Laser cooling of molecules

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Why laser cooling (usually) fails for moleculesLaser cooling relies on repeated absorption – spontaneous-emission events

How many cycles are required?Example – Rb-87 atom with initial speed of 100m/s.

M vh 17000

For some atoms (e.g. alkalis), this is possible due to a “closed” energy level structure.This situation is special.

LaserLaser

Ground state

Excited state

AbsorptionSpontaneous emission

Imperial College London1st December 2008

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Cold neutral atomic gases

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Why laser cooling (usually) fails for moleculesFollowing excitation, the molecule can decay to a multitude of other vibrational states.

Note – it’s the vibrations that cause all the trouble.The rotations are governed by selection rules

Need to scatter ~10,000 photons for laser cooling.Most molecules scatter 1, start to vibrate, and decouple from the laser

0 1 2 3 4

0 0.964 0.036 0.000 0.000 0.000

1 0.035 0.895 0.070 0.000 0.000

2 0.001 0.065 0.830 0.103 0.001

3 0.000 0.004 0.092 0.767 0.136

4 0.000 0.000 0.008 0.117 0.704

Some molecules are better…

Excited state

Ground state

Example: Franck-Condon factors for CaF

Many other molecules with almost “diagonal” Franck-Condon matrices, e.g. SrF, AlF, YbF, BeH, MgH, CaH, SrH, BaH, AlH, NH, BH, AlCl, YO

Mean number of photons scatteredExcited state

r1-r

Every molecule scatters the first photon.A fraction r scatter a second photon.A fraction r2 scatter a third photon etc.

Mean number of scattered photons, Ng = 1 + r + r2 + r3 +…. = 1/(1-r)

• When r = 0.99, Ng = 100• When r = 0.999, Ng = 1000• When r = 0.9999, Ng = 10000

No excitation out of this state

rotationalangular momentum parity

01

2

3

+-+

-

01

2

3

+-+

-

How to apply laser cooling to molecules

J=1

J=0

M=-1 M=0 M=+1

Dark states

There are sub-levels that cannot couple to the laser polarization

Solve this by:• Rapid modulation of the laser polarization, or• Apply a magnetic field to rotate the dark states into bright states

Laser cooling scheme for CaF

“The orange transition”

“The red transition”

Electronic ground state

Electronic excited state

For CaF, the A-X(0-0) Franck-Condon factor is ~0.97 Upper state decay rate is G = 2p × 8.3 MHz

v = 2

Small leak (≈0.05%)

Demonstration of laser cooling CaF

Pulsed CaF beam600m/s, 5K

Laser beam – 8 frequencies

Probe laser(detects v=0,v=1 & v=2)

Source

DetectorB

0.1 ms

0.5 ms

1.0 ms

1.4 ms

1.8 ms

PRA 89, 053416 (2014)

Transverse laser cooling of SrF

SrF beam

Cooling lasers (12 frequencies)

Doppler cooling Sisyphus cooling

Nature 467, 820 (2010)

2D MOT of YO molecules

i – No coolingii – 1D MOTiii – 2D MOT

PRL 110, 143001 (2013)

3D MOT of SrF molecules

Nature 512, 285 (2014)

~ 300 SrF molecules in the MOT.Temperature ~ 2mK.Lifetime ~ 60ms.

Future directions

• Extending techniques to many more species

• Zeeman slowing of molecules

• Much larger 3D MOTs

• Laser-cooled molecular fountain for precision measurements

• Ultracold molecules in optical lattices – a quantum simulator

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