Mesoscopic Quantum Phenomena

Quantum physics was first developed to understand the properties of small individual objects such as photons (► light quantum), atoms and molecules. And many features of quantum physics, such as the discreteness of energy levels, the ► superposition of mutually exclusive states, quantum interference or ► entanglement are usually not directly accessible to our human senses. Colloquially we therefore often separate between microscopic and macroscopic in the sense of ‘being observable or unobservable by the unaided eye’ rather than in the more physical sense where microscopic would refer to objects in the micrometer size range. In physics, the notion of mesoscopic quantum phenomena is generally used for systems with dimensions somewhere in the middle (in Greek: meso = middle) between the microscopic and the macroscopic world. In practice, mesoscopic systems mostly range between a few and a few hundred nanometers. They are large enough to contain many particles and can therefore be described by average properties, such as density or conductivity. On the other hand they are small enough for their lateral extensions to match characteristic lengths, such as the coherence length or the mean free path. Mesoscopic quantum systems therefore often exhibit unique physical properties such as size-dependent electronic properties, transport phenomena and more. The following examples select some of the most quoted mesoscopic quantum phenomena [6,9–11].