[en] The coherence in quantum superposition states of protons (and chemically similar particles, the positive muons) has been studied in some condensed matter environments. It is shown that if the proton systems and the experimental techniques used to study them are carefully selected, it is possible to observe quantum delocalization states of single particles and to understand the mechanisms for their loss of coherence. Quantum correlated two- and multiparticle states of protons lose coherence very fast when coupled to condensed matter environments, but new sub-femtosecond techniques have made them accessible to experimental studies. The degree of decoherence can be measured as function of time and the decoherence mechanisms can, at least in certain cases, be identified. Although less clean than in corresponding studies of quantum optical systems, these studies can be seen as a first step towards understanding the conditions for preservation of quantum correlation and entanglement in massive systems. Some consequences and some suggestions for future work are discussed. (author)