Billiard Ball Fredkin Gate
A Fredkin gate with binary inputs returns the inputs unchanged if , but switches inputs 2 and 3 to give if . In this Demonstration, the gate is shown with billiard balls bouncing off fixed dominoes and each other. You can choose from seven possible inputs. The two interesting cases are and , which transform into one another. All other cases return their inputs.
 M. A. Nielsen and I. L. Chuang, Quantum Computation and Quantum Information, Cambridge University Press, 2000 p. 156. mmrc.amss.cas.cn/tlb/201702/W020170224608149940643.pdf.