Unavoidable Decoherence in Matter Wave Interferometry

Abstract

Quantum mechanics with massive particles has developed into an important tool for fundamental and applied research since many previously proposed “Gedanken” experiments have now become feasible. Neutrons are massive particles which couple to gravitational, nuclear and electro-magnetic interactions and, furthermore, are sensitive to topological effects. Spatially widely separated coherent beams of neutrons can be produced by means of perfect crystal interferometers and these beams can be influenced individually. Spinor symmetry, spin superposition and quantum beat effect experiments have been performed and topological phases have also been observed. To achieve an interference pattern of high quality, the experimenter is, most of the time, fighting against various decoherencing effects. However, decoherence phenomena are fundamental features of quantum physics and deserve detailed investigations. In post-selection experiments it is, in many cases, possible to retrieve a substantial part of the lost coherence features. The latter are inherently connected to unavoidable losses which stem partly from the theory and partly from an imperfect environment. In existing experiments neither a completely coherent nor a completely incoherent situation can be achieved since decoherence sets in with the first interaction experienced by the quantum system.