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. 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.
Highlights
Interaction-free measurements allow one to measure whether or not an object is present without ever interacting with it.[1]
A single photon used in a Mach-Zehnder interferometer (MZI) sometimes reveals whether or not an absorbing object had been placed in one of the interferometer arms, without any interaction between the photon and the bomb
We experimentally implement a counterfactual communication (CFC) protocol where information can propagate without being carried by physical particles
Summary
Interaction-free measurements allow one to measure whether or not an object is present without ever interacting with it.[1]. Some authors have argued that the presence of the “weak trace” renders the counterfactuality of the protocol invalid,[16,17,18,19] but others have dismissed the weak trace as a consequence of the unwanted weak measurement’s disturbance.[20,21,22] (4) Unless operated in the theoretical limit of infinite optical operations, this scheme requires post-selection to remove the CFC violations.[13,21,23] It has recently been shown that a classical communication protocol can be counterfactual if postselection is allowed.[14]
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