Figure 2.

The principle of the two-color, three-field experiment and the concatenated logic gates that can be built by using only one of the fields. The actual experimental results are shown in Fig. 1. These show that two or even all three fields can be used for the building of logic circuits. More elaborate schemes will be discussed below. (Left) A two-photon absorption leading to a high Rydberg state via an intermediate state, which is the v₆ vibration of the first electronically excited state (S₁). The two photons are of different colors. The Rydberg state is accessed in a region in which a weak DC electrical field, F_ℓ, is present. This field is switched off after 10 ns but can be switched on again, several microseconds later (see Fig. 1 and ref. 26). Next, a second DC field, F_m, which is in a direction perpendicular to the first field, is or is not switched on. The measured spectrum can be used to indicate whether the field F_m has been “on” or “off.” Thereby, the output of the first AND gate (shown in the usual notation as a half-moon, with its truth table below it) can be fed to a second AND gate, and the output depends on whether the field F_m was or was not on. Note that each AND gate is equivalent to two switches (25) so that even this simple scheme has a single-molecule response equivalent to four switches. As discussed for Fig. 3, using two photons and one field is really a four-gate logic circuit. We use the simpler circuit shown in this figure to introduce the ideas.