In laser cooling, the Boltzmann constant times the recoil temperature is equal to the recoil energy deposited in a single atom initially at rest by the spontaneous emission of a single photon. The recoil temperature is
T
r
e
c
o
i
l
=
ℏ
2
k
2
2
m
k
B
,
since the photon's momentum is
p
=
ℏ
k
(here
k
is the wavevector of the light,
m
is the mass of an atom,
k
B
is Boltzmann's constant and
ℏ
is Planck's constant). The recoil temperature for the D2 lines of alkali atoms is typically on the order of 1 μK, and thus lower than the Doppler temperature. An example of a process where the recoil temperature can be reached is Sisyphus cooling.