Physical principles of low temperature detectors: Ultimate performance limits and current detector capabilities

The rapid development of low temperature thermal detectors since the early 1980s has resulted in remarkable improvements in the sensitivity and precision of many types of measurements. We will discuss the operating principles of these detectors in the most general possible terms. We will try to show how the physics of some popular thermometer systems introduces performance limits, and how the different figures of merit and optimizations that result affect various applications.