Inhibition of the Measurement of the Wavefunction of a Single Quantum System and the Projection Postulate Orly

Recently, with the technological developments which allow manipulation of single quantum systems, such as atoms, electrons and photons, in laboratories allover the world , there has been an interest in the quantum theory of a single system. This interest raises several fundamental questions, one of which is the question of the meaning of the quantum wavefunctioD. Quantum mechanics describes a single system by a corresponding wavefunction. The wavefunction contains all relevant information about the single physical system. However, in order to obtain this information , and determine the wavefunction, one needs to consider the statist ics of the results of measurements performed on an ensemble of identical systems. Therefore, the wavefunction is said to have a statistical (or epistemological) meaning. Naturally, one would ask the question: Can we give the quantum wavefunction a deeper physical (or ontological) meaning? Aharonov, Anandan and Vaidman (1,2) showed recently that the wavefunction of a single system could be determined from the results of a series of "protective measurements" performed on the system. In the protective measurement scheme, a-priori know ledge of the wavefunction of the system is used in order to measure this system and protect its wavefunction from changing at the same time. Aharonov , Anandan and Vaidman argued that the protective measurement accounts for the physical reality of the wavefunctioD. Yet, it seems that one should be able to measure the wavefunction of a single system without any a-priori knowledge, if the wavefunction were real. Considering a measurement of the spin of a single spinl /2 system, with an unknown wavefunction, Royer showed (3) recently that this measurement can be physically reversed, so that the system is back in its unknown init ial state, with a finite success probability. However, as Huttner showed (4) , the statistics of a series of successful "reversible measurements" , performed on a single spin-1/2 system, are independent of the initial spin wavefunction of the system. Therefore, reversible measurements cannot be used for the measurement of the wavefunctioD of a single system. According to the projection postulate , a precise measurement performed