The NIST quantum key distribution (QKD) testbed uses single photons (the smallest particles of light) to produce a continuous binary code, or "key," for encrypting information. In the fiber-optic system that set the latest speed record, the photons are sent and received in four different orientations. Each photon is sent in one of two modes, either vertical/horizontal, representing the bit values 1 and 0, respectively; or plus 45 degrees/minus 45 degrees, representing the bit values 1 and 0, respectively.
Each photon fits best through one of two types of detectors, or "mailboxes," one with slots shaped like +, the other with slots shaped like X. The sender randomly chooses both a mode and an orientation for each photon. The receiver randomly chooses one of the two mailboxes to receive each photon. If the receiver chooses the same mode used in sending a particular photon, the detector always records the correct orientation and bit value. But if the receiver chooses the other mailbox, then the detector will record the wrong bit value about half the time. For this reason, the sender and receiver only use data sent and received in the same mode when they are making a shared key to encrypt information.
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