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arXiv:1703.05055

Comment on "Franson Interference Generated by a Two-Level System"

Jonathan Jogenfors, Adán Cabello, Jan-Åke Larsson

Published Mar 15, 2017• 2 citations
arXivHTMLPDF

Abstract

In a recent Letter [Phys. Rev. Lett. 118, 030501 (2017)], Peiris, Konthasinghe, and Muller report a Franson interferometry experiment using pairs of photons generated from a two-level semiconductor quantum dot. The authors report a visibility of 66% and claim that this visibility "goes beyond the classical limit of 50% and approaches the limit of violation of Bell's inequalities (70.7%)." We explain why we do not agree with this last statement and how to fix the problem.

DOI
10.48550/arxiv.1703.05055

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In a recent Letter [Phys. Rev. Lett. 118, 030501 (2017)], Peiris, Konthasinghe, and Muller report a Franson interferometry experiment using pairs of photons generated from a two-level semiconductor quantum dot. The authors report a visibility of 66% and claim that this visibility "goes beyond the classical limit of 50% and approaches the limit of violation of Bell's inequalities (70.7%)." We explain why we do not agree with this last statement and how to fix the problem.
We report on a two-photon interference experiment proposed by Franson [Phys. Rev. Lett. 62, 2205 (1989)], in which sinusoidal fringes with visibilities greater than 70.7%, such as those predicted by quantum mechanics, violate a Bell inequality. We observe visibility of 80.4\ifmmode\pm\else\textpm\fi{}0.6%, implying a violation of the inequality by 16 standard deviations. Here the elements of reality under consideration are energy and time rather than spin components. Any classical field models describing separate beams in a Franson interferometer are limited to visibilities less than 50%, and hence ruled out as well, without the need for any supplementary assumptions.
We have performed an EPR-type experiment first proposed by Franson,1 resulting in two-particle interference that is both nonclassical and nonlocal. We used photon pairs produced in an entangled state of energy via spontaneous down-conversion. Each photon enters an unbalanced interferometer, and we detect singles and coincidence rates at the output ports. If the long- and short-arm path-length difference is much greater than the photons’ coherence lengths, no fringes are seen in singles as the arm lengths are varied. Yet when the difference between the path-length differences of the two interferometers is less than these coherence lengths, fringes are observed in coincidence, resulting from an interference of the (indistinguishable) “long–long” and “short–short" coincidence possibilities. If the electronic resolution is sufficient to eliminate the background of "short–long" and "long–short” coincidences, the visibility may be as high as 100%.2 For V > 71%, with certain reasonable auxiliary assumptions, Bell’s inequalities can be violated. The interpretation is that no local, realistic model can yield the observed results. The use of this type of entangled state is significant because it is much closer to the original EPR paradox than the spin-singlet case commonly considered.
We demonstrate optical interferometry beyond the limits imposed by the photon wavelength using 'triggered' entangled photon pairs from a semiconductor quantum dot. Interference fringes of the entangled biphoton state reveals a periodicity half of that obtained with the single photon, and much less than that of the pump laser. High fringe visibility indicates that biphoton interference is less sensitive to decoherence than interference of two sequential single photons. The results suggest that quantum interferometry may be possible using a semiconductor LED-like device. (C) 2007 Optical Society of America
The two-photon interferometric experiment proposed by J. D. Franson [Phys. Rev. Lett. 62, 2205 (1989)] is often treated as a ``Bell test of local realism.'' However, it has been suggested that this is incorrect due to the 5050%50 postselection performed even in the ideal gedanken version of the experiment. Here we present a simple local hidden variable model of the experiment that successfully explains the results obtained in usual realizations of the experiment, even with perfect detectors. Furthermore, we also show that there is no such model if the switching of the local phase settings is done at a rate determined by the internal geometry of the interferometers.