Uncertainty Relations in Pre- and Post-Selected Systems

Till date, no uncertainty relation has been reported for two incompatible observables in a pre- and post-selected (PPS) system which can express the impossibility of jointly sharp preparation of pre- and post-selected quantum states for measuring those observables. Here, we derive such an uncertainty relation for a PPS system by defining an uncertainty (standard deviation) of an observable measured in the given pre- and post-selected states. We provide here physical interpretations of the newly defined standard deviation and the uncertainty relation. Zero uncertainty condition, which is also the condition for optimized Fisher information in quantum metrology using the PPS system, is derived. It is shown that joint sharp preparation of a quantum state for non-commuting observables is possible when the standard quantum system is transformed into a PPS system with certain conditions, an impossible task in standard quantum system. We provide here applications of uncertainty and uncertainty relation in the PPS system. Namely, $(i)$ detection of mixedness of the given pre-selection using two different definitions of the uncertainty, $(ii)$ stronger uncertainty relation in the standard quantum system (i.e., the uncertainty relation can not be made trivial or the lower bound can not be null for almost all possible choices of initially prepared systems) using the uncertainty relation in the PPS system, ($iii$) genuine quantum mechanical uncertainty relation can be found using the first definition of uncertainty when the pre-selection is a mixed state.Comment: 15 pages, 3 figure