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Propagation of the space-time entanglement of twin photons and twin beams

Alessandra Gatti*

Enrico Brambilla

  • *Contact author: Alessandra.Gatti@cnr.it
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Phys. Rev. Research 8, 033165 – Published 10 August, 2026

DOI: https://doi.org/10.1103/jv56-c2nf

Abstract

We investigate how the space-time entanglement of photon pairs generated by multimode parametric down-conversion (PDC) propagates away from the source. We show that the entangled two-photon probability, tightly confined in space and time at the source, quickly broadens and decays out of a focal plane, behaving as a focused coherent beam of approximately twice the wavelength of the twin photons. This behavior is in strong contrast with the much slower decay rate of the classical mutual coherence of the PDC radiation, which follows the prescription of the generalized Van Cittert-Zernicke theorem. Besides their fundamental interest, our results could be relevant for the design of experiments, such as quantum-enhanced two-photon processes, that exploit the spatiotemporal entanglement of the pairs.

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Phys. Rev. Research 8, 033165– Published 10 August, 2026
  • Received 22 December 2025
  • Accepted 2 July 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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