  When a magnetic field pierces a multiple-connected quantum system, the
corresponding wavefunction is altered although no net Lorentz force acts upon
its carriers. This is the so called Aharonov-Bohm effect. The most simple
multiply-connected quantum system is a quantum ring QR. Nowadays it is possible
to obtain QRs in the nanoscopic range providing spectroscopic data vs. and
applied external magnetic field. We describe here the most significant quantum
effects induced by the magnetic field in a QR by means of simple quantum
mechanical models.
