If anyone is interested in example of a reversible operation, the simplest one is logical negation of a single bit. The second-simplest is CNOT, or controlled-not, where one bit is the "control" bit and another bit is the "target" bit. If the control bit is 1, the target bit is flipped. If the control bit is 0, the target bit is unchanged. The control bit is always left unchanged.

A very important reversible gate is the CCNOT gate (also called a Toffoli gate) which has two control bits that only flip the target bit if both are 1. This is a universal gate: any reversible logical operation can be constructed from a series of Toffoli gates.

The article refers to reversible computers as "quantum computing's practical cousin" because all operations on a quantum computer are reversible (except for measurement). You may recall the recent pop science headline "scientists reverse time using a quantum computer"[1], which was a hideous mangling of this concept.

[1] https://phys.org/news/2019-03-physicists-reverse-quantum.htm...