Lecture 3. Bose-Einstein condensation Ultracold molecules

Transcription

1 Lecture 3 Bose-Einstein condensation Ultracold molecules 66

2 Bose-Einstein condensation Bose 1924, Einstein 1925: macroscopic occupation of the lowest energy level db h 2 mk De Broglie wavelength d 1/3 n B T mean interparticle distance 67 (related to density)

3 Bose-Einstein condensation Absorption images BEC of mm 87 Rb atoms 400 nk 200 nk 50 nk Science 269, 198 (1995) Physics Nobel Prize 2001: Eric Cornell, Wolfgang Ketterle, Carl Wieman Remark: Metastable systems, true ground state of Rb at 1 mk is a piece of solid! Bose-Einstein condensates (BEC) with bosons Degenerate Fermi gas (DFG) with fermion both are called quantum degenerate gases 68

4 Experimental signature of BEC Bimodal distribution Anistropic expansion 1 ms 5 ms 10 ms 20 ms 30 ms 45 ms Mewes et al, PRL

5 BEC: demonstrated species 70

6 Early key experiments BEC Interference between two BEC s Atom laser Andrew et al, Science 1997 Bloch et al, PRL

7 Degenerate Fermi Gases (DFG): demonstrated species 72

8 Experimental signature of DFG Truscott et al, Science 2001 Fermi pressure 73

9 BEC in optical lattice Greiner et al, Nature 2002 Superfluid to Mott insulator transition Bose-Hubbard model 74

10 Quantum gas microscope Bakr et al, Nature 2009 Weitenberg et al, Nature

11 Wim VU 76

12 77

13 From atoms to molecules U(r) many decay channels (vibrational levels with rotational and hyperfine structure) r Impossible? 78

14 Why ultracold molecules? Ultracold chemistry Control with external fields F+H 2 (n=0,j=0)->hf(n )+H High resolution spectroscopy Example: time variation m e /m p Molecular BEC Superchemistry Dipolar quantum gases Quantum processing 1 mk 1 K 79

15 Cooling strategies direct: cooling molecules broad class of molecules not ultracold indirect: cooling atoms, associate to molecules narrow class of molecules ultracold (near degenerate) 80

16 Direct: Stark deceleration polar molecules (e.g. ND 3, CO,...) add laser cooling (e.g. for SrF) 81

17 Direct: Buffer gas cooling Weinstein et al, Nature 1998 paramagnetic molecules (magnetic trap) 82

18 Indirect: photo-association s+p laser s+p s+s laser 2 laser 1 photo-association one-color free atoms s+s controlled photo-association two-color Deiglmayr et al, PRL

19 Indirect: Feshbach association atomic quantum gas few nk molecular quantum gas Feshbach molecules Herbig et al., Science 301, 1510 (2003) 84

20 V(r) Feshbach resonance bound state a coupling incident channel hyperfine interaction r a 3 S u- : triplet Feshbach, Ann. Phys. 5, 357 (1958) in ultracold quantum gases: Tiesinga, Verhaar, Stoof, PRA 47, 4114 (1993) X 1 S g+ : singlet 85

21 Magnetically induced Feshbach resonance (2,1)+(2,1) F=2 E Zeeman effect m F (1,1)+(1,1) F= E B B 86

22 scattering length a/a bg N atoms (x 10 5 ) Feshbach resonance L 3 ~ a 4 E 0 B two atoms molecule B a( B) a bg 1 B B 0 Feshbach resonances in ultracold gases Chin, Grimm, Julienne, Tiesinga, Rev. Mod. Phys. 82, 1225 (2010) B(G) Inouye et al, Nature 392, 151 (1998) 87

23 scattering length Feshbach molecules E 0 B two atoms molecule Feshbach resonance B E Ultracold Feshbach Molecules Ferlaino, Knoop, Grimm arxiv: Chapter of Cold Molecules: Theory, Experiment, Applications two atoms molecule (Taylor & Francis, London, 2009) 88 B

24 Adiabatic magnetic field ramp E two atoms dissociation molecule B Imaging: fast magnetic field backramp Separation: e.g. magnetic field gradient 89

25 Adiabatic magnetic field ramp E two atoms molecule B Imaging: fast magnetic field backramp Separation: e.g. magnetic field gradient (Stern-Gerlach imaging) 90

26 Purification E B resonant laser light / microwave pulse 91

27 First Feshbach molecules from BECs Cs 2 Na 2 Herbig et al, Science 301, 1510 (2003) 87 Rb 2 Xu et al, PRL 91, (2003) Dürr et al, PRL 92, (2004) 92

28 Making ultracold ground-state molecules a 3 S u- : triplet S+P n=0, J=0 X 1 S g+ : singlet Recipe make Feshbach molecules coherent two-photon transfer (STIRAP) polar molecules -> atomic mixture S+S r 93

29 Ultracold ground state polar molecules STIRAP (Stimulated Raman Adiabatic Passage) KRb Ni et al, Science

30 Key experiments with ultracold polar molecules Quantum-State Controlled Chemical Reactions Ospelkaus et al, Science

31 Key experiments with ultracold polar molecules Dipolar collisions of polar molecules in the quantum regime Ni et al, Nature 2010 De Miranda et al, Nature Physics

32 Stability ultracold ground state polar molecules collisional stability KRb+KRb -> K 2 +Rb 2 Zuchowski & Hutson, PRA 2010 atom exchange exo- or endothermic? 97

33 Ultracold ground state polar molecules Non-bialkali molecules in progess (e. g. UvA, Florian Schreck) 98

34 Lecture 1: Overview of lectures Introduction Laser cooling of atoms Lecture 2: Atom traps Evaporative cooling Lecture 3: Bose-Einstein condensation (BEC) Ultracold molecules 99

35 What to take from this lecture? Basic principles of laser cooling Doppler cooling How does a Zeeman slower and MOT work Magnetic and optical dipole traps (MT and ODT) Basic properties (which atoms/states are trapped?) Principle of evaporative cooling and how applied in MT and ODT Difference between bosonic and fermionic atoms Bose Einstein condensation (BEC) Criteria, signature, and typical numbers for atomic gases Difference with degenerate Fermi gases (DFG) Ultracold molecules Different methods Feshbach association & STIRAP to ground state 100