Quantum transport in two dimensions

Formation of two-dimensional electron gas in heterojunction devices

Given all the advantages of superconducting structures, with their tunability, intrinsic protection against decoherence, well-understood physics and well-developed fabrication and experimental techniques, it would seem superfluous even to consider other possibilities for quantum engineering. Nevertheless, it would be short-sighted to neglect other possibilities, especially such rich ones as provided by devices based on a two-dimensional electron gas (2DEG). Here one has a normal electron system, which, nevertheless, maintains quantum coherence over comparatively large distances, and can literally be shaped into the desirable form (two- one- or zero-dimensional) during an experiment by a simple turn of a knob. It can use both charge and spin degrees of freedom, and serve as a basis for qubits, sensitive quantum detectors, quantum interferometers or other interesting devices. It is edifying, showing that one does not necessarily need a macroscopic quantum state (like superconductivity) to observe macroscopic quantum coherence.

As a bonus, there are interesting and useful effects that can be realized in hybrid, superconductor-2DEG structures. Probably, if we had discussed these devices first, we would have asked, who needs superconductors?