making cold molecules from cold atoms
DESCRIPTION
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 PresentationTRANSCRIPT
Laser cooling of molecules
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
Imperial College London1st December 2008
3
Cold neutral atomic gases
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
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
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
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.
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)
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