Laser cooling of molecules

2 Why laser cooling (usually) fails for molecules Laser cooling relies on repeated absorption – spontaneous-emission events How many cycles are required? Example – Rb-87 atom with initial speed of 100m/s. For some atoms (e.g. alkalis), this is possible due to a “closed” energy level structure. This situation is special. Laser Ground state Excited state Absorption Spontaneous emission

Imperial College London 1 st December Cold neutral atomic gases

4 Why laser cooling (usually) fails for molecules Following 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

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 scattered Excited state r 1-r Every molecule scatters the first photon. A fraction r scatter a second photon. A fraction r 2 scatter a third photon etc. Mean number of scattered photons, N  = 1 + r + r 2 + r 3 +…. = 1/(1-r) When r = 0.99, N  = 100 When r = 0.999, N  = 1000 When r = , N  = No excitation out of this state

rotational angular momentum parity How to apply laser cooling to molecules

J=1 J=0 M=-1M=0M=+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  = 2  × 8.3 MHz v = 2 Small leak (≈0.05%)

Demonstration of laser cooling CaF Pulsed CaF beam 600m/s, 5K Laser beam – 8 frequencies Probe laser (detects v=0, v=1 & v=2) Source Detector B 0.1 ms 0.5 ms 1.0 ms 1.4 ms 1.8 ms PRA 89, (2014)

Transverse laser cooling of SrF SrF beam Cooling lasers (12 frequencies) Doppler coolingSisyphus cooling Nature 467, 820 (2010)

2D MOT of YO molecules i – No cooling ii – 1D MOT iii – 2D MOT PRL 110, (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 cryogenic beam source