Gates analogous to conventional electronic logic gates in conventional computers but different in that the system follows the strange rules of quantum mechanics. An early realization of a quantum logic gate uses a single trapped beryllium ion to demonstrate a two-bit quantum logic gate. One bit, the control bit, is specified by the (quantized) external vibrations of the ion in the atom trap; the two lowest vibrational levels correspond to values 0 and 1. The other bit (the target bit) is specified by an internal state of one of the ion's electrons; it has a "spin-down" state (0) and a "spin-up" state (1). Shooting laser pulses at the single ion causes it to act as a two-bit "controlled NOT" gate. If the control bit is 0 then the target bit is left alone. If the control bit is 1 then the target bit flips its spin.
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