Trace-free counterfactual communication with a nanophotonic processor

• Published in: npj quantum information Vol. 5; no. 1; pp. 1 - 5
• Main Authors: Alonso Calafell, I., Strömberg, T., Arvidsson-Shukur, D. R. M., Rozema, L. A., Saggio, V., Greganti, C., Harris, N. C., Prabhu, M., Carolan, J., Hochberg, M., Baehr-Jones, T., Englund, D., Barnes, C. H. W., Walther, P.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 23.07.2019; Nature Publishing Group
• Subjects: 639/766/483/2802; 639/766/483/3925; 639/766/483/481; Classical and Quantum Gravitation; Communication; Nanotechnology; Photons; Physics; Physics and Astronomy; Protocol; Quantum Computing; Quantum Field Theories; Quantum Information Technology; Quantum Physics; Relativity Theory; Spintronics; String Theory
• ISSN: 2056-6387; 2056-6387
• DOI: 10.1038/s41534-019-0179-2

In standard communication information is carried by particles or waves. Counterintuitively, in counterfactual communication particles and information can travel in opposite directions. The quantum Zeno effect allows Bob to transmit a message to Alice by encoding information in particles he never interacts with. A first remarkable protocol for counterfactual communication relied on thousands of ideal optical operations for high success rate performance. Experimental realizations of that protocol have thus employed post-selection to demonstrate counterfactuality. This post-selection, together with arguments concerning a so-called “weak trace” of the particles traveling from Bob to Alice, have led to a discussion regarding the counterfactual nature of the protocol. Here we circumvent these controversies, implementing a new, and fundamentally different, protocol in a programmable nanophotonic processor, based on reconfigurable silicon-on-insulator waveguides that operate at telecom wavelengths. This, together with our telecom single-photon source and highly efficient superconducting nanowire single-photon detectors, provides a versatile and stable platform for a high-fidelity implementation of counterfactual communication with single photons, allowing us to actively tune the number of steps in the Zeno measurement, and achieve a bit error probability below 1%, without post-selection and with a vanishing weak trace. Our demonstration shows how our programmable nanophotonic processor could be applied to more complex counterfactual tasks and quantum information protocols.