Our colleague dr. Milan Radonjić (Faculty of Physics, University of Vienna, Austria; Institute of Physics Belgrade) on November 2 was lecturer on SCL seminar of the Center for the Study of Complex Systems in the library reading room “Dr. Dragan Popović” of the Institute of Physics Belgrade.
The talk was entitled: “Macroscopic superpositions as quantum ground states”
In this talk we will study the question of what kind of a macroscopic superposition can(not) naturally exist as a ground state of some gapped local many-body Hamiltonian . We will derive an upper bound on the energy gap of an arbitrary physical Hamiltonian provided that its ground state is a superposition of two well-distinguishable macroscopic “semiclassical” states. For a large class of macroscopic superposition states we will show that the gap vanishes in the macroscopic limit. This in turn will show that preparation of such states by simple cooling to the ground state is not experimentally feasible and requires a different strategy. Our approach is very general and can be used to rule out a variety of quantum states, some of which do not even exhibit macroscopic quantum properties. Moreover, we will show how our methods and results shed some light on quantum marginal related problems.
 Borivoje Dakić and Milan Radonjić, Phys. Rev. Lett. 119, 090401 (2017).