  It is shown that it is possible to measure the Schr\"odinger wave of a single
quantum system. This provides a strong argument for associating physical
reality with a quantum state of a single system in sharp contrast with the
usual approach in which the physical meaning of a quantum state is related only
to an ensemble of identical systems. An apparent paradox between measurability
of a quantum state of a single system and the relativistic causality is
resolved.
