Actually, Wheeler says, quantum phenomena are neither waves nor particles but are intrinsically undefined until the moment they are measured. Unfortunatelly the jargon of quantum mechanics is rather less lively. But as soon as an observer makes a measurement capable of distinguishing between these states, the wave function 'collapses,' and the entity is forced into a single state. Yet even this deliberately abstract language contains some misleading implications. One is that measurement requires direct physical intervention.
Most experiments do in fact involve intrusive measurements.
But an experiment done last year by Mandel's team at the University of Rochester shows that a photon can be forced to switch from wavelike to particielike behavior by something much more subtle than direct intervention. The experiment relies on a parametric down-converter, an unusual tens that splits a photon of a given energy into two photons whose energy is half as great. In the experiment, a laser fires light at a beam splitter. Reflected photons are directed to one downconverter, and transmitted photons go to another down-converter. The two down-converters are arranged so that the two idler beams merge into a single beam.