The Physics of Information Technology is a physics text, not a work of popular science: it assumes the reader has done a physics degree or the larger part of one. The connection with information technology is threefold: Gershenfeld takes an information-theoretic approach at a fundamental level, focuses on areas of physics relevant to information technology, and uses examples from computing systems. The result is dense but richly rewarding, covering an immense range of material and often providing a different perspective to more traditional physics textbooks. (The Physics of Information Technology might be suitable as a text for an advanced electrical engineering course.) Enhancing the work's utility for students, each chapter has a "selected references" section, which lists maybe half a dozen books along with one sentence descriptions, and a set of problems, with full worked solutions.

Gershenfeld starts with chapters on noise and information in physical systems, covering noise mechanisms, the equipartition and fluctuation-dissipation theorems, channels, Shannon's theorems, and Fisher information. A rapid electromagnetism refresher is followed by a chapter on circuits, transmission lines, and waveguides, and another on antennas. A general review of optics is followed by a chapter "Lensless Imaging and Inverse Problems", covering matched filters, coherent imaging, computed tomography, and magnetic resonance imaging. Turning towards solid state physics, a quick overview of quantum statistical mechanics and electronic structure leads to an explanation of the operation of junctions, diodes, and transistors and various kinds of semiconductor logics; a chapter on opto-electronics looks at systems for the generation, detection, and modulation of light; and a chapter covers magnetic materials and recording. Two chapters then link this back to the information theory, covering instrumentation and signal modulation, detection, and coding and, adding complexity, many-body effects (superconductivity), non-equilibrium thermodynamics (thermo- and piezo-electricity), and relativity. And a long final chapter offers a solid introduction to quantum computing and communications, starting with an explanation of the necessary quantum mechanics.

Gershenfeld packs a huge amount into The Physics of Information Technology. Though he does review background theory, he does so rapidly and then cuts straight to the essentials. The section on coding, for example, explains arithmetic and Huffmann compression in just a paragraph each, while two and a half pages on thermoelectricity explain thermocouples and Peltier coolers. The mathematics is perhaps an exception, with the bits Gershenfeld chooses to treat in detail (and it gets quite involved in places) sometimes rather arbitrary — the mathematics can usually be skipped without too much loss. So the discussion of ferro- and ferri-magnetism includes a page and a half of mathematics deriving the Heisenberg Hamiltonian and J coupling, but then drops out of "mathematics mode" pretty much entirely (with one paragraph here quoted as an example of the style):

"In an antiferromagnet such as Mn or Cr the exchange energy is negative, therefore neighbouring spins alternate orientation and there is no net movement even though there is long-range magnetic order. A ferrimagnet is a ceramic oxide that has a spontaneous moment but is a good insulator. The moment arises because it has an antiferromagnetic coupling, but there are interpenetrating spin-up and spin-down lattices that have different moments but do not cancel. Most common ferrimagnets are made from materials containing iron oxides, called ferrites. Because they do not conduct, they do not screen electric fields or have eddy current heading, and so they are useful for a range of microwave applications as well as guiding flux in coils. One example is the microwave equivalent of optical Faraday rotation, which is used in a "magic T" to steer microwave signals in different directions depending on whether they arrive at the input or the output port. This apparent violation of reversability is possible because magnetic interactions break time reversal invariance, since the sign of time appears in the velocity in the basic vxB law. Cables are often wrapped around ferrites, such as the beads on computer monitor cables, to add inductance to filter out unwanted high-frequency components."

This also illustrates the use of examples from computer hardware.

September 2001

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