  Multi-valued logic gates, which can handle quaternary numbers as inputs, are
developed by exploiting the ballistic transport properties of quantum point
contacts in series. The principle of a logic gate that finds the minimum of two
quaternary number inputs is demonstrated. The device is scalable to allow
multiple inputs, which makes it possible to find the minimum of multiple inputs
in a single gate operation. Also, the principle of a half-adder for quaternary
number inputs is demonstrated. First, an adder that adds up two quaternary
numbers and outputs the sum of inputs is demonstrated. Second, a device to
express the sum of the adder into two quaternary digits [Carry (first digit)
and Sum (second digit)] is demonstrated. All the logic gates presented in this
paper can in principle be extended to allow decimal number inputs with high
quality QPCs.
