Trace-free counterfactual communication with a nanophotonic processor

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.npj Quantum Information (2019) 5:61 ; https://doi.

Universal programmable photonic architecture for quantum information processing

“…Such devices have a wide range of applications in classical information processing [4,[6][7][8][9][10], and integrated universal photonic circuits provides an especially promising hardware platform for high-throughput, energy-efficient machine learning. [11][12][13][14] These devices also have promising applications in quantum information processing: recent demonstrations of boson sampling [15], quantum transport dynamics [16], photonic quantum walks [17], counterfactual communication [18], and probabilistic two-photon gates [19] have all been performed on this type of programmable photonic hardware. Photonic systems offer a range of unique advantages over other substrates for quantum information processing: optical quantum states have long coherence times and can be maintained at room temperature, since they interact very weakly with their environment; photonic qubits are optimal information carriers for distant nodes within quantum networks; and MZIs provide simple, high-fidelity implementations of single-qubit operations which can be integrated into a photonic chip.…”