Why ultracold molecules?

Transcription

1 Cold & ultracold molecules new frontiers J. Ye, JILA Michigan Quantum Summer School, Ann Arbor, June 18, 2008 Quantum dipolar gas Precision test QED ee- eehco OH H2O H2CO Quantum measurement Chemical reactions

2 Why ultracold molecules? J. Doyle et al., Eur. Phys. J. D 31, 149 (2004). Electric dipole moments: Orientation is a big deal! E A. Avdeenkov and J. L. Bohn, Phys. Rev. Lett. 90, (2003). + - R - Manifested at or below µk temperatures

3 Ultracold molecules a hard challenge A diatomic molecule is a molecule with one atom too many! Nobel Laureate Arthur Schawlow, co-inventor of laser and co-founder of laser spectroscopy Atoms Basketball 2> 1> Molecules American football

4 Ways to make cold polar molecules Pairing ultracold atoms (Magneto-Photo-association) Direct cooling of ground-state molecules Buffer gas cooling Stark or magnetic slowing

5 Ultracold molecules: quantum physics Quantum information (strong dipolar interactions, long coherence time) Quantum degeneracy (e.g. BEC) (anisotropic interactions) Dipolar phase transition (Condensed matter system) DeMille, Phys. Rev. Lett. 88, (2002). H.P. Buchler et al., PRL 98, (2007). T. Koch et al., Nature Phys. 4, 218 (2008). Micheli, Brennen, Zoller, Nature Physics 2, 341 (2006).

6 Dipolar quantum gas Long range Orientation-specific interactions Cigar BEC Pancake BEC

7 Ultracold molecules: Test fundamental principles Excited electronic state Ultrahigh resolution spectroscopy Standards One system, in wide spectral ranges Molecular interferometry Precision measurement two different fundamental forces! Ground electronic state Electronic ~ α QED e- e- e- e- Vibration ~ m e /m p (mass on a spring) Strong interactions

8 Ultracold molecules: Precision Chemistry OH + HBr H 2 O + Br Reaction rate vs. temp. Controlled molecular collisions Ultracold chemical reactions Molecules in single quantum states, under precise control, for internal & external motions OH Stereo-Chemistry Electric field HCO Unprecedented study of fundamentally important reactions (Dial the rates): OH + HBr, OH + H 2 CO, CN + O 2, OH + NO, OH + OH, CN + NH 3, OH + H H 2 CO H 2 O Higher reaction rate at lower temperature (10 K, importance for interstellar chemistry)

9 Quantum gas of polar molecules

10 Towards quantum gas of polar molecules Feshbach molecule creation + Coherent two-photon state transfer Single initial quantum state Weakly bound, non-polar Single final quantum state Deeply bound, polar Dense ultracold deeply bound molecules (T ~ 100nK, n ~ /cm 3 )

11 Magnetic-field Feshbach resonance Field-tunable scattering resonance Bound state in channel 2 B Colliding atoms (continuum states) in channel 1 Channels coupled by hyperfine interaction

12 KRb Feshbach molecules Near-degenerate mixture of 40 K & 87 Rb (T ~100 nk) RF association of molecules Binding energy (MHz) Universal binding energy Experiment K 9/2,-7/2> Coupled channel -5 RF photon ~80MHz K 9/2,-9/2> h E b = 2µ a Magnetic Field (G) 2 2 Number (10 3 ) K 9/2,-7/2>+Rb 1,1> K 9/2,-9/2>+Rb 1,1> -E B /h E/h ν (khz)

13 Ultracold trapped KRb position (µm) f radial = 174(3) Hz Oscillations inside trap time (ms) 0 ms 1 ms 3 ms rms cloud size (µm) Confined in an optical trap x 10 4 molecules Density ~10 12 /cm 3 Expansion from the optical trap T=200(80)nK 6 ms expansion time (ms)

14 Fermionic molecule collisional properties Zirbel et al., Phys. Rev. Lett. 100, (2008). Collisional processes: Fermionic 40 K Molecule-molecule collisions Atom-molecule collisions Bosonic 87 Rb Suppressed at ultralow temperatures (fermionic character) Bosonic enhancement Ferminoic suppression Distinguishable particles

15 KRb photoassociation efficiency single quantum state vs. continuum atom / molecule number (10 4 ) mol. number x2 time x100 atom photoassociation τ Α =2.5(1.1)ms molecule photoassociation τ Μ =3.9(7)µs time(ms)

16 Two photon spectroscopy a 3 Σ + (v= 3) Feshbach molecules E B /h=-270khz EIT/Townes doublet

17 Coherent Transfer - STIRAP Ospelkaus et al., Nature Physics, in press (2008) GHz 84% transfer efficiency

18 KRb potentials

19 Frequency comb-assisted transfer Sr2 ν 1 Julienne Kotochigova ν 2 ν 2 ν 1 ν Reaching Reached A 1 Σ a 3 (v=0) Σ + (v=0) Stwalley, Eur. Phys. J. D 31, (2004)

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22 Where are we? Efficient coherent transfer, > 7 THz in a single step v = 0 (J = 0) in 3 Σ, N = 2 x 10 4, n = /cm 3, No heating, 300 nk, T/T F = 3 Experimentally observed dipole moment ~0.1 Debye Expect to reach v = 0 in 1 Σ, (120 THz), ~ 1 Debye Introduce anisotropic & long-range interactions

23 Frequency comb spectroscopy Thorpe et al., Science 311, 1595 (2006); Opt. Exp. 16, 2387 (2008). Absorption (x10-6 cm -1 ) H 2 O CO 2 CH 4 H 2 O Wavelength (µm) 1.70

24 Tomography of all degrees of freedom Thorpe and Ye, Appl. Phys. B 91, 397 (2008). Ro-vibration distribution & Translational temp. Pulsed valve Molecules + Kr

25 Cold ground-state molecules (from precision measurement to cold molecular collisions)

26 Test of fundamental constants α: fine structure constant Modern epoch Blatt et al., Phys. Rev. Lett. 100, (2008). Paris Clock measurements are consistent with zero α/α = -3.3 (3.0) x /yr Early universe Not so clear Webb et al., PRL 87, (2001). Astron. Astrophys. 415, L7 (2004). Conflicting results

27 Cold OH molecules to constrain α / α F = 2 F = 1 Hyperfine interactions ~ α 4 OH megamasers 2 Π 3/2 Lambda doubling ~ α 0.4 F= 2 F= 1 High redshift z > 1 Darling, Phys. Rev. Lett 91, (2003). Chengalur et al., Phys. Rev. Lett. 91, (2003). Kanekar et al., Phys. Rev. Lett. 93, (2004). Multiple transitions from the same gas cloud (Self check on systematics)

28 Molecular electronic state labeling Heteronuclear diatomic molecules possess only axial symmetry - different good quantum numbers than for atoms J L Ω Λ S Σ N Ω = Λ + Σ J = Ω + N Electronic potentials are labeled as 2Σ+1 Λ Ω - Σ, Π,,... states for Λ = 0, 1, 2,... (i.e., 2 Π 3/2 state has Λ=1, Σ=1/2, Ω=3/2) Good quantum # s are Λ, Σ, Ω, J, m J (or just Ω, J, m J )

29 Basic energy structure of OH F = 2 2 Σ 1/2Pump: 282 nm F = v = 11 Basics: 2 Π 3/2 Frank-Condon ~ 72% -Ground State 2 Π 3/2 2 Π 1/2 F= 2 F= 1 v = 0 Detect: 313 nm v = 1 2 Π 3/2 v = 0 -λ doublet spacing ~1 GHz - Electric dipole (1.67 D) - magnetic moment (µ B )

30 Stark deceleration Direct manipulation of ground state molecules v v + Electrode Cooling by supersonic expansion (~ 1 K in a moving frame) F net p + - p + - Phase space selection (~ 10 mk) Applicable to a large variety of molecules Energy - Position Electrode Bethlem, Berden, Meijer, Phys. Rev. Lett (1999).

31 Stark Decelerator Slower electrodes Stark energy Position

32 Experiment & Theory φ o = 0 o OH LIF Signal [arb.u.] data simulations Time from discharge [ms]

33 Slowed molecular packet φ o = 0 o, 380 m/s φ o = 10 o, 312 m/s 2500 OH LIF signal Time from Discharge (ms)

34 Slowed molecular packet φ o = 0 o, 380 m/s φ o = 10 o, 312 m/s φ o = 20 o, 224 m/s OH LIF signal Time from Discharge [ms]

35 Cold ground-state molecules Bochinski, Hudson, Lewandowski, Meijer, Ye, Phys. Rev. Lett. 91, (2003). Hudson et al., Phys. Rev. A 73, (2006) m/s v OH OH density (arb.) m/s 300 m/s 259 m/s 211 m/s φ 0 = m/s 33 m/s Density: /cm Time (ms) φ H 2 CO 550 m/s to rest 1 K to 10 mk molecules

36 Cold molecule based precision spectroscopy Hudson, Lewandowski, Sawyer, Ye, PRL 96, (2006). Lev, Meyer, Hudson, Sawyer, Bohn, Ye, PRA 74, (2006). Decelerator High resolution and precision Systematic evaluations Hexapole PMT Detection can Microwave Interrogation cavity Excitation laser

37 Magnetic trapping of OH Sawyer, Lev, Hudson, Stuhl, Lara, Bohn, & Ye, Phys. Rev. Lett. 98, (2007). decelerator Electric quadrupole Imaging Magnetic trap coil Electrodes can be used as quadrupole lens

38 End view Trapping Scheme 20 m/s 0 A Side view 2000 A Magnetic Quadrupole Electric Quadrupole z

39 Trapping Scheme ~30 mk, 5x10 3 cm -3 Magnetic Quadrupole + Electric Quadrupole A 1500 A z

40 Permanent-Magnet Trap 10mm NdFeB (N42SH) T op = 120 o C B res = 1.24 T

41 Trap Loading 0 V +12 kv -12 kv

42 Trap Loading 0 V 0 V 0 V

43 Permanent magnetic trap of OH ~ 2 x 10 6 cm mk Loading Lifetime

44 Collision inside a trap OH molecules E cm ~ 60 cm cm -1

45 Trap Scattering OH He or OH D 2 colliding beam arrival

46 Absolute collision cross sections OH 2 Π 3/2 D 2 : (1) J = 1 quadrupole moment (2) J = 1 J =3 (300 cm -1 ) J = 5/2 84 cm -1 J = 3/2 E cm (cm -1 )

47 External electric field tunes reaction barrier Hudson, Ticknor, Sawyer, Taatjes, Lewandowski, Bochinski, Bohn, Ye, Phys. Rev. A 73, (2006). IP only 8.1 ev External electric field adds: E 1.8µ Control of cold chemical reactions; Unique dipolar interaction dynamics OH r E Maximum experimentally attainable shift around 10 K

48 Special thanks OH and H 2 CO KRb B. Sawyer B. Stuhl M. Yeo D. Wang E. Hudson (Yale) B. Lev (Illinois) H. Lewandowski (JILA) J. Bochinski (NC State) S. Ospelkaus A. Pe er K.-K. Ni J. Zirbel B. Neyenhuis M. Miranda D. S. Jin D. Jin, J. Bohn (JILA) P. Julienne (NIST), S. Kotochigova (Temple) H. Ritsch (Innsbruck)