**December 20, 2014**

Yesterday we had another paper out on the arXiv (the last of 2014 for me) : https://arxiv.org/abs/1412.5953

As I have written about in previous posts, quantum physics allows for correlations which are in a certain sense stronger than any which are possible in classical physics. These so-called nonlocal correlations can be exploited cryptography and for generation of random numbers, as I’ve explained here http://jonatanbohrbrask.dk/2014/10/13/a-self-testing-quantum-random-number-generator/ and here http://jonatanbohrbrask.dk/2014/10/29/92/. They are also interesting in their own right as a natural phenomenon which is hard to grasp and goes against intuition based on our everyday experience of the world.

While applications of nonlocality are already being developed, there are still many things about it which we have not fully understood. In this paper we study the robustness of nonlocal correlations to loss. We do this for a certain type of highly entangled quantum states known as Dicke states. These states can be understood e.g. as a state of excitations stored in atoms. Take a large number of atoms which are all in their lowest energy state. Now put some of the atoms in an excited state with higher energy. Dicke states are states where the number of excitations are fixed, but all possible combinations of which atoms are in the ground state and which are excited are explored (the atomic ensemble is in a coherent superposition of all these posibilities). Dicke states give rise to nonlocal correlations, and we study how robust this nonlocality is to particle loss – i.e. how many atoms can one lose before the nonlocality disappears? – and to loss of excitations – i.e. if an excited atom has some probability to decay back to the ground state, how much decay can the nonlocality tolerate?

** Published paper**:

*https://journals.aps.org/pra/abstract/10.1103/PhysRevA.91.032108*