1. Laser cooling of molecules

2. 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

3. Imperial College London 1 st December 2008 3 Cold neutral atomic gases

4. 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

5. 01234 00.9640.0360.000 10.0350.8950.0700.000 20.0010.0650.8300.1030.001 30.0000.0040.0920.7670.136 40.000 0.0080.1170.704 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

6. 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 = 0.9999, N  = 10000 No excitation out of this state

7. rotational angular momentum parity 0 1 2 3 + - + - 0 1 2 3 + - + - How to apply laser cooling to molecules

8. 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

9. 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%)

11. 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, 053416 (2014)

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

13. 2D MOT of YO molecules i – No cooling ii – 1D MOT iii – 2D MOT PRL 110, 143001 (2013)

14. 3D MOT of SrF molecules Nature 512, 285 (2014) ~ 300 SrF molecules in the MOT. Temperature ~ 2mK. Lifetime ~ 60ms.

15. 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