In classical physics and general relativity, that describe the macroscopic world, an event is a well defined state in space and time, and it’s possible to measure it without changing its history. On the other hand, in quantum mechanics, that describes the microscopic world, the problem of measurement is the subject of a long debate between all the fathers of the theory, including Einstein, Bohr, Schrodinger, that is still lively nowadays.

In quantum mechanics indeed outcomes of measurements are determined by the measurement itself, and do not correspond to properties of the system that existed previously and independently. In this sense the measurement shapes a flow of events, a property that has been termed as the lack of realism.

A big step forward was made when Bell derived his famous inequality, that opened the possibility to test the principle of local-realism in actual experiments, in order to exclude that quantum mechanics could be thought as a statistical version of a more fundamental realistic theory. However this test was intrinsecally designed to involve only microscopic systems, thus leaving an open the debate on the ontological status of a quantum object at larger scales.

Later, in the same spirit of Bell, Leggett and Garg derived an inequality that was meant to look deeper at the scale at which possibly lies the boundary between a classical and a quantum description of nature. They designed a test for the principles of macrorealism and non invasive measurability, that allows to exclude classical explanations for macroscopic events correlated in time. However to date there is still no satisfactory experimental answer to such test in macroscopic systems.

Here we propose to performing an experimental test of LG inequalities in a macroscopic ensemble of atoms, exploiting the features of repeated Quantum Non Demolition measurements followed by macroscopic rotations of the atomic ensemble. In this way we can disentangle the two features that nowadays bound the classical with quantum world: the size of the system and the measurement itself, aiming to identify a new regime of where quantum mechanical interpretation of Nature is necessary even at macroscopic scale.

Giorgio Colangelo

giorgio.colangelo@icfo.es