Abstract

The complementary nature of wave-like and particle-like behavior is commonly interpreted as follows: Due to the uncertainty principle, any attempt to measure the position (particle aspect) of a quantum will lead to an uncontrollable, irreversible disturbance in its momentum, thereby washing out any interference pattern (wave aspect).¹ However, in some circumstances no "state reduction" is necessary to destroy wave-like behavior, and measurements can be reversible in a certain sense. To fully understand this phenomenon, one must view the loss of coherence as arising from an entanglement of the system wave function with that of the measuring apparatus (MA). Through this entanglement, previously interfering paths can become distinguishable such that no interference is observed. The mere possibility of obtaining welcher Weg ("which way") information is sufficient to destroy interference. It may be regained, however, if one somehow manages to "erase" the distinguishing information through a suitable measurement on the MA, and correlates these results with measurements on the original quantum. This is the physical content of quantum erasure.² There are four basic measurements possible on the MA—two of which yield which-path information, one that recovers the initial interference fringes, and one that yields interference anti-fringes. The decision to measure wave-like or particle-like behavior may be delayed until after detection of the quantum, an irreversible process. In all cases one must correlate the results of measurements on the MA with this detection.

© 1994 IEEE