Laser CoolingLaser Cooling

1. Doppler Cooling – optical molasses.

2. Magneto-optical trap.

3. Doppler temperature.

Doppler Cooling: How can a laser cool?Doppler Cooling: How can a laser cool?Lab frame

vground

excited

Lab frame

vground

excited

Atom’s frame

' '

Doppler Cooling: How can a laser cool?Doppler Cooling: How can a laser cool?

Lab frame

vground

excited

Atom’s frame

' '

Lab frame, after absorption

v-vrecoil

Doppler Cooling: How can a laser cool?Doppler Cooling: How can a laser cool?

Lab frame

vground

excited

Atom’s frame

' '

Lab frame, after absorption

v-vrecoil

Doppler Cooling: How can a laser cool?Doppler Cooling: How can a laser cool?

Lab frame

vground

excited

Atom’s frame

' '

Absorb a photon atom gets momentum kick.

Repeat process at 107 kicks/s large deceleration.

Emitted photons are radiated symmetrically do not affect motion on average

Absorb a photon atom gets momentum kick.

Repeat process at 107 kicks/s large deceleration.

Emitted photons are radiated symmetrically do not affect motion on average

k

Lab frame, after absorption

v-vrecoil

Doppler Cooling: How can a laser cool?Doppler Cooling: How can a laser cool?

Lab frame

vground

excited

Atom’s frame

' '

Absorb a photon atom gets momentum kick.

Repeat process at 107 kicks/s large deceleration.

Emitted photons are radiated symmetrically do not affect motion on average

Absorb a photon atom gets momentum kick.

Repeat process at 107 kicks/s large deceleration.

Emitted photons are radiated symmetrically do not affect motion on average

k

Lab frame, after absorption

v-vrecoil

m/s

m/s2 87Rb: = - I = Isat

Vdoppler ~ 10 cm/s

Vrecoil = 6 mm/s

Doppler Cooling: How can a laser cool?Doppler Cooling: How can a laser cool?

Magneto-Optical Trap (MOT)Magneto-Optical Trap (MOT)

Problem:

Doppler cooling reduces momentum spread of atoms only.

Similar to a damping or friction force (optical molasses).

Does not reduce spatial spread.

Does not confine the atoms.

Magneto-Optical Trap (MOT)Magneto-Optical Trap (MOT)

Solution:

Spatially tune the laser-atom detuning with the Zeeman shift from a spatially varying magnetic field.

zB,

~10 G/cm~14 MHz/cm

Problem:

Doppler cooling reduces momentum spread of atoms only.

Similar to a damping or friction force (optical molasses).

Does not reduce spatial spread.

Does not confine the atoms.

Magneto-Optical TrapMagneto-Optical Trap

E

e

g

2-level atom2-level atom

E

e

g

2-level atom2-level atom

magnetic gradientmagnetic gradient

+

zBB

Magneto-Optical TrapMagneto-Optical Trap

E

e

g

4-level atom

|g “F=0”, |e “F=1”

4-level atom

|g “F=0”, |e “F=1”

magnetic gradientmagnetic gradient

+

z

mF=0

mF=0

mF=-1

mF=+1

BB

Magneto-Optical TrapMagneto-Optical Trap

E

e

g

4-level atom

|g “F=0”, |e “F=1”

4-level atom

|g “F=0”, |e “F=1”

magnetic gradientmagnetic gradient

+

mF=0

mF=0

mF=-1

mF=+1

zBB

Magneto-Optical TrapMagneto-Optical Trap

E

e

g

4-level atom

|g “F=0”, |e “F=1”

4-level atom

|g “F=0”, |e “F=1”

magnetic gradientmagnetic gradient

+

mF=0

mF=0

mF=-1

mF=+1

zBB

Magneto-Optical TrapMagneto-Optical Trap

E

e

g

4-level atom

|g “F=0”, |e “F=1”

4-level atom

|g “F=0”, |e “F=1”

magnetic gradientmagnetic gradient

+

mF=0

mF=0

mF=-1

mF=+1

zBB

-

+

Magneto-Optical TrapMagneto-Optical Trap

E

e

g

4-level atom

|g “F=0”, |e “F=1”

4-level atom

|g “F=0”, |e “F=1”

magnetic gradientmagnetic gradient

+

mF=0

mF=0

mF=-1

mF=+1

zBB

-

+

Magneto-Optical TrapMagneto-Optical Trap

Magneto-Optical Trap (MOT)Magneto-Optical Trap (MOT)

Magneto-Optical Trap (MOT)Magneto-Optical Trap (MOT)

Magneto-Optical Trap (MOT)Magneto-Optical Trap (MOT)

~ 100 K

Magneto-Optical Trap (MOT)Magneto-Optical Trap (MOT)

Magneto-Optical Trap (MOT)Magneto-Optical Trap (MOT)

109 87Rb atoms

Francium MOTFrancium MOT

PROBLEM: Accelerator produces only 106 Fr atoms/s.

Very difficult to work with.

SOLUTION:SOLUTION: Attach a Francium Magneto-Optical Trap to the accelerator.

Cold Francium is concentrated in ~1 mm3 volume.

With T < 100 K, Doppler broadening is negligible.

Long integration times.

Minimally perturbative environment (substrate free).

Francium MOTFrancium MOT

PROBLEM: Accelerator produces only 106 Fr atoms/s.

Very difficult to work with.

SOLUTION:SOLUTION: Attach a Francium Magneto-Optical Trap to the accelerator.

Cold Francium is concentrated in ~1 mm3 volume.

With T < 100 K, Doppler broadening is negligible.

Long integration times.

Minimally perturbative environment (substrate free).

MOT with ~105 210Fr atoms

MOT collection efficiency ~ 1 %