A ternary computer (also called trinary computer) is a computer that uses ternary logic (three possible values) instead of the more common binary logic (two possible values) in its calculations.
One of the earliest calculating machines, built by Thomas Fowler entirely from wood in 1840, was a ternary computer. The only modern, electronic ternary computer Setun was built in the late 1950s in the Soviet Union at the Moscow State University by Nikolay Brusentsov, and it had notable advantages over the binary computers which eventually replaced it (such as lower electricity consumption and lower production cost). In 1970 Brusentsov built an enhanced version of the computer, which he called Setun-70.
Ternary computing is commonly implemented in terms of balanced ternary, which uses the three digits −1, 0, and +1. The negative value of any balanced ternary digit can be obtained by replacing every + with a − and vice versa. It is easy to subtract a number by inverting the + and − digits and then using normal addition. Balanced ternary can express negative values as easily as positive ones, without the need for a leading negative sign as with decimal numbers. These advantages make some calculations more efficient in ternary than binary.
I often reflect that had the Ternary instead of the binary Notation been adopted in the Infancy of Society, machines something like the present would long ere this have been common, as the transition from mental to mechanical calculation would have been so very obvious and simple. (Fowler, 1840)
With the advent of mass-produced binary components for computers, ternary computers have diminished to a small footnote in the history of computing. However, ternary logic’s elegance and efficiency is predicted by Donald Knuth to bring them back into development in the future. Possible ways on how this can happen is by the combination of an optical computer with the ternary logic system. A ternary computer using fiber optics could use dark as 0 and two orthogonal polarizations of light as 1 and −1. This future potential has also been remarked by certain companies as Hypres which is actively engaged in ternary computing. IBM also reports infrequently on ternary computing topics (in its papers), but it is not actively engaged in it.
The Josephson junction has been proposed as a balanced ternary memory cell, using circulating superconducting currents, either clockwise, counterclockwise, or off.
The advantages of the proposed memory circuit are capability of high speed computation, low power consumption and very simple construction with less number of elements due to the ternary operation.
In 2009, a ternary quantum computer was proposed which thus uses qutrits rather than qubits. When the number of basic states of quantum element is d, it is called qudit.