Post-selection-loophole-free Bell violation with genuine time-bin entanglement

I. Capraro¹, A. Tomaello¹, A. Dall’Arche¹, F. Gerlin¹, R. Ursin², G. Vallone¹, and P. VilloresiF. Vedovato¹, C. Agnesi¹,M. Tommasin¹,M. Avesani¹,Jan-Ake Larsson²,G. Vallone^1,3, P. Villoresi^1,3₁
Phys. Rev. Lett. 121, 190401 (2018)
DOI:10.1103/PhysRevLett.121.190401

¹Department of Information Engineering, University of Padova, via Gradenigo 6/B, 35131 Padova, Italy
²Institutionen for systemteknik, Linkopings Universitet, 581 83 Linkoping, Sweden
³Istituto di Fotonica e Nanotecnologie, CNR, Padova, Italy

Entanglement is an invaluable resource for fundamental tests of physics and the implementation of quantum information protocols such as device-independent secure communications. In particular, time-bin entanglement is widely exploited to reach these purposes both in free-space and optical fiber propagation, due to the robustness and simplicity of its implementation. However, all existing realizations of time-bin entanglement suffer from an intrinsic post-selection loophole, which undermines their usefulness. Here, we report the first experimental violation of Bell’s inequality with “genuine” time-bin entanglement, free of the post-selection loophole. We modify the setup by replacing the first passive beam-splitter in each measurement station with an additional interferometer acting as a fast optical switch synchronized with the source. Using this setup we obtain a post-selection-loophole-free Bell violation of more than nine standard deviations. Since our scheme is fully implementable using standard fiber-based components and is compatible with modern integrated photonics, our results pave the way for the distribution of genuine time-bin entanglement over long distances.