**In a recently published Emerging Talents paper in Journal of Physics A: Mathematical and Theoretical Florian Fröwis investigates Lower bounds on the size of general Schrödinger-cat states from experimental data. Florian spoke to JPhys+ to explain more about Schrödinger-cat-like states.
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Recent experiments have been able to show quantum effects in large systems involving many atoms, electrons or photons. These fascinating developments confront us with new conceptual challenges. Among all possible ways to qualify large quantum systems, we have to identify the most important features that are particularly relevant for foundations and applications of quantum mechanics. Only then we are in the position to assess the experimental success more quantitatively.

For example, many physicists are interested in “Schrödinger-cat-like states”, that is, superpositions of states that are highly distinguishable on a macroscopic, coarse-grained scale. Several proposals to formalize this basic intuition have recently been put forward [arXiv:1706.06173]. Our idea was to use the quantum Fisher information (which plays a prominent role in quantum metrology), as it measures the spread of quantum coherence in a well-chosen spectrum. Besides favorable theoretical properties, a big advantage is the experimentally feasible ways to lower-bound the quantum Fisher information with real data from the lab. The key insight is that widespread quantum coherence for a given observable is directly related to the sensitivity of the system to small influences generated by the very same operator. Hence the experimental observation of a strong response to a weak perturbation can be used to bound the presence of widespread quantum coherence.

In my recent contribution in JPhysA, I analyze various experiments that aim to create Schrödinger-cat-like states. The following graph is an example with data taken from [Phys. Rev. Lett. 116, 140402 (2016)], where the rapid oscillations with high amplitude are a signature of widespread quantum coherence.

The analysis of several recent experiments with many atoms or photons shows that quantum coherence can be generated that is up to 70 times more spread than what can be achieved with semi-classical quantum states (i.e., this corresponds to a 70-qubit GHZ state).

This work contributes to a better assessment of large-scales quantum experiments. In the future, we hope to apply this framework to other experiments such as superconducting devices and also to find further “key identifiers” of large quantum systems.

This paper was originally reported in Florian Fröwis 2017 *J. Phys. A: Math. Theor.* **50** 114003.

**About the Author
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Florian Fröwis is a postdoctoral researcher at the University of Geneva in the Group of Applied Physics. He works on a wide range of large-scale quantum problems, from very abstract questions to practical contributions to the quantum memory experiments in the group.

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This work is licensed under a Creative Commons Attribution 3.0 Unported License.

Author image owned by Florian Fröwis, used with permission. Figures and featured image from Florian Fröwis 2017 *J. Phys. A: Math. Theor.* **50** 114003 © IOP Publishing Ltd.

Categories: Journal of Physics A: Mathematical and Theoretical