Heisenberg’s famous uncertainty principle, formulated in the early days of quantum mechanics in 1927, states that pairs of certain properties of a quantum particle cannot be measured with arbitrary accuracy. In the case of successive measurements this is due to the unavoidable disturbances caused by the first measurement on of any further measurement. A particular interesting scenario of successive measurements is studied by the so-called Leggett–Garg inequalities (LGIs), named after Anthony James Leggett and Anupam Garg. LGIs study temporal correlations of a system but assume that the underlying measurements do not change the system, which - especially in quantum mechanics - is an unjustified assumption. Aim of the project is consequently a critical revision of the underlying assumptions associated with the traditional LGI tests. This highlights the necessity of reformulating the usual approach, taking into account the unavoidable disturbance in invasive measurability, a signature of quantumness.

  Neutron interferometry, where an interference effects of matter waves passing through a perfect silicon-crystal interferometer is observed, and neutron polarimetry, also known as spin-interferometry, are two methods that will be used to study this non-classical behavior in successive quantum measurements. Furthermore, experiments applying a so-called spin-echo instrument, are also foreseen within the scope of the proposal. This type of interferometer can be seen as a hybrid system, coupling different degrees of freedom of the neutron, particularly suited to test the reformulated LGI approach that takes the unavoidable disturbances caused by the quantum measuring process into account.