举个最简单的例子，当发送方将一束光子流传送给接受方，必须从两种模式中的选取一种对其中的光子进行编码（译码者通常称发送方为爱丽丝，接受方为鲍勃）。

The simplest example is when the sender (whom cryptographers usually call Alice) dispatches a stream of them to the receiver (who is known as Bob). These photons will have one of two modes.

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