'Data-in, data-out' signals quantum breakthrough

 作者:乔诩     |      日期:2019-03-02 08:05:08
By Will Knight A trick for transferring quantum information from atoms to photons and back again could be used to create impenetrable global communication networks and computers that work at astounding speeds. Two research groups, one led by Mikhail Lukin at Harvard University and the second headed by Alex Kuzmich of Georgia Institute of Technology, both in the US, separately demonstrated the feat using similar methods. Both teams employed powerful laser pulses to extract quantum information from a cloud of atoms in the form of a single photon. That photon was then transmitted through a normal optical fibre before its quantum state was transferred to a second atomic cloud. Creating communication links between such “quantum memories” – the clouds of atoms – is crucial to building complex networks that exploit quantum phenomena, such as entanglement and superposition. Quantum networks are extremely sensitive to interference, but hold great promise for secure communications and superfast computing. “To me this is a major step forward,” says Bill Munro, an expert in quantum communications at Hewlett Packard’s research laboratory in Bristol, UK. “Getting light coherently into a memory and then out again is key.” In both experiments, powerful laser beams were used to excite a cloud of rubidium atoms and generate a single photon carrying the quantum state of the excited atoms. The individual photon was then transmitted along a fibre optic cable about 100 metres long to another rubidium cloud. There, the quantum state was transferred using further strong laser pulses. Matthew Eisaman, a member of the Harvard team, says the crucial step is filtering the individual photon from the various laser pulses. His team managed this by using crystals to separate photons based on their polarity, reflectivity and absorption. The technique could lead to the development of long-distance optical-quantum communications channels. These promise flawless privacy, because any attempt at eavesdropping will disturb the quantum nature of the information sent. Currently, photons carrying quantum information can only travel tens of kilometres through fibre optics cables before deteriorating. But a “quantum repeater” capable of storing a photon’s quantum information before retransmitting it, could be used to transmit quantum data over much greater distances. Munro says the technique could also be harnessed to build a quantum computer, by passing information from one part of the machine’s memory to another. Quantum particles can exist in more than one state at once, meaning quantum computers should, in theory, be able to perform billions of calculations simultaneously. However, both research teams warn that further refinements must be made before their experiments can be used for practical purposes. Eisaman says it is necessary to increase the time that quantum information can be stored from the atom clouds from millionths of a second to thousandths. Only then, he says, will it be possible to use it for communications. Kuzmich, at Georgia Institute of Technology, adds that the experiments are “an important step toward distributed quantum networks” but adds: “It will take a lot of steps and several more years for this to happen in a practical way.” Journal reference: Nature (vol 438,