Christopher Jarzynski, University of Maryland
March 25, 2014

Devices that perform feedback control range from the flyball governors of 18th-century steam engines to modern computerized control systems. The ability of one physical system to control another, however, is subject to limits imposed by the laws of physics. These limits become particularly interesting in the context of microscopic machines, operating on scales at which thermal fluctuations dominate. After a brief historical tour Jarzynski will discuss recent developments related to this topic, from two different perspectives. The first involves the manipulation of a microscopic system by an external agent - or "Maxwell's demon" - capable of performing measurements and feedback control at the level of thermal fluctuations. He will describe generalizations of the second law of thermodynamics, which quantitatively relate the agent's information-gathering abilities to its thermodynamic efficiency. The second perspective concerns the capacity of an autonomous physical device to use information processing in order to rectify thermal fluctuations, so as to bring about apparent violations of the second law. Jarzynski will present an explicit mechanical model - a hypothetical contraption of paddles and axles immersed in a gas of particles - that systematically withdraws energy from the gas and delivers it to raise a mass against gravity, and he will discuss the thermodynamic limits that apply to such systems.

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