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The Physics Educator

A Classical Experiment for the Quantum Eraser: A Comparison Between Model and Experimental Results

Marco Di Mauro, L M Gratton, Pasquale Onorato, Stefano Oss

Published Mar 1, 2023• 1 citation
Publisher

Abstract

We describe a simple experimental apparatus which allows the quantitative study of the classical analog of a quantum eraser. The apparatus consists of a laser diode, a thin wire, two polarizers providing the which-way information, and a third polarizer which erases that information. The experimental results are analyzed with the aid of a digital camera and the Tracker application, and are compared with a novel classical computation of the interference patterns, which is presented as well.

DOI
10.1142/s2661339523500026

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We describe a simple experimental apparatus which allows the quantitative study of the classical analog of a quantum eraser. The apparatus consists of a laser diode, a thin wire, two polarizers providing the which-way information, and a third polarizer which erases that information. The experimental results are analyzed with the aid of a digital camera and the Tracker application, and are compared with a novel classical computation of the interference patterns, which is presented as well.
This experiment explores the quantum phenomenon known as the Quantum Eraser, using a variation of Young’s Double Slit experiment. Young’s Double Slit experiment demonstrates that light acts as a wave by creating an interference pattern when diffracted through two slits. If one measures which of the two slits the photons pass through, then the interference pattern is replaced by a single bright spot, as would be expected for particle-like behaviour. The “Quantum Eraser” eliminates the measurement on the photons, thereby reintroducing the interference pattern observed in Young’s original experiment. The experiment’s first stage saw Young’s Double Slit experiment recreated and an interference pattern was observed. Upon adding two orthogonally polarized filters, the photon’s path was measured, and the interference pattern was removed. By then adding a third filter which was polarized 45O relative to both other polarisers, the interference pattern was somewhat restored. For each experiment, the heights of the peaks in the interference patterns were compared to each other to examine the quality of the reproduced interference pattern based on the original double slit interference pattern. This comparison gave a quantitative result that demonstrated that the Quantum Eraser was able to restore the interference pattern to within 5 standard errors, thereby exemplifying the effect that changing the measurement conditions affects the final measurement.
We have demonstrated the operation of a quantum eraser based on a polarization interferometer. Which-path information is erased not by modifying the interferometer apparatus, but instead by modifying the information obtained from measurements performed on a second beam, whose polarization is correlated with that of the interferometer beam. We compare the results obtained when the two beams are in an entangled state (quantum correlations) and in a mixed state (classical correlations). We find that classical correlations can mimic most, but not all, of the quantum-mechanical behavior.
In a delayed-choice quantum eraser, interference fringes are obtained by erasing which-way information after the interfering particle has already been irreversibly detected. Following an introductory review of delayed-choice experiments and quantum erasure, we describe the experimental realization of an optical delayed-choice quantum eraser, suitable for advanced undergraduates, based on polarization-entangled pairs of single photons. In our experiment, the delay of the erasure is implemented using two different setups. The first setup employs an arrangement of mirrors to increase the optical path length of the photons carrying which-way information. In the second setup, we use fiber-optic cables to elongate the path of these photons after their passage through the polarization analyzer but prior to their arrival at the detector. We compare our results to data obtained in the absence of a delay and find excellent agreement. This shows that the timing of the erasure is irrelevant, as also predicted by quantum mechanics. The experiment can serve as a valuable pedagogical tool for conveying the fundamentals of quantum mechanics.
The quantum eraser involves a two-slit experiment in which an individual chooses, after the particles have passed through the slits, whether to do an interference experiment or a which-way experiment. If the experimenter chooses to erase the which-way information, the interference is restored. In one design, an excited atom enters one or the other cavity and emits a photon. By choosing to observe the photon’s position or to let it be absorbed by a detector atom, the investigator can pick the type of experiment to be done. The analysis uses the change-of-basis method, in which the form of wave function used depends on the experiment performed. A similar experiment can be performed by using polarizers at the slits and at the screen.
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.