Optimizing quantum communication

As quantum computing moves closer to real-world application, engineers are considering how to meet the challenge of transmitting quantum information faithfully down long, noisy optical fibers. The fidelity with which an entangled photon pair can be sent down a fiber decreases exponentially with distance, requiring repeater nodes spaced 10 km apart. Liang Jiang et al. report an approach to finding an optimal protocol to carry out the many operations necessary to make quantum information transfer practical. The process of optimization, they explain, is analogous to the ‘‘nesting problem’’ of multiplying a string of large matrices with the minimal number of calculations. Also, the system is self-similar, so the same protocol applies at any length scale. The authors optimize the process for two major types of repeater architectures (BDCZ and CTSL) by adjusting three parameters: the time required to generate entangled pairs, the internodal distances, and the number of steps taken to improve fidelity by ‘‘entanglement purification.’’ Even for optimized protocols, quantum information transfer remains more than two orders of magnitude slower than classical; the authors say that further experimental advances are necessary to make this mode of communication viable. — K.M.