One of the fundamental principles in quantum mechanics is that certain properties of a quantum-mechanical object, such as a photon or electron, cannot be measured simultaneously with precision. The position of these particles, for example, cannot be determined at the same time as its momentum: measuring one property causes a certain ‘fuzziness’ in the determination of the other.
A few years ago, an experiment in which both the path of photons and their interference patterns were measured simultaneously drew considerable attention. “This was because the experiment seemingly overcame the fundamental restrictions of quantum mechanics. Simultaneous measurements of the path information and interference picture are impossible in standard quantum theory, like the simultaneous determination of the coordinates and momentum of a particle.”
The results of the two-slit interference experiment—as it was known—were brought into line with quantum mechanics by arranging the measurements such that the results were averaged over several experiments conducted using a number of photons. This means that the precise position of a single photon was not actually measured. Instead, its properties were retrospectively deduced by making many measurements on identical particles.
Read more at: Phys.org