Early in the twentieth century, cloud chamber traces and scintillograms merely offered transitory post-mortem images of quantum objects that lacked reproducibility. More recently, individual atomic ions have been prepared for extended and repeated observation and manipulation.
Moreover, quantum measurements on individual objects are unique: They
result in eigenvalues, not in expectation values, classical quantities
that appear as the outcome from measured ensembles. Consequently, those
quantum measurements offer more than just recordings of unprecedented
precision. They even allow insights — sometimes unexpected or
counter-intuitive ones — in the very nature of quantum interactions and
in the peculiarities of quantum measurement. This potential is laid open
in characteristic examples.