making cold molecules from cold atoms

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Laser cooling of molecules

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Making cold molecules from cold atoms. (AB)*. Ground state A + B. Photoassociation. Photoassociation is resonant in the photon energy Usually achieved by adding an extra laser to a MOT – the PA laser Very little heating involved – the molecules are ultracold - PowerPoint PPT Presentation

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Page 1: Making cold molecules  from cold atoms

Laser cooling of molecules

Page 2: Making cold molecules  from cold atoms

2

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 17000For 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

Page 3: Making cold molecules  from cold atoms

Imperial College London1st December 2008

3

Cold neutral atomic gases

Page 4: Making cold molecules  from cold atoms

4

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

Page 5: Making cold molecules  from cold atoms

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

Page 6: Making cold molecules  from cold atoms

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

Page 7: Making cold molecules  from cold atoms

rotationalangular momentum parity

01

2

3

+-+

-

01

2

3

+-+

-

How to apply laser cooling to molecules

Page 8: Making cold molecules  from cold atoms

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

Page 9: Making cold molecules  from cold atoms

97%

3%

0.08%606 nm

628 nm

Laser cooling scheme for CaF

J=1/2, F=1

J=1/2, F=0J=3/2, F=1J=3/2, F=2

0

76123148MHz

628 nm

X 2S+ (N=1)

v = 0

v = 1

v = 2

A 2P1/2 (J=1/2, p=+1)v = 0

v = 1

72

72

48

EOM+AOM

Page 10: Making cold molecules  from cold atoms
Page 11: Making cold molecules  from cold atoms

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)

Page 12: Making cold molecules  from cold atoms

Transverse laser cooling of SrF

SrF beam

Cooling lasers (12 frequencies)

Doppler cooling Sisyphus cooling

Nature 467, 820 (2010)

Page 13: Making cold molecules  from cold atoms

2D MOT of YO molecules

i – No coolingii – 1D MOTiii – 2D MOT

PRL 110, 143001 (2013)

Page 14: Making cold molecules  from cold atoms

3D MOT of SrF molecules

Nature 512, 285 (2014)

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

Page 15: Making cold molecules  from cold atoms

RF MOTs-s+

s- s+

0

+1

-1

s+ s-

0

+1

-1

0

-1

+1

s- s+

0

-1

+1

• Solves dark state problem• Net restoring force is large

arXiv:1511.00930 (2015)

Page 16: Making cold molecules  from cold atoms

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

cryogenicbeamsource