Shape of things to comeQuantum communication over long distance, flawed networks possible

Published 17 December 2007

Chinese scientists offer possible breakthrough in quantum communication — overcoming the problem of quantum entanglement between photons at long distances; the scientists show a quantum-communications network in which producing entanglement over a long distance is conceptually possible

Here is an important discovery which, if successfully implemented, would make quantum communication an exceedingly secure method of transmitting information. Recently, physicists suggested a way, at least in theory, to overcome one of the biggest problems: Making quantum communication possible over real-life networks with serious imperfections, such as leakage, and across distances greater than ten kilometers. All of the problems hobbling the progress of quantum communication have to do with the foundation of quantum communication, a phenomenon called quantum entanglement. Quantum entanglement occurs when two quantum-information carriers, such as photons, are aware of each other’s existence and know each other’s particular quantum state despite never having previously interacted and being physically separate. It is one peculiar effect of the strange, mysterious — and exciting — world of quantum physics. Currently, photon channels, such as fiber-optic cables, are the only feasible choice for quantum communication. Creating high-fidelity quantum entanglement between photons at two distant locations, however, becomes exponentially more difficult as the distance between them increases, seriously impeding the real-life implementation of quantum communication. Extending the range to practical distances remains a challenge on many levels.

Here is the breakthrough: In a recent paper in Physical Review A, physicists from Nanjing University in China propose a quantum-communications network in which producing entanglement over a long distance is conceptually possible. The basic network they suggest is made of a sending node and receiving node coupled to a quantum channel (such as a fiber optic cable) that contains an optical circulator, a fiber-optic component that allows signals to simultaneously travel in both directions down a fiber. Inside the sending and receiving nodes are a quantum dot (typically a tiny cluster of atoms that behaves as a single atom in the quantum sense) in a microcavity. Each dot can be in one of three quantum states: a ground state, an excited state, and an intermediate state. Each state is a qubit, or quantum bit, the most basic piece of quantum information, like how a “0” or “1” form a bit of computer storage. These qubits are stationary. The scheme also includes a “flying qubit,” a mobile piece of quantum information, that moves between them. The flying qubit in this case is a pulse of light with a specific shape. The pulse acts as something like a middle man, initially being entangled with the sending qubit but swapping its entanglement with the receiving qubit, thus leaving the sending and receiving qubits entangled.

This scheme, when its parameters are properly and carefully tweaked, avoids some of the issues that arise in other methods which have been proposed and, the Chinese scientists say, can yield fidelities that are almost perfect.

-read more in Fang-Yu Hong and Shi-Jie Xiong, “Quantum Communication in a Network with Serious Imperfections,” Physical Review A 76 (6 December 2007) (DOI: 10.1103/PhysRevA.76.052302): 052302 1-6 (sub. req.)