The Second Law and Quantum Physics

10/04/2007 10:20 AM Broad InstituteCharles Bennett, IBM Fellow, IBMDescription: In this often droll lecture on a very abstract subject, Charles Bennettexplores entropy, "one of my long loves," and how it relates to quantum information. He first reminds his audience that such information is reducible to qubits, a two"state system that can exist in a superposition of states -- such as the polarized photon. Bennett believes that "quantum mechanics helps resolves the paradox or puzzle of the origin of the second law" of thermodynamics-the irreversible increase of entropy. Classical science might invoke chaos dynamics or environmental effects to explain entropy. The quantum way of viewing it involves entanglement.

In classical mechanics, when two subsystems in a definite state interact "by some deterministic reversible interaction," there will be a definite output for each subsystem. "The entropy of the whole thing will be 0+0 =0." But while the entropy output of two quantum systems interacting might be 0, the individual subsystems manage to have "as much entropy as they could possibly have." This is due to entanglement, "a state of the whole system that cannot be described by attributing states to its parts. Two entangled photons can be said to be in a definite state of sameness even though neither has a polarization of its own." Bennett acknowledges "this is an idea that's hard to explain to many people," although he believes that back in 1967, during the Summer of Love, many people "could understand this from an intuitive sense, if not mathematically."

Bennett plays with the famous evanescence of quantum information, noting that the photons illuminating him fill up the room with "optical replicas of the shape of my nose." But where do they go? He says, "If no record is made of which path a photon follows through an interferometer, or if a record is made but then unmade, the photons will have followed a superposition of both paths. Putting it in slightly theological terms, after the experiment is over, even God doesn't remember which path it followed."

Most classical information, such as "a pattern of snowflakes or grains of rice in last night's dinner," is impermanent, though occasionally frozen by a fossil"like process, Bennett says. It's like a medallion he saw in a flea market: "In 1832, on this spot, nothing happened." But even if information in our physical world is doomed to vanish, in spite of our digital"age efforts to duplicate everything, "the particular physics of our universe" viewed from the perspective of quantum dynamics, seems to "evolve in a complexity"increasing manner, under appropriate conditions," concludes Bennett.
About the Speaker(s): Charles H. Bennett received his Ph.D. from Harvard University in 1970 for molecular dynamics studies (computer simulation of molecular motion). He worked at the Argonne Laboratory for two years following graduation.
He has been at IBM since 1972, where he has explored various aspects of the relation between physics and information. He developed a practical system of quantum cryptography, in collaboration with Gilles Brassard and John Smolin. Other research interests include algorithmic information theory, and the physics of computation. In 1993, Bennett and Brassard with other collaborators discovered "quantum teleportation."

Bennett is a Fellow of the American Physical Society, and a member of the National Academy of Sciences.Host(s): School of Engineering, Department of Mechanical Engineering


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