Ultracold (or ultra-low-temperature) is a state of matter in which atoms and molecules have very little thermal energy. Thermal energy is the kinetic energy of atoms and molecules in a substance. The lower the temperature, the less thermal energy the particles have. At extremely low temperatures, atoms and molecules move very slowly and can even stop moving altogether.
The study of ultracold matter began in the early 1900s when scientists developed ways to cool gases to very low temperatures. In 1995, researchers achieved temperatures below one millionth of a degree above absolute zero (-273 degrees Celsius or 0 Kelvin). This was an important milestone because it opened up the possibility of studying quantum mechanical effects that are only observable at these incredibly low temperatures.
Since then, advances in cooling techniques have allowed scientists to reach even lower temperatures. In 2003, researchers achieved a record temperature of 100 picokelvins—just 10^-10 Kelvin above absolute zero! This work was done using a technique called laser cooling, which traps atoms using laser light and slows them down by absorbing and reemitting photons (particles of light).
Today, ultracold research is an active area of investigation with many different applications. For example, ultracold atoms can be used to study quantum phenomena such as superfluidity (a fluid with zero viscosity), Bose-Einstein condensation (a state of matter where particles occupy the same quantum state), and quantum magnetism. Ultracold molecules can be used to create new materials with novel properties such as superconductivity (zero electrical resistance) or insulating behavior at extremely high densities. And ultracold plasmas (ionized gases) may help us unlock the mysteries of astrophysical phenomena such as planetary formation and stellar evolution.