Quantum entanglement, dephasing and decoherence effects in neutron experiments

Fundamental quantum properties like quantum coherence and entanglement are amoung the most interesting features of quantum mechanics which are studied nowadays. They form the basis for the new, fast developing fields of quantum information and computation. The project aims at a deeper understanding of quantum physics, by investigating especially two peculiar features: entanglement and decoherence. The physical system of interest is the (massive) neutron subjected to interferometric and polarimetric measurements. The study of quantum features in massive systems, in contrast to photonic systems, is appealing but also challenging. Neutrons are proper objects for a study of quantum mechanical behaviour of spin-1/2 systems: they allow for rather easy experimental control and the neutron spin is the simplest two-level system with easy manipulation by magnetic fields. As seen in the past, many quantum phenomena have first been observed with neutrons and afterwards studied in other systems. In combination with interferometric measurements the system has enough intrinsical richness to show interesting quantum features such as entanglement. The coupling of the neutron to an external magnetic field allows for selective manipulations of the spinor quantum states. This can be used, on the one hand, to create entangled states where the entanglement occurs between different degrees of freedom (e.g., spin and path) and, on the other hand, one can introduce dephasing and decoherence by varying magnetic fields. Both aspects of quantum physics manifested in neutron interferometry are of importance and will be investigated in detail during the research project. The results obtained in this research proposal are of fundamental interest. It is clear that neutrons are very suitable particles to test and implement the basic features of quantum mechanics because they can be easily manipulated and controlled during the experimental procedure without disturbances in contrast to atoms or molecules. Besides analysis of fundamental quantum physical questions such as entanglement and decoherence, the project delivers answers on how a particle with spin behaves and interacts with a magnetic field. The obtained results have a wide range of applications, e.g. in atom optics where the behaviour of atoms and their coupling to magnetic fields is of interest and the theoretical results can be directly applied to these systems. Beyond the theoretical work of the proposal it is possible via the intensive contact with experimental physicists to develop experimental strategies to check the results directly in suitably designed experiments. The experimental work is done at the Institute Laue-Langvine (ILL) in Grenoble, France.

Entanglement is the fundamental quantum property and a resource for quantum information.For constructing a possible quantum computer, scientists need to answer two questions:How do we create entanglement?How do we protect entanglement from environmental influences?In this project we did address both questions. We studied them with neutrons - small, neutral, massive particles which are part of the atomic nucleus. They can be produced in the research reactor at the ATI in Vienna.The larger part of the project was devoted to the possible ways of creation of entanglement. In neutron physics we do not create entanglement between different neutrons (like with photons) but we entangle different properties (degrees of freedom as they are called) of one single neutron.We have two different tools to create entanglement: the interferometer and the polarimeter.In an interferometer setup the neutron beam is split up into two possible paths and recombined again. We use the path as the first degree of freedom and the spin of the neutron as the second degree of freedom. We can establish entanglement between path and spin of single neutrons.Now we are able to add a third degree of freedom to be entangled with, namely the energy of the neutron. For this tripartite entanglement we develop tools (so called Bell inequalities) which measure how much entanglement is present in the system. We get very good results which tells us that our entangled states are nearly perfect.In the polarimeter setup we have no spatial splitting of the neutron beam. We work with the spin, the energy and the momentum of the neutron as degrees of freedom for entanglement.With our setup it is possible to create more than two energy levels which has promising applications in quantum information processing as a mulitlevel quantum system.In the second part of the project we focused on the destruction of entanglement. We need to know what influences the amount of entanglement present in a system in order to protect it.All mechanisms which lead to a reduction of entanglement, are called decoherence. First we study decoherence induced by geometric phases. We see that we can undo the effects by special arrangement of our measurement procedures. When decoherence is caused by time varying magnetic fields, our results show that even very small quantum magnetic fields leave a significant footprint on the state of the neutron.Most of the work was done in close collaboration between theoretical and experimental physicists, where people benefit from each other and their different points of view.