2018).
 Hensen, Reiserer et al.: Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres. Nature 526, 682 (2015).
 Rančić, Hedges, Ahlefeldt, and Sellars: Coherence time of over a second in a telecom-compatible quantum memory storage material. Nature Physics 14, 50 (2018).
 Merkel, Ulanowski, and Reiserer: Coherent and Purcell-Enhanced Emission from Erbium Dopants in a Cryogenic High-Q Resonator, Phys. Rev. X 10, 041025 (2020).