A single digital signal has only two states - on or off. However, if you use two inputs you can double the number of possible states, to four.

Building logic gates out of silicon is old hat, as is building them from discrete transistors, 555 chips, LEGO, and even gears. [Yukio-Pegio Gunji] and [Yuta Nishiyama] from Kobe University ...

A chip that contains one logic gate or a small number of logic gates. Although thousands of gates are routinely placed on a single chip, discrete logic chips with only one or two gates are also ...