A model of the photon avalanche in a four level coupled system is proposed.

提出了四能级耦合系统的光子雪崩上转换模型。

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In fact, real benefit levels were lower in the 1970s than they were in the 1940s, and in most states the maximum benefit was below the poverty level for a family of four.

事实上，20世纪70年代的实际福利水平比20世纪40年代的要低，而且在大多数州，四口之家的最高福利都低于贫困线。

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According to a study by Australian National University, Australian children and Chinese children have the same level of eyesight before they start school, but once they enter primary school, Chinese children only spend about an hour a day outside, while Australian children spend three to four hours each day in the sunshine.

澳大利亚国立大学的一项研究显示，澳大利亚的孩子和中国的孩子在入学前的视力水平相同。但是一旦他们进入小学，中国的孩子每天只会在户外度过一个小时，而澳大利亚的孩子每天在阳光下待三到四个小时。

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Phase four means that a virus has gained the ability to cause community-level outbreaks.

VOA: special.2009.05.06

They now face perennial power Brazil, a four-time champion at this level, but Tetteh says he is not concerned by the formidable opponent.

VOA: standard.2009.10.15

Water in the South Korean portion of the Imjin River surged Sunday more than four and a half meters, about twice its usual level.

VOA: standard.2009.09.07

Samsung trumpeted the much-anticipated phone's arrival Thursday at an event accompanied by a live orchestra while an audience of thousands watched the theatrics unfold on a four-level stage at Radio City Music Hall.

WSJ: Samsung refreshes iPhone-challenging Galaxy line

Smith, 36, who has a level four coaching badge, has already been helping with the club's academy.

BBC: Smith signs dual deal at New Road

Figures released by the Education Department showed more pupils achieved a level four or five grade at Key Stage Two, compared to the previous year.

BBC: Alan Brown