When a group of atoms is super-cooled to temperatures near absolute zero, they form a

Bose–Einstein condensate In condensed matter physics, a Bose–Einstein condensate (BEC) is a state of matter that is typically formed when a gas of bosons at very low densities is cooled to temperatures very close to absolute zero (−273.15 °C or −459.67&n ...

, a

state of matter In physics, a state of matter is one of the distinct forms in which matter can exist. Four states of matter are observable in everyday life: solid, liquid, gas, and Plasma (physics), plasma. Many intermediate states are known to exist, such as ...

where quantum effects can be observed in the

macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifying optical instruments. It is the opposite of microscopic. Overview When applied to physical phenomena ...

system. In this state, the

trajectories A trajectory or flight path is the path that an object with mass in motion follows through space as a function of time. In classical mechanics, a trajectory is defined by Hamiltonian mechanics via canonical coordinates; hence, a complete traj ...

of the atoms can be manipulated using light. If a super-cooled atomic gas is placed in a standing light wave produced by two counter-propagating

lasers A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The firs ...

of certain frequency, the atoms are diffracted in different order. A cold atom absorbs a photon from one of the laser beams and emits a photon in the other beam receiving a net momentum of

2\hbar k

in the direction of the absorbed

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are Massless particle, massless ...

. Here

k

is the magnitude of the wave vector of the

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The firs ...

. This is called two-photon recoil process. In such situation, a BEC cloud sitting at the center of a trap is split into two identical clouds. The clouds then travel in opposite directions with a velocity,

v = 2\hbar k / m

in the direction of the

References

Atoms {{AMO-physics-stub