In Fredkin and Toffoli’s model, each possible atomic collision performs AND, OR, NOT, or COPY operations on suitably defined input and output bits. By assigning the proper initial positions and velocities to atoms in a gas, it is a straightforward matter to “wire up” any desired logic circuit. Atoms bouncing in a gas are, in principle, capable of universal digital computation. In practice, of course, it is rather difficult to make a gas of atoms perform a computation. Even if we did have control over the position and velocity of individual atoms, quantum mechanics limits the accuracy to which
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