Genuine Counterfactual Communication with a Nanophotonic Processor

TL;DR

This paper demonstrates a scalable, high-fidelity counterfactual communication protocol using a programmable nanophotonic processor, eliminating weak traces and enabling complex quantum information tasks.

Contribution

It introduces a novel implementation of counterfactual communication on a nanophotonic chip, overcoming previous scalability and trace issues with a versatile, stable platform.

Findings

Achieved bit error probability below 1% without post-selection.

Implemented a tunable number of Zeno measurement steps.

Demonstrated trace-free counterfactual communication in a practical setup.

Abstract

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. The first suggested protocol not only required thousands of ideal optical components, but also resulted in a so-called "weak trace" of the particles having travelled from Bob to Alice, calling the scalability and counterfactuality of previous proposals and experiments into question. Here we overcome these challenges, implementing a new 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 genuinely trace-free counterfactual communication, allowing us to actively tune the number of steps in the Zeno measurement, and achieve a bit error probability below 1%, with neither post-selection nor a weak trace. Our demonstration shows how our programmable nanophotonic processor could be applied to more complex counterfactual tasks and quantum information protocols.