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Physical Review A

Three proposed ‘‘quantum erasers’’

Paul G. Kwiat, Aephraim M. Steinberg, R. Y. Chiao

Published Jan 1, 1994• 97 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.49.61

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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.
We present various experiments demonstrating the mutual exclusivity of observing interference and which-path information, as demanded by Bohr's complementarity principle. Using photon pairs created in parametric down-conversion, no which-path measurements need to be performed on the interfering photon itself. Instead the other photon can be used to introduce distinguishability, which consequently destroys interference. However, a suitable measurement erases this distinguishability and interference can be recovered.
We have observed an effect known as a quantum eraser, using a setup similar to one previously employed to demonstrate a violation of Bell's inequalities. In this effect, an interfering system is first rendered incoherent by making the alternate Feynman paths which contribute to the overall process distinguishable; with our apparatus this is achieved by placing a half wave plate in one arm of a Hong-Ou-Mandel interferometer so as to rotate the polarization of the light in that arm by 90\ifmmode^\circ\else\textdegree\fi{}. This adds information to the system, in that polarization is a new parameter which serves to label the path of a given photon, even after a recombining beam splitter. The quantum ``eraser'' removes this information from the state vector, after the output port of the interferometer, but in time to cause interference effects to reappear upon coincidence detection. For this purpose, we use two polarizers in front of our detectors. We present experimental results showing how the degree of erasure (which determines the visibility of the interference) depends on the relative orientation of the polarizers, along with theoretical curves. In addition, we show how this procedure may do more than merely erase, in that the act of ``pasting together'' two previously distinguishable paths can introduce a new relative phase between them.
In the conventional interpretation of quantum mechanics the interference of particles in a two-beam interferometer is closely related to the problem of which-way information. One of the mysteries of quantum mechanics relies on the assumption that the wavefunction of each photon propagates simultaneously along both classically allowed paths, and that interference arises as a consequence of the indistinguishability of those paths. Any attempt to obtain which-way information by putting individual labels on the photons in each pathway inevitably destroys interference. However, even in cases in which the photons carry which-way labels, it is possible to erase those labels after the particle has left the interferometer. The erasing process (partly or completely) destroys the which-way information, and thereby restores interference. This phenomenon is known as quantum erasing. Here we present a lecture demonstration experiment of quantum erasing based on a Mach–Zehnder interferometer operated with single photons.
We have implemented an optical quantum eraser with the aim of studying this phenomenon in the context of state discrimination. An interfering single photon is entangled with another one serving as a which-path marker. As a consequence, the visibility of the interference as well as the which-path information are constrained by the overlap (measured by the inner product) between the which-path marker states, which in a more general situation are non-orthogonal. In order to perform which-path or quantum eraser measurements while analysing non-orthogonal states, we resort to a probabilistic method for the unambiguous modification of the inner product between the two states of the which-path marker in a discrimination-like process.
We report a quantum eraser experiment which actually uses a Young double slit to create interference. The experiment can be considered an optical analogy of an experiment proposed by Scully, Englert, and Walther [Nature (London) 351, 111 (1991)]. One photon of an entangled pair is incident on a Young double slit of appropriate dimensions to create an interference pattern in a distant detection region. Quarter-wave plates, oriented so that their fast axes are orthogonal, are placed in front of each slit to serve as which-path markers. The quarter-wave plates mark the polarization of the interfering photon and thus destroy the interference pattern. To recover interference, we measure the polarization of the other entangled photon. In addition, we perform the experiment under ``delayed erasure'' circumstances.