When a quantum state is subjected to a measurement and the state is not
an eigenstate of the dynamical variable being measured, the outcome is unpredictable.
Only the probabilities of the various possible outcomes are predicted by
theory. This phenomenon is sometimes discussed in terms of ‘wavefunction collapse.’?
Trapped ions can be used for real, as opposed to gedanken demonstrations
of this basic process. In the experiments described here, a single ion, or a few
identical ions, were prepared in well defined superpositions of two internal energy
eigenstates. The populations of the energy levels were then measured. When
the state amplitudes were equal, the population fluctuations were greater than
when one of the amplitudes was nearly zero, in agreement with the predictions of
quantum mechanics. In other experiments, such as those with atomic beams, the
number of atoms under observation fluctuates, and this obscures the fluctuations
from other sources. However, if the number of atoms is small and constant, the
fundamental quantum mechanical fluctuations can be observed.