Making cold molecules from cold atoms

1. Making cold molecules from cold atoms

2. (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 • Molecules will be in low-lying rotational states • Produces molecules in highly excited vibrational states

3. Decay to free atoms Decay to bound state (AB)* Ground state A + B Photoassociation – decay channels

4. Photoassociation properties • Occurs at outer turning point of excited vibrational state • Extremely inefficient for tightly bound states – always forms “long-range molecules” • Decay to a deeply bound state of the molecular ground state is highly unlikely • Decay to two free atoms is accompanied by a large release of energy – trap loss Li2, Na2, K2, Rb2, Cs2, Sr2, Ca2 , LiCs....

5. Photoassociation - advantages and limitations • Molecules are formed at ultracold temperatures • Limited to constituents that can be laser-cooled • High experimental complexity, particularly for heteronuclears • Production rate is usually very slow • Difficult to reach the vibrationalground-state (but this can be done)

6. Feshbach resonance

7. Reminder about s-wave scattering • At ultracold temperatures, scattering is described by the s-wave scattering length, a • All other partial waves are suppressed by the angular momentum barrier V=∞ Toy example: V=0 V=V0 Zero-energy scattering state Uppermost bound state u(R)  exp(-R/a) u(R)  (R – a)

8. Feshbach resonance

9. Reminder about level crossings 

10. Atoms into molecules via a Feshbach resonance Bound molecules Free atoms B / B0

11. Molecules (converted back to atoms) Atoms With Stern-Gerach field Without Cold molecules via Feshbach resonance Experimental procedure Ramp B through the resonance – create some molecules Apply a Stern-Gerlach field – separate the atoms and molecules Ramp B back through the resonance – convert molecules back to atoms Image! Li2 – Hulet group – PRL 91, 080406 (2003) & Ketterle group – PRL 91, 250401 (2003) Na2 – Ketterle group – PRL 91, 210402 (2003) K2 – Jin group – Nature 424, 47 (2003) & Nature 426, 537 (2003) Rb2 – Wieman group – Nature 417, 529 (2002) & Rempe group – PRL 92, 020406 (2004) Cs2 – Grimm group – Science 301, 1510 (2003)

12. Reminder about STIRAP 2  23 12 Coupling strength 23 12 3 1 Time

13. Coherent transfer to deeply bound states • Form loosely bound Feshbach molecules • Stimulated Raman adiabatic passage to coherently transfer molecules to the ground state • Ultracold ground state polar molecules with high phase space density KRb Science 322, 231 (2008)

14. Can also do it all in an optical lattice.... Cs2 Nature Physics 6, 265 (2010)

15. Feshbach - advantages and limitations • Direct formation of quantum degenerate molecules • Fully coherent, highly efficient • Excellent system for studying BEC-BCS crossover (fermionic atoms) • Polar molecules can be formed by coherent transfer to ground state • Limited to molecules made from atoms that can be laser cooled • High experimental complexity, particularly for heteronuclears • Often short-lived (~ms), though long-lived states (several seconds) are possible (Fermi stats)