Abstract
The first lesson began with a discussion of the physical limits of information-processing machines. We know that the problem of the efficiency of thermal machines led, following the pioneering work of Sadi Carnot, to the development of thermodynamics. In Charlie Bennett's provocative phrase, a computer is a machine that " converts free energy into mathematical work and waste heat ". But how much energy is required for an elementary calculation operation ? What is its minimum duration ? Landauer and Bennett have taught us that, in principle, the energy cost of a calculation is only determined by the heat lost when the machine's output register is erased, an erasure that becomes necessary at a given moment to store new data in the register. It is this principle of memory erasure cost that resolves Maxwell's Demon paradox. This brings us to superconducting circuits, capable of reversible information processing, and their quantum version, the subject of the lecture, in which one bit of information is encoded by a single microwave photon. We have recalled the switching laws of the collective circuit variables flux and branch charge, generalizations of the magnetic flux through a loop and the electric charge of an isolated electrode. Flux at the terminals of a Josephson tunnel element is a particularly subtle variable, as it describes a non-linear inductance whose energy is a periodic function of flux. Because of the energy bandgap of the excitations in the superconductivity of the Josephson junction electrodes, this inductance has no dissipative part, at least according to current knowledge, when the junction is in thermodynamic equilibrium at temperatures well below that associated with the bandgap.
