Franson Interference Generated by a Two-Level System

Abstract

We report a Franson interferometry experiment based on correlated photon pairs generated via frequency-filtered scattered light from a near-resonantly driven two-level semiconductor quantum dot. In contrast to spontaneous parametric down conversion and four-wave mixing, this approach can produce single pairs of correlated photons. We have measured a Franson visibility as high as 66%, which goes beyond the classical limit of 50% and approaches the limit of violation of Bell's inequalities (70.7%).Introduction-Entanglement is perhaps the most intriguing of all physical phenomena, and was famously challenged by Einstein, Podolsky and Rosen (EPR) in 1935 [1]. The EPR paradox led to Bell's inequalities [2] which were since tested by numerous experiments [3] including recent "loophole-free" tests [4][5][6] that have conclusively demonstrated that entanglement cannot be explained using local hidden variable theories.Most experiments have employed photons that are entangled in the energy and polarization degrees of freedom [7][8][9][10]. However, this method is known to be sensitive to polarization mode dispersion in optical fibers [11]. As an alternative, entanglement in the time-energy basis may be employed, wherein quantum information is encoded in the arrival time of photons, and long-distance fiber transmission over 300 km is possible [12]. The approach of using time-energy entangled photons was first formulated by Franson, with photon pairs created in an atomic decay process and two unbalanced interferometers [13]. Franson-type experiments have stimulated a plethora of research activities and have been extensively used for Bell tests in quantum optics [14][15][16].Today, entangled photon pairs for Franson experiments are primarily produced via spontaneous parametric down conversion (SPDC) or four-wave mixing (FWM), following two main methods. In time-energy experiments [17,18], a nonlinear medium is pumped by a continuous monochromatic laser and emission times of the photons have an uncertainty equal to the coherence time of the pump laser. On the other hand in time-bin experiments [14,19], a nonlinear medium is pumped by pulses that have previously passed through an unbalanced interferometer, leading to a "which-path" ambiguity in emission time. However, either method provides a nondeterministic pair generation, leading to limitations on the accuracy and security of quantum communication.Single-photon sources, based on single atoms, ions, impurity centers and quantum dots (QDs) [20], have emerged as alternatives [21,22], emitting no more than a single pair during any given cascade decay. In particular, intense research efforts using InAs semiconductor QDs have enabled the generation of triggered photon pairs at

Breaking the Unbreakable: Exploiting Loopholes in Bell’s Theorem to Hack Quantum Cryptography

“…The same assertion is made in a recent publication by Peiris et al [58], which states that their achieved visibilities of 66 % in a Franson setup "approach the visibility required to violate Bell's inequalities (70.7 %)". However, this conclusion is inaccurate for the Franson interferometer, as our discussion on the postselection loophole in section 6.2 shows.…”

Breaking the Unbreakable: Exploiting Loopholes in Bell’s Theorem to Hack Quantum Cryptography

“…We consider the lack of security in the standard Franson interferometric setup to be proven beyond doubt, but it is obvious we still have ways to go in educating the scientific community on this issue. In fact, publication F is a comment on an experimental paper that was published as recently as 2017 [58].…”

“…We submitted our comment to arXiv and PRL on the 15 th of March 2017 [59]. The comment was not accepted by PRL, but our contribution was acknowledged in the form of an erratum to the original paper published on the 18 th of August 2017 [60]. This erratum references our comment and thanks us for bringing the issues to their attention.…”

“…There, we briefly reviewed the complications of measuring the Bell value in a system with 50 % postselection and interpreting the results as a violation of local realism. While our paper was not accepted by PRL for publication, the authors published an erratum [60] to their paper where the quote in the previous paragraph was replaced with "with improved experimental capabilities our approach can lead to a violation of Bell's inequalities". The erratum references our comment (as published on arXiv) and thanks us for bringing the issue to their attention.…”

“…Publication F: Comment on "Franson Interference Generated by a Two-Level System" This is a comment to a paper by Peiris et al [58] published by Physical Review Letters (PRL) on the 19 th of January 2017. We submitted our comment to arXiv and PRL on the 15 th of March 2017 [59].…”