Quantum repeaters based on atomic ensembles and linear optics

TLDR

Photon loss limits long‑distance quantum state distribution, and because amplification is forbidden by the no‑cloning theorem, quantum repeaters that generate long‑distance entanglement via entanglement swapping are required. This review surveys the current theoretical and experimental status of quantum repeaters based on atomic ensembles and linear optics. The protocols rely on heralded entanglement generation, quantum memory storage, and entanglement swapping, implemented with atomic ensembles, linear optics, and photon counting. Quantitative comparisons of various approaches show which can surpass direct photon transmission, highlighting the most promising strategies.

Abstract

The distribution of quantum states over long distances is limited by photon loss. Straightforward amplification as in classical telecommunications is not an option in quantum communication because of the no-cloning theorem. This problem could be overcome by implementing quantum repeater protocols, which create long-distance entanglement from shorter-distance entanglement via entanglement swapping. Such protocols require the capacity to create entanglement in a heralded fashion, to store it in quantum memories, and to swap it. One attractive general strategy for realizing quantum repeaters is based on the use of atomic ensembles as quantum memories, in combination with linear optical techniques and photon counting to perform all required operations. Here the theoretical and experimental status quo of this very active field are reviewed. The potentials of different approaches are compared quantitatively, with a focus on the most immediate goal of outperforming the direct transmission of photons.