Bose–Einstein condensates in dilute atomic gases, which were first realized experimentally in 1995 for rubidium, sodium, and lithium, provide unique opportunities for exploring quantum phenomena on a macroscopic scale. These systems differ from ordinary gases, liquids, and solids in a number of respects, as we shall now illustrate by giving typical values of some physical quantities.

The particle density at the centre of a Bose-Einstein condensed atomic cloud is typically 1013–1015 cm−3. By contrast, the density of molecules in air at room temperature and atmospheric pressure is about 1019 cm−3. In liquids and solids the density of atoms is of order 1022 cm−3, while the density of nucleons in atomic nuclei is about 1038 cm−3.

To observe quantum phenomena in such low-density systems, the temperature must be of order 10−5 K or less. This may be contrasted with the temperatures at which quantum phenomena occur in solids and liquids. In solids, quantum effects become strong for electrons in metals below the Fermi temperature, which is typically 104–105 K, and for phonons below the De bye temperature, which is typically of order 102 K. For the helium liquids, the temperatures required for observing quantum phenomena are of order 1 K. Due to the much higher particle density in atomic nuclei, the corresponding degeneracy temperature is about 1011 K.

The path that led in 1995 to the first realization of Bose–Einstein condensation in dilute gases exploited the powerful methods developed over the past quarter of a century for cooling alkali metal atoms by using lasers.