Sub-Doppler cooling of 40K in three-dimensional gray optical molasses

Transcription

1 Sub-Doppler cooling of 40K in three-dimensional gray optical molasses Diogo Rio Fernandes

2 Outline - Experimental apparatus - Motivation for sub-doppler cooling - Gray molasses in a nutshell - Discussion of experimental results - Final remarks

3 Experimental Apparatus 40 K 2D-MOT 6 Li Zeeman Slower MOT Science Cell Magnetic Transport ~1m Typical flux for 6 Li and 40 K: ~10 9 at/s Best number of atoms achieved in MOT: ~8x10 9 at for each species Ridinger, A. et al. EPJ D 242, (2011)

4 Experimental Apparatus Magnetic Transport of 40 K Injection 1 st arm Injection MOT Science Cell T 0 = 300 µk b 0 = 140 G cm 2 nd arm Temperature Atom number Hx10 8 L Transport distance H cml Transport distance HcmL Heating ~10/2=5% Δt (s) τ (s) Injection To Elbow nd arm Total 4 Efficiency 40% Runaway evaporation not yet observed in the Science cell

5 Improving the initial conditions Sub-Doppler cooling Narrow transition MOT atoms 40 K MOT λ (nm) TD (μk) 6 Li K Molasses Modugno, G. et al. PRA 60, R3373 (1999) 6 Li: Duarte, P. et al. PRA 84, (2011) 40 K: McKay, D. et al. PRA 84, (2011)

6 Gray molasses: a simple picture Cooling transition F F =F-1 involves 2 dark states F =F Bright manifold 1 Dark manifold F Sisyphus-type cooling in gray molasses: 1. (Motional) coupling to the bright manifold in the valley 2. Loss of kinetic energy by climbing the potential hill 3. Optical pumping back to the dark manifold Ol'shani M. and Minogin V. Opt. Commun. 89, 393 (1992) Grynberg, G. and Courtois, J.-Y. EPL 27, 41 (1994) Cohen-Tannoudji, C. - Collège de France

7 Numerical solving of the OBE lin? lin 6 5 Light shift s + s - -9ê2 9ê2 e HMHzL ê2-5ê2-3ê2-1ê2 1ê2 3ê2 5ê zêl Optical pumping rate 7ê2 5ê2 3ê2 1ê2-1ê2-3ê2-5ê2 Positive correlation g Hms -1 L e HMHzL g Hms -1 L Dark manifold I cool = 20I sat I repump = I cool /8 = zêl Courtesy of Saijun Wu

8 Experimental Sequence MOT C-MOT I cool = 13I sat I repump = I cool /20 b 0 =9G cm 1 b 0 =9 5ms! 60 G cm 1 N 0 = T 200 µk I sat =1.75 mw/cm 2 T 1 4 mk rms =1.4mm D1 Molasses I cool = 14I sat I repump = I cool /8 ~B =0 3D gray molasses in Cesium: Boiron, D. et al. (Y.C. & C.S.) PRA 53, R3734 (1996)

9 Fast cooling dynamics Temperature HmKL D 1 molasses time t m HmsL Fluorescence Hrel. unitsl Time HmsL Log scale! Two dynamics Fast cooling: 4mK to ~100μK in <1ms Slow cooling: 100uK to 30uK in 6ms

10 Fast cooling dynamics Atom number Hâ10 8 L D 1 molasses time t m HmsL Temperature HmKL Fluorescence Hrel. unitsl Time HmsL Two dynamics Fast cooling: 4mK to ~100μK in <1ms Slow cooling: 100uK to 30uK in 6ms Capture efficiency in molasses = 100% Plateau of low fluorescence atoms trapped in dark states

11 Temperature determination TOF=3.5ms no molasses 4 T 3mK T 120μK τm = 1.5 ms TOF=17ms svert HmmL T 36μK Time of flight HmsL τm = 6.0 ms 1.5 cm

12 Light intensity dependence Varying light intensity Atom number Hâ10 8 L % capture efficiency D 1 cooling intensity HIêIsatL Temperature HmKL Atom number Hâ10 8 L I 6 ms 2 ms τm Two-step sequence Temperature HmKL Final D 1 cooling intensity HIêIsatL

13 Optimal parameters Atom number Hâ10 8 L Detuning d HGL Temperature limit of this scheme? ambient magnetic field bias compensated - no atomic density dependence observed - what is the limiting factor? - role of the off-resonant excitation? Temperature HmKL Temperature HmKL duration (ms) I cool (I sat ) capture phase 6 14 cooling phase 2 14!1 cool = repump =+2.3 I repump = I cool /8 T final 20 µk Magnetic field bias HGL

14 Loading into a Quadrupole Trap b 0 = 76 G cm 1 N = T = 80 µk D 2 molasses [1] Blue MOT [2] N T(µK) n 0 (cm 3 ) PSD ( 10 5 ) PSD [1] Modugno, G. et al. PRA 60, R3373 (1999) [2] McKay, D. et al., PRA 84, (2011) p-wave s-wave Good starting point for evaporation coll 23 s 1 DeMarco, B. et al. PRL 82, 4208 (1999)

15 Future Steps - Test the improvement on transport efficiency to the science cell - Evaporation in a hybrid trap - Loading into a 1D optical lattice - Study of a system with mixed dimensions Lin, Y.-J. et al. PRA 79, (2009) Nishida, Y. and Tan, S. PRL 101, (2008) Nishida, Y. PRA 82, (R) (2010)

16 D1 molasses for Lithium isotopes (preliminary results) 6 Li 7 Li Vertical lin lin 1D gray molasses no molasses with molasses TOF = 2 ms Tvert = 1 mk Tvert = 60 μk T HmKL n R -n P HMHzL F 0 =2i F =1i Repumper Principal MHz F =2i Thanks to Ulrich E. and Andrea B. for powerful laser source

17 Fermix-Termodynamix team Norman K. Christophe S. Franz S. Saijun W. Lev K. Daniel S. Frédéric C. DRF Thanks for listening arxiv: [cond-mat.quant-gas]