Absolute zero is the lowest temperature that can be obtained in any macroscopic system and corresponds to 0 K. In fact it can be shown from the laws of thermodynamics that the temperature can never be exactly absolute zero, although it is possible to achieve temperatures arbitrarily close to it. At absolute zero the molecules and atoms in a system are all in the ground state (i.e. the lowest possible energy state) and the system has the least possible amount of kinetic energy allowed by the laws of physics. This minimum energy corresponds to the zero point energy encountered in the quantum mechanical Particle in a box problem.
For the case of free atoms at temperatures approaching absolute zero, all of the energy is in the form of translational motion and the temperature can be measured in terms of the speed of this motion with slower speeds corresponding to lower temperatures. A temperature below absolute zero would imply a speed of less than zero, which is not possible. Actually, due to quantum mechanical effects the speed at absolute zero is not precisely zero, but depends, as does the energy, on the size of space within which the atom is confined.
For some special systems and specific definitions of temperature, it is possible to obtain a negative temperature. See Temperature for more information.
At very low temperatures in the vicinity of absolute zero, matter exhibits many unusual properties including superconductivity, superfluidity, and Bose-Einstein condensation. In order to study such phenomena, scientists have worked to obtain ever lower temperatures. As of 2001 the lowest temperature ever acheived was 20 nK (20 billionths of a degree above absolute zero). This record-setting low temperature was achieved in 1995 by scientists working for NIST in Boulder, Colorado. The work is described in the July 14, 1995 edition of the journal Science.