## Macroscopic Quantum Systems

Since the early days of quantum mechanics physicists have speculated about the possibility of seeing quantum effects at a macroscopic scale. Schrödinger’s cat, a macroscopic system that is in a coherent superposition state of two macroscopically distinguishable states (“dead” and “alive”), is perhaps the best known example where the puzzling features of quantum mechanics are highlighted. With the experimental progress in many fields, such entertaining thought experiments have actually become a real possibility.

We are interested in studying and assessing such large-scale entangled states with respect to their effective size (i.e., to obtain a measure for macroscopicity), their stability under noise and decoherence, and the possibility to actually prepare and detect them. The very same features that make a state a macroscopic quantum superposition seem to fundamentally hinder their experimental detection. We would like to better understand this and find possible ways to circumvent such problems, e.g., by using encodings.

We are interested in studying and assessing such large-scale entangled states with respect to their effective size (i.e., to obtain a measure for macroscopicity), their stability under noise and decoherence, and the possibility to actually prepare and detect them. The very same features that make a state a macroscopic quantum superposition seem to fundamentally hinder their experimental detection. We would like to better understand this and find possible ways to circumvent such problems, e.g., by using encodings.

[1] W. Dür and H. J. Briegel,
Stability of macroscopic entanglement under decoherence, Phys. Rev. Lett. 92, 180403 (2004) [arXiv:quant-ph/0307180].[2] F. Fröwis and W. Dür, Measures of macroscopicity for quantum spin systems, New J. Phys. 14, 093039 (2012) [arXiv:1205.3048].[3] F. Fröwis, M. van den Nest, and W. Dür, Certifiability criterion for large-scale quantum systems, New J. Phys. 15, 113011 (2013) [arXiv:1306.0370].[4] P. Sekatski, N. Gisin, and N. Sangouard, How difficult it is to prove the quantumness of macroscropic states? Phys. Rev. Lett. 113, 090403 (2014) [arXiv:1402.2542]. |

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