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arXiv:1209.4191 · Physical Review Letters

Entanglement Swapping between Photons that have Never Coexisted

E. Megidish, A. Halevy, Tomer Shacham, Tom Dvir, Liat Dovrat, H. S. Eisenberg

Published May 22, 2013• 118 citations
arXivHTMLPDF

Abstract

The role of the timing and order of quantum measurements is not just a fundamental question of quantum mechanics, but also a puzzling one. Any part of a quantum system that has finished evolving can be measured immediately or saved for later, without affecting the final results, regardless of the continued evolution of the rest of the system. In addition, the nonlocality of quantum mechanics, as manifested by entanglement, does not apply only to particles with spacelike separation, but also to particles with timelike separation. In order to demonstrate these principles, we generated and fully characterized an entangled pair of photons that have never coexisted. Using entanglement swapping between two temporally separated photon pairs, we entangle one photon from the first pair with another photon from the second pair. The first photon was detected even before the other was created. The observed two-photon state demonstrates that entanglement can be shared between timelike separated quantum systems.

DOI
10.1103/physrevlett.110.210403

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Entanglement swapping between photons that never coexist in laboratory time provides a clean test of the temporal ordering of quantum measurements without invoking retrocausal signaling. We review the 2013 Physical Review Letters experiment by Megidish et al., correcting several common secondary-source simplifications: the temporal separation was implemented with a 31.6 m free-space delay line (105 ns), the final state of photons 1 and 4 was characterized by quantum-state tomography, and the reported result was a conditional entangled state with approximately 77% fidelity to the target Bell state rather than a standalone Bell-inequality test. We then integrate this experimental basis with a DQ-12 interpretation in which photonic excitations are modeled phenomenologically as phase modes of a higher-dimensional continuous state substrate. Within that interpretation, detection is treated as a local relaxation map, while the Bell-state projection on the intermediate photons is represented as a conditional phasebridging operation. Crucially, the standard quantum-mechanical account is sufficient for the observed experiment; DQ-12 is therefore presented not as a confirmed explanation but as an additional ontology that acquires scientific content only through differential, falsifiable predictions. We propose residual observables, delay-dependent tests, matched controls, and a null hypothesis that reduce the DQ-12 claim to an experimentally decidable extension.Keywords: entanglement swapping; temporal entanglement; Bell-state measurement; quantum-state tomography; timelike separation; DQ-12; phase connectivity; emergent time; quantum foundations.
We report the experimental realization of entanglement swapping over long distances in optical fibers. Two photons separated by more than 2km2\phantom{\rule{0.3em}{0ex}}\mathrm{km}2km of optical fibers are entangled, although they never directly interacted. We use two pairs of time-bin entangled qubits created in spatially separated sources and carried by photons at telecommunication wavelengths. A partial Bell-state measurement is performed with one photon from each pair, which projects the two remaining photons, formerly independent onto an entangled state. A visibility high enough to infer a violation of a Bell inequality is reported, after both photons have each traveled through 1.1km1.1\phantom{\rule{0.3em}{0ex}}\mathrm{km}1.1km of optical fiber.
We experimentally entangle freely propagating particles that never physically interacted with one another or which have never been dynamically coupled by any other means. This demonstrates that quantum entanglement requires the entangled particles neither to come from a common source nor to have interacted in the past. In our experiment we take two pairs of polarization entangled photons and subject one photon from each pair to a Bell-state measurement. This results in projecting the other two outgoing photons into an entangled state.
Entangled quantum systems can connect to the environment via a Bell state measurement. This applies, for example, to teleportation and entanglement swapping. Although the results are well understood, it is not entirely clear whether they involve nonlocal action or whether they are predetermined. This can best be decided from a model, provided it predicts the key measurement results. Models based on the fact that the partners of an entangled pair have the same value of a statistical parameter cannot be applied here. This is because the partner particles of the resulting entangled states after a teleportation or an entanglement swapping never had contact before. The question then is, what connects entangled photons? Therefore, this paper presents a local realistic model that reproduces the quantum mechanical predictions for expectation values with polarization measurements, but is not based on shared statistical parameters. Instead, the coupling of the entangled particles is based on initial conditions and conservation of spin angular momentum. The model refutes Bell's theorem and also explains teleportation and entanglement swapping in a local way. The manuscript is thus a step forward toward a complete theory describing quantum physical reality as thought possible by Einstein, Podolsky, and Rosen.