QUANTUM HEAT. The movement of electrons down a wire becomes a quantum affair when the electron wavelength (the size of the quantum wave counterpart of the particulate electron) is comparable in size to the width of the wire. Theorists have thought the same would be true of "particles" of heat (phonons) moving down a wire. In the case of electrons, quantum reality manifests itself in the form of quantization: the electrons can only have conduction values in multiples of a basic unit equal to 2 times the electric charge squared, divided by Planck's constant. In the case of heat, the unit of thermal conduction would equal the temperature times pi squared times the square of Boltzmann's constant, divided by three times Planck's constant. Such quantized thermal conduction has now been seen for the first time by physicists at Caltech (Michael Roukes, roukes@caltech.edu), where heat added to a tiny (4x4 micron) silicon nitride "phonon cavity" can depart only across narrow bridges, essentially wires only 500 atoms wide (Schwab et al., Nature, 27 April /pnu/2000/). Heat is added, and the temperature of the cavity monitored, by tiny gold circuits leading to SQUIDs (superconducting quantum interference devices). With further refinements, the researchers hope to explore the particle nature of heat, in effect a sort of "quantum phonon optics." In the same issue, commentators Leo Kouwenhoven and Liesbeth Venema refer to the Caltech observations as "the first demonstration of quantum physics in nanomechanical structures."