Ultimate physical limits to computation

Computers are physical systems: the laws of physics dictate what they can a nd cannot do. In particular, the speed with which a physical device can pro cess information is limited by its energy and the amount of information tha t it can process is limited by the number of degrees of freedom it possesse s. Here I explore the physical limits of computation as determined by the s peed of light c, the quantum scale (h) over bar and the gravitational const ant G. As an example, I put quantitative bounds to the computational power of an 'ultimate laptop' with a mass of one kilogram confined to a volume of one litre.