Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

arXiv:2508.19207 · Physical Review Letters

Unquestionable Bell Theorem for Interwoven Frustrated Downconversion Processes

Paweł Cieśliński, Jan-Åke Larsson, Marcin Markiewicz, Konrad Schlichtholz, Marek Żukowski

Published Jan 31, 2026• 1 citation
arXivHTMLPDF

Abstract

Interwoven frustrated parametric down-conversion (PDC) processes produce interference effects based on path identity [Krenn et al., Entanglement by path identity, Phys. Rev. Lett. 118, 080401 (2017)PRLTAO0031-900710.1103/PhysRevLett.118.080401]. In this Letter we show Bell nonclassicality of the processes: a proper violation of the Clauser-Horne inequality when the local measurements are controlled by on-off switching of the final local PDC processes. Such a nonstandard approach is needed because if only the local phase shifts are used for the measurement settings, as done in the experiment reported in Wang et al. [Violation of Bell inequality with unentangled photons, Sci. Adv. 11, eadr1794 (2025)SACDAF2375-254810.1126/sciadv.adr1794], there exists a local realistic model of the interference, which we present in this Letter. Nevertheless, the reported destructive interference is deep enough to violate the inequality when using on-off switching, so our result establishes a firm footing for nonclassicality of the new interferometry, in both theory and experiment. The on-off approach to Bell analysis of path-identity-based interference forms a new platform for seeking new highly counterintuitive quantum phenomena.

DOI
10.1103/h2l1-fcz9

Related papers

Multipartite Bell-Greenberger-Horne-Zeilinger nonclassicality from interwoven frustrated down-conversion

Marek Żukowski, Paweł Cieśliński, Marcin Markiewicz +1 more

We present a theory of an interference process that starts with $N$ coherently pumped two-mode parametric down-conversion (PDC) sources, whose output modes are directed to $N$ observers such that each observer receives modes from two different source crystals. Each observation station is equipped with a locally controlled PDC crystal, coherently pumped with the source crystals, whose output modes are perfectly aligned with the input modes from the source PDCs. By varying the local phases of the input modes, perfect $2N$-photon interference can be observed in $2N$ single-photon registrations, one in each output mode of these $N$ local PDCs. The interference results from the indistinguishability of the origins of the detected $2N$ photons: either they all originate from the source PDCs or from the local PDCs. Bell-GHZ nonclassicality of the process emerges when one also considers situations in which at least one of the local PDC processes is blocked. In such cases, the $2N$-photon interference disappears. A ''lifted`` Clauser-Horne inequality is violated when its sole negative term, involving all observers with all local pumps active, is tuned to maximal destructive interference, while all other terms involve settings in which one of the local pumps is off.

quant-pharXiv:2602.18381•Physical Review A•Feb 23, 2026•0 citations

Violation of Bell inequality with unentangled photons

Kai Wang, Zhaohua Hou, Kaiyi Qian +6 more

Violation of local realism via Bell inequality-a profound and counterintuitive manifestation of quantum theory that conflicts with the prediction of local realism-is viewed to be intimately linked with quantum entanglement. Experimental demonstrations of such a phenomenon using quantum entangled states are among the landmark experiments of modern physics and paved the way for quantum technology. Here, we report the violation of the Bell inequality that cannot be described by quantum entanglement in the system but arises from quantum indistinguishability by path identity, shown by the multiphoton frustrated interference. By analyzing the measurement of four-photon frustrated interference within the standard Bell-test formalism, we find a violation of Bell inequality by more than four SDs. Our work establishes a connection between quantum correlation and quantum indistinguishability, providing insights into the fundamental origin of the counterintuitive characteristics observed in quantum physics.

arXiv:2507.07756•Science Advances•Aug 1, 2025•9 citations

Phase-shifted Bell states

J. J. Joshua Davis, Carey L. Jackman, R. Leonhardt +2 more

Inspired by previous studies and pioneers of the field, we present new results on an extensive Einstein, Podolsky, and Rosen (EPR)–Bell experiment using photons generated by parametric down-conversion, where one of the photons is deliberately phase-shifted. Our experiments show some surprising results for particular angles of this phase shift, where, for some states, the Clauser, Horne, Shimony, and Holt inequality shows no violations, requiring us to review our understanding of “entangled” states vis-à-vis the correlations obtained via experimental coincidence counts in agreement with quantum mechanics theoretical predictions. We give some possible explanations about the correlated states that show no violations of these inequalities.

Journal of the Optical Society of America B•Apr 9, 2025•4 citations

Simple explanation of apparent Bell nonlocality of unentangled photons

Antoni Wójcik, Jan W. Wojcik

Wang et al. [Science Advances, 1 Aug 2025 Vol 11, Issue 31] recently reported an experiment that they interpret as demonstrating a violation of Bell's inequality using unentangled photons. Such a claim is highly controversial, since it is well established that product states cannot surpass the bounds set by local hidden variable models. In this manuscript, we analyze the essential features of the experiment through simplified, idealized scenarios. Our analysis shows that the apparent violation of Bell's inequality originates from two key elements: postselection and an unconventional normalization procedure. These steps produce quantities that formally mimic Bell-type correlations but lack the operational meaning required in a genuine Bell test. We therefore argue that the reported violation does not demonstrate nonlocality without entanglement, but rather reflects a misinterpretation of otherwise valid and interesting multiphoton interference effects.

arXiv:2509.03127•Sep 3, 2025•0 citations

Entangled and Factorized States in Parametric Down-Conversion Sources

Augusto Garuccio, Augusto Garuccio

In the last few years the parametric down-conversion of light has attracted the attention of the researchers in foundations of quantum physics. The photons that come out from the non-linear crystal are highly correlated in many variables. One of the most important consequences of entanglement is the possibility of violating the so called Einstein locality expressed in Bell's inequality. In general factorizable state vectors do not violate Bell's inequality, but we will show that the factorizable state vector produced with a beam-splitter set on the paths of parametric down-conversion photons violates a new inequality that can be deduced from local realism. This inequality is more stringent than those used up to now to test the locality.

Fortschritte der Physik•Nov 1, 1998•0 citations

An energy–time Bell inequality in two-photon interferometry

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

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.

Annual Meeting Optical Society of America•Jan 1, 1992•0 citations
Interwoven frustrated parametric down-conversion (PDC) processes produce interference effects based on path identity [Krenn et al., Entanglement by path identity, Phys. Rev. Lett. 118, 080401 (2017)PRLTAO0031-900710.1103/PhysRevLett.118.080401]. In this Letter we show Bell nonclassicality of the processes: a proper violation of the Clauser-Horne inequality when the local measurements are controlled by on-off switching of the final local PDC processes. Such a nonstandard approach is needed because if only the local phase shifts are used for the measurement settings, as done in the experiment reported in Wang et al. [Violation of Bell inequality with unentangled photons, Sci. Adv. 11, eadr1794 (2025)SACDAF2375-254810.1126/sciadv.adr1794], there exists a local realistic model of the interference, which we present in this Letter. Nevertheless, the reported destructive interference is deep enough to violate the inequality when using on-off switching, so our result establishes a firm footing for nonclassicality of the new interferometry, in both theory and experiment. The on-off approach to Bell analysis of path-identity-based interference forms a new platform for seeking new highly counterintuitive quantum phenomena.
We present a theory of an interference process that starts with NNN coherently pumped two-mode parametric down-conversion (PDC) sources, whose output modes are directed to NNN observers such that each observer receives modes from two different source crystals. Each observation station is equipped with a locally controlled PDC crystal, coherently pumped with the source crystals, whose output modes are perfectly aligned with the input modes from the source PDCs. By varying the local phases of the input modes, perfect 2N2N2N-photon interference can be observed in 2N2N2N single-photon registrations, one in each output mode of these NNN local PDCs. The interference results from the indistinguishability of the origins of the detected 2N2N2N photons: either they all originate from the source PDCs or from the local PDCs. Bell-GHZ nonclassicality of the process emerges when one also considers situations in which at least one of the local PDC processes is blocked. In such cases, the 2N2N2N-photon interference disappears. A ''lifted`` Clauser-Horne inequality is violated when its sole negative term, involving all observers with all local pumps active, is tuned to maximal destructive interference, while all other terms involve settings in which one of the local pumps is off.
Violation of local realism via Bell inequality-a profound and counterintuitive manifestation of quantum theory that conflicts with the prediction of local realism-is viewed to be intimately linked with quantum entanglement. Experimental demonstrations of such a phenomenon using quantum entangled states are among the landmark experiments of modern physics and paved the way for quantum technology. Here, we report the violation of the Bell inequality that cannot be described by quantum entanglement in the system but arises from quantum indistinguishability by path identity, shown by the multiphoton frustrated interference. By analyzing the measurement of four-photon frustrated interference within the standard Bell-test formalism, we find a violation of Bell inequality by more than four SDs. Our work establishes a connection between quantum correlation and quantum indistinguishability, providing insights into the fundamental origin of the counterintuitive characteristics observed in quantum physics.
Inspired by previous studies and pioneers of the field, we present new results on an extensive Einstein, Podolsky, and Rosen (EPR)–Bell experiment using photons generated by parametric down-conversion, where one of the photons is deliberately phase-shifted. Our experiments show some surprising results for particular angles of this phase shift, where, for some states, the Clauser, Horne, Shimony, and Holt inequality shows no violations, requiring us to review our understanding of “entangled” states vis-à-vis the correlations obtained via experimental coincidence counts in agreement with quantum mechanics theoretical predictions. We give some possible explanations about the correlated states that show no violations of these inequalities.
Wang et al. [Science Advances, 1 Aug 2025 Vol 11, Issue 31] recently reported an experiment that they interpret as demonstrating a violation of Bell's inequality using unentangled photons. Such a claim is highly controversial, since it is well established that product states cannot surpass the bounds set by local hidden variable models. In this manuscript, we analyze the essential features of the experiment through simplified, idealized scenarios. Our analysis shows that the apparent violation of Bell's inequality originates from two key elements: postselection and an unconventional normalization procedure. These steps produce quantities that formally mimic Bell-type correlations but lack the operational meaning required in a genuine Bell test. We therefore argue that the reported violation does not demonstrate nonlocality without entanglement, but rather reflects a misinterpretation of otherwise valid and interesting multiphoton interference effects.
In the last few years the parametric down-conversion of light has attracted the attention of the researchers in foundations of quantum physics. The photons that come out from the non-linear crystal are highly correlated in many variables. One of the most important consequences of entanglement is the possibility of violating the so called Einstein locality expressed in Bell's inequality. In general factorizable state vectors do not violate Bell's inequality, but we will show that the factorizable state vector produced with a beam-splitter set on the paths of parametric down-conversion photons violates a new inequality that can be deduced from local realism. This inequality is more stringent than those used up to now to test the locality.
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.