Experimental implementation of collision-based gates in Belousov–Zhabotinsky medium Benjamin De Lacy Costello a, * , Andrew Adamatzky b a Faculty of Applied Sciences, Centre for Research in Analytical, Material and Sensor Sciences, University of the West of England, Coldharbour Lane, Bristol BS16 1QY, UK b Faculty of Computing, Engineering and Mathematical Sciences, University of the West of England, Bristol BS16 1QY, UK Accepted 26 November 2004 Abstract We experimentally demonstrate that excitation wave-fragments in a Belousov–Zhabotinsky (BZ) medium with immobilised catalyst can be used to build elementary logical gates and circuits. Following our previous theoretical con- structions [Adamatzky A. Collision-based computing in Belousov Zhabotinsky medium. Chaos, Solitons & Fractals 2004;21:1259–64] on embedding logical schemes in BZ medium, we represent True/False values of logical variables by presence/absence of wave-fragments. We show that when wave-fragments collide with each other they may annihi- late, fuse, split and change their velocity vectors. Thus the values of logical variables represented by the wave-fragments change and certain logical operations are implemented. In the paper we provide examples of experimental logical gates, and present pioneer results in dynamic, architectureless computing in excitable reaction–diffusion systems. Ó 2005 Elsevier Ltd. All rights reserved. 1. Introduction Belousov–Zhabotinsky (BZ) [27] chemical system—a typical example of excitable chemical media—is proved to be capable for specialised computation. The medium belongs to a class of reaction–diffusion processors, namely to its sub-class of reusable processors. A reaction–diffusion processor (RD) is a real chemical medium, typically a thin-layer solution, gel or film, which transforms data to results in a predictable, sensible and pre-programmable way. In RD pro- cessors data are represented by perturbances of mediumÕs characteristics, e.g. inhomogeneous concentration profile of reagents, possibly induced by non-uniform illumination. Excitation waves—in reusable processors, or diffusive waves— in disposable processors, travel from the disturbances, interact with each other, and produce either stationary, precip- itate concentration profile (disposable processors), or a dynamic, oscillatory field, or a dissipative structure (reusable processors). The final state of the mediumÕs spatial dynamics represent a result of the RD computation. Recently exper- imental prototypes of RD processors were applied to solve a wide range of computational problems, including image processing [20,2], path planning [24,10,19,4], robot navigation [5,8], computational geometry [6], counting [15], memory units [17]. 0960-0779/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.chaos.2004.11.056 * Corresponding author. Tel.: +44 117 344 2977; fax: +44 177 344 2904. E-mail addresses: ben.delacycostello@uwe.ac.uk (B. De Lacy Costello), andrew.adamatzky@uwe.ac.uk (A. Adamatzky). Chaos, Solitons and Fractals 25 (2005) 535–544 www.elsevier.com/locate/chaos