One of the main research interests in the group is the study of entanglement, a form of correlation between quantum systems that is in some sense stronger than correlations between classical systems can be. Entanglement is a resource for a variety of quantum-communication protocols such as quantum teleportation or dense coding and lies at the heart of many applications in quantum technologies (e.g., entanglement-based quantum key distribution based on the Ekert-91 protocol). At the same time, the presence of entanglement is necessary for the violation of Bell inequalities (a phenomenon commonly referred to as `non-locality’), which is both of foundational interest but also of significance for device independent approaches to certification. 

A particular focus of the group’s activities in this direction lies on exploring the theoretical and practical aspects of  characterization, detection, and quantification of entanglement in high-dimensional and multipartite settings (see, e.g., [1]). Recent work of the group in this direction for instance  includes  aspects such as imprecise measurement apparata [2], scenarios with access to multiple copies [3] and the distribution of entanglement in networks assisted by parameter-estimation techniques [4].

In addition, we are engaged with connecting theoretical results to practical applications of entanglement certification in a variety of platforms, ranging from ultracold atomic ensembles [5], photon pairs entangled in their spatial (orbital angular momentum) [6,7] and in their energy-time [8] degrees of freedom, to trapped-ion quantum simulators [9].

References

1. Nicolai Friis, Giuseppe Vitagliano, Mehul Malik, and Marcus Huber, Entanglement certification from theory to experiment, Nat. Rev. Phys. 1, 72 (2019) [arXiv:1906.10929].
2. Simon Morelli, Hayata Yamasaki, Marcus Huber, and Armin Tavakoli, Entanglement Detection with Imprecise Measurements, Phys. Rev. Lett. 128, 250501 (2022) [arXiv:2202.13131].
3. Hayata Yamasaki, Simon Morelli, Markus Miethlinger, Jessica Bavaresco, Nicolai Friis, and Marcus Huber, Activation of genuine multipartite entanglement: beyond the single-copy paradigm of entanglement characterisation, Quantum 6, 695 (2022), [arXiv:2106.01372]
4. Simon Morelli, David Sauerwein, Michalis Skotiniotis, and Nicolai Friis, Metrology-assisted entanglement distribution in noisy quantum networks, Quantum 6, 722 (2022) [arXiv:2110.15627].
5. Matteo Fadel, Ayaka Usui, Marcus Huber, Nicolai Friis, and Giuseppe Vitagliano, Entanglement quantification in atomic ensembles, Phys. Rev. Lett. 127, 010401 (2021), [arXiv:2103.15730].
6. Jessica Bavaresco, Natalia Herrera Valencia, Claude Klöckl, Matej Pivoluska, Paul Erker, Nicolai Friis, Mehul Malik, and Marcus Huber, Measurements in two bases are sufficient for certifying high-dimensional entanglement, Nat. Phys. 14, 1032 (2018) [arXiv:1709.07344].
7. Natalia Herrera Valencia, Vatshal Srivastav, Matej Pivoluska, Marcus Huber, Nicolai Friis, Will McCutcheon, and Mehul Malik, High-Dimensional Pixel Entanglement: Efficient Generation and Certification, Quantum 4, 376 (2020) [arXiv:2004.04994].
8. Sebastian Ecker, Frédéric Bouchard, Lukas Bulla, Florian Brandt, Oskar Kohout, Fabian Steinlechner, Robert Fickler, Mehul Malik, Yelena Guryanova, Rupert Ursin, and Marcus Huber, Overcoming Noise in Entanglement Distribution, Phys. Rev. X 9, 041042 (2019) [arXiv:1904.01552].
9. Nicolai Friis, Oliver Marty, Christine Maier, Cornelius Hempel, Milan Holzäpfel, Petar Jurcevic, Martin B. Plenio, Marcus Huber, Christian Roos, Rainer Blatt, and Ben Lanyon, Observation of Entangled States of a Fully Controlled 20-Qubit System, Phys. Rev. X 8, 021012 (2018) [arXiv:1711.11092].