news and views Obituary career, he witnessed the failure of many Rolf Landauer (1927–99) promising technologies, such as early attempts to construct computers using Josephson junctions and tunnel diodes. Head and heart of the IBM Having seen them come and go, Landauer physics of information was sceptical of new computer technologies, noting that most proposals It is well known that scientific progress gravely underestimated the difficulty in requires intellect. Less advertised is constructing a viable device. Over the past science’s need for conscience. Intellect 30 years, he wrote a series of articles creates new ideas; conscience examines pointing out deficiencies in various them for consistency and correctness. In alternative computing proposals, other words, a scientific field requires not including quantum computation. When a only ‘head’ but ‘heart’ as well. Rolf researcher failed to pay adequate attention Landauer was both head and heart to the to his published admonitions, he would field of physics of information. He died of warn them in person. For example, he cancer on 27 April this year at the age of 72. suggested that all papers on quantum Landauer based his research on a computing should carry a footnote: “This simple rule: information is physical. That proposal, like all proposals for quantum is, information is registered by physical could acquire information about the computation, relies on speculative systems such as strands of DNA, neurons velocities of individual molecules of a gas technology, does not in its current form and transistors; in turn, the ways in which could shepherd fast molecules in one take into account all possible sources of systems such as cells, brains and direction and slow molecules in another, noise, unreliability and manufacturing computers can process information is thereby decreasing the gas’s entropy. But error, and probably will not work.” governed by the laws of physics. the second law of thermodynamics says Landauer’s footnote is, of course, Landauer’s work showed that the that entropy does not decrease. Because of correct. In any field of technology, let alone apparently simple and unproblematic its pernicious effect, William Thompson one that attempts to store information on statement of the physical nature of and Lord Kelvin dubbed this being subatomic particles, most ideas will information had profound consequences. ‘Maxwell’s demon’. In order to preserve the probably not work. It is by identifying Born in Stuttgart, Landauer was raised second law of thermodynamics, scientists these wrong ideas that scientists and in New York from the age of eleven. He earlier this century postulated that getting engineers winnow out those few ideas that received both undergraduate and graduate and processing information must are potentially right. Yet, despite the degrees from Harvard, where he worked necessarily involve an increase in entropy. importance of mistakes in science, most under Léon Brillouin and Wendell Furry. Landauer’s principle shows that, whereas published papers report work that is In 1952 he joined IBM, where he was Maxwell’s demon can, in principle, use potentially right, rather than provably employed for the rest of his life. In the late information about the gas to do work wrong. In such an environment, it is 1950s, after constructing an influential without increasing entropy, when it erases important for a field to have a mechanism theory of electrical conductance based on that information it must pay a price of kln2 for remembering past mistakes and for the scattering of electrons, Landauer in entropy per bit, exactly cancelling the preventing them from recurring. In the turned his attention to the physics of advantage it obtained in the first place. field of physics of information, Landauer computation. In the 1970s, working from Landauer’s supplied such a mechanism by functioning In 1961, Landauer discovered that result, Bennett (and independently Ed as the field’s conscience. logical operations that get rid of Fredkin and Tom Toffoli of MIT) showed Landauer’s bracing criticism had a information, such as erasure, necessarily that all computation could, in principle, be highly positive and healthy effect on require the dissipation of energy. Erasure carried out in a logically reversible fashion: quantum computing. His critiques were transforms information from an accessible as a result, computation does not require always accompanied by advice, support to an inaccessible form, known as entropy, dissipation. The fact that modern and suggestions for making the suspect whereas logical operations that can be computers dissipate large amounts of heat proposal work. Researchers explicitly reversed do not lead to a rise in entropy. arises from practical engineering concerns addressed his concerns and developed Landauer also identified the minimum rather than from physical necessity. The methods for coping with noise, amount of entropy increase required for possibility of reversible computation was unreliability and manufacturing error, to each bit erased as kln2, where k is central to Paul Benioff’s discovery at the extent that, by the end of his life, Boltzmann’s constant. Landauer’s Argonne National laboratory in 1980 that although he still believed that large-scale principle transformed the physics of quantum systems could perform quantum computers would not be built, he information by identifying the basic trade- computations in a coherent fashion, a admitted that they were considerably more off between information and physical result that led to quantum computation. plausible than they had been a few years quantities. Although he may be considered the before. It is not yet clear what the future In the 1980s, Landauer’s colleague at godfather of quantum computation, physical basis for computation will be, but, IBM, Charles Bennett, applied his Landauer was fundamentally critical of the whatever its successes, they will be in no principle to provide a self-consistent field. During the 1960s, he was a senior small part due to Rolf Landauer. solution to the century-old problem of manager in IBM’s research division, and Seth Lloyd Maxwell’s demon. In his research into the played a role in many of the laboratory’s Seth Lloyd is in the Department of Mechanical molecular nature of heat in the mid- successes, including the development of Engineering 3-160, Massachusetts Institute of nineteenth century, James Clerk Maxwell the injection laser and of large-scale Technology, Cambridge, Massachusetts 02139, USA. had noted that a hypothetical being that integrated memory circuits. 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