A team of physicists at the University of Innsbruck
performed an experiment that seems to contradict the foundations of quantum
theory – at first glance. The team led by Rainer Blatt reversed a quantum
measurement in a prototype quantum information processor. The experiment is
enabled by a technique that has been developed for quantum error correction in
a future quantum computer.
Measurements on quantum systems have puzzled generations of
physicists due to their counterintuitive properties. One of them is the fact
that measurements on a quantum system are in general non-deterministic. This
means that even if the state of the system is completely known, it is
impossible to determine the outcome of a single measurement. Furthermore, the
measurement alters the system’s state so that a previous measurement will
certainly return the same result as the first measurement. Thus the system is
irreversibly altered by a measurement.