What is a preparation?
I have observed for myself what experimentalists, particularly those working in quantum optics, actually do, and not how they talk about it.
Physicists often say that they have prepared particles in a particular state.
What does this usually mean? The experimental setup contains a source of particles, about whose state one knows fairly little. By means of a system of screens and filters, one uses this to produce a beam of particles containing only particles in the required state.
This state (let us say spin up) is confirmed, for example, by measuring spin in the up-direction with a Stern-Gerlach magnet, and finding a spot in the expected place.
From the point of view of the thermal interpretation, however, the subsequent measurement of the spot is only a measurement of the macroscopic properties of the screen, and only indirectly connected with the objective state of the system.
From this point of view the same preparation looks like this: the source, that is the beam, is prepared, but not the individual particle.
At the quantum field theory level it is not at all clear whether one has particles or fields in the beam. The thermal interpretation, however, speaks only about precisely defined mathematical objects, and is therefore unable to give a judgement on this matter.
Stationary sources have a well-defined, measurable quantum state, given by a density matrix. With this interpretation there are no problems with preparation.
Arnold Neumaier (Arnold.Neumaier@univie.ac.at) A theoretical physics FAQ