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arXiv:0911.2072 · American Journal of Physics

Entanglement, which-way measurements, and a quantum erasure

Christian Ferrari, Bernd Braunecker

Published Jul 14, 2010• 16 citations
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Abstract

We present a didactical approach to the which-way experiment and the counterintuitive effect of the quantum erasure for one-particle quantum interferences. The fundamental concept of entanglement plays a central role and highlights the complementarity between quantum interference and knowledge of which path is followed by the particle.
DOI
10.1119/1.3369921

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Interaction with a measuring apparatus can cause otherwise interfering paths of a particle to become distinguishable. Under some conditions, however, it is possible to ``erase'' the distinguishability, thereby restoring interference. The concept of a quantum eraser is useful for understanding the role of entanglement in interference. Several attempts to investigate the phenomenon have been made using the correlated photons from spontaneous parametric down-conversion. However, none of the experiments previously performed in connection with quantum erasure has provided an optimal demonstration. We propose three improved down-conversion schemes, each of which satisfies all the criteria for a true quantum eraser. As the proposed schemes are all modifications or combinations of previously completed experiments, they are deemed to be feasible.
Quantum mechanics is very puzzling. A particle can be delocalized, it can be simultaneously in several energy states and it can even have several different identities at once. This schizophrenic behavior is encoded in its wavefunction, which can always be written as a superposition of quantum states, each characterized by a complex probability amplitude. Interferences between these amplitudes occur when the particle can follow several indistinguishable paths. Any attempt to determine which trajectory it “actually takes” destroys these interferences. This is a manifestation of wave—particle complementarity, which has recently been illustrated in textbook fashion by several beautiful experiments.
Photons are discrete particles of light, yet they undergo interference, a phenomenon typically demonstrated by waves. This phenomenon, known as “wave-particle duality,” is inexplicable in classical physics. In this chapter, we discuss two experiments. The first experiment demonstrates how photons can undergo interference and how measurement can force photons to give up their wave nature and behave like particles that cannot interfere. The second experiment adds to the surprise, showing that it is not only the measurement of the photon under consideration that can force it to change its nature but also that of its entangled twin.