Figure 4.

The implementation of a half-adder. Note that by monitoring the intermediate excited state (S₁) one is able to get an Exclusive OR (denoted as XOR) gate. This generates the “sum,” which is a sum of two Boolean variables, modulo 2. In Boolean notation, the “sum” of A and B equals AB̄ + ĀB, where the bar denotes the logical negation. The forward carry bit is generated only when both laser colors are on by a ZEKE signal, which is obtained via pulsed-field ionization. Note that an XOR gate, shown with an additional semicircle, is a universal gate. The concatenation here is achieved without external fields by using only the intramolecular dynamics of S₁. By adding the role of the field, one can do more, as shown in Fig. 5. A full adder is shown in Figs. 6 and 7.