Quantum theory describes peculiar fundamental properties of matter such as the double nature of a single object, which can behave as either a wave or a particle, and the existence of strong correlations at distance between systems of microscopic particles (named by Einstein “spooky action at distance”), including photons. These two properties, known respectively as “wave-particle duality” and “entanglement”, have been verified separately in several experiments, while no direct joint observation has been ever reported.

A recent study, published in Nature Communications, has shown that these two fundamental aspects of quantum mechanics can coexist in the same system. Such work is the result of an international collaboration, coordinated on the experimental side by the Quantum Information Lab of the Physics Sapienza Department, lead by Fabio Sciarrino, and on the theoretical side by Rosario Lo Franco from University of Palermo, together with researchers from universities in China and Vietnam. The Sapienza team is fully composed of under 40 researchers.

The international team realized experimentally a theoretical scheme that exploits polarized photons, that is, quanta of light having a specific direction of oscillation. The first step of this work was to demonstrate that a single particle can simultaneously exist as a wave and a particle, being in a quantum superposition state of the two behaviors.
Then, the physicists have created and measured a two-photon state presenting specific wave-particle correlations.

More specifically, in such state the wave or particle behavior of one of the photons is determined by the corresponding behavior of the other one, independently from their distance. “We have demonstrated – Lo Franco explains – that it is possible to generate entanglement in the dual wave-particle nature of the photons. Analogously to the peculiar definition by Einstein, this phenomenon can be named ‘wave-particle duality action at a distance’. Furthermore, the simplicity of the theoretical scheme and the effectiveness of the experimental apparatus permits to extend such results to larger number of photons”.

Indeed, the Sapienza team has implemented an experimental apparatus with several points of strength, including a high degree of stability, a modular approach that can be reproduced for increasing number of photons and a completely automated acquisition system. “We are now working in two new directions – Sciarrino says. – On the conceptual side, we aim at developing theoretically and experimentally new quantifiers (“witness”) of quantum resources.

On the technological side, we are developing an integrated platform based on photonics circuits. In such a way, we expect to increase the complexity of the generated states, that can be applied in different contexts such as quantum cryptography and quantum communication”.

References:

Adil S. Rab, Emanuele Polino, Zhong-Xiao Man, Nguyen Ba An, Yun-Jie Xia, Nicolò Spagnolo, Rosario Lo Franco & Fabio Sciarrino - Entanglement of photons in their dual waveparticle nature, Nature Communications 8, Article number: 915 (2017), DOI: 10.1038/s41467-017-01058-6.

Further Information

Fabio Sciarrino
Quantum Information Lab
Department of Physics, Sapienza Universiy of Rome 
Email: fabio.sciarrino@uniroma1.it