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Unformatted text preview: Chapter 1 Introduction and Overview The course has a website at http://www.theory.caltech.edu/ preskill/ph229 General information can be found there, including a course outline and links to relevant references. Our topic can be approached from a variety of points of view, but these lectures will adopt the perspective of a theoretical physicist (that is, its my perspective and Im a theoretical physicist). Because of the interdisciplinary character of the subject, I realize that the students will have a broad spectrum of backgrounds, and I will try to allow for that in the lectures. Please give me feedback if I am assuming things that you dont know. 1.1 Physics of information Why is a physicist teaching a course about information? In fact, the physics of information and computation has been a recognized discipline for at least several decades. This is natural. Information, after all, is something that is encoded in the state of a physical system; a computation is something that can be carried out on an actual physically realizable device. So the study of information and computation should be linked to the study of the underlying physical processes. Certainly, from an engineering perspective, mastery of principles of physics and materials science is needed to develop state-of-the- art computing hardware. (Carver Mead calls his Caltech research group, dedicated to advancing the art of chip design, the Physics of Computation (Physcmp) group). 1 2 CHAPTER 1. INTRODUCTION AND OVERVIEW From a more abstract theoretical perspective, there have been noteworthy milestones in our understanding of how physics constrains our ability to use and manipulate information. For example: Landauers principle . Rolf Landauer pointed out in 1961 that erasure of information is necessarily a dissipative process. His insight is that erasure always involves the compression of phase space, and so is irreversible. For example, I can store one bit of information by placing a single molecule in a box, either on the left side or the right side of a partition that divides the box. Erasure means that we move the molecule to the left side (say) irre- spective of whether it started out on the left or right. I can suddenly remove the partition, and then slowly compress the one-molecule gas with a piston until the molecule is definitely on the left side. This procedure reduces the entropy of the gas by S = k ln2 and there is an associated flow of heat from the box to the environment. If the process is isothermal at temperature T , then work W = kT ln2 is performed on the box, work that I have to provide. If I am to erase information, someone will have to pay the power bill....
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This note was uploaded on 01/24/2012 for the course PHYS 219 taught by Professor Johnpreskill during the Fall '11 term at Caltech.
- Fall '11