Manipulating Rydberg atoms and molecules in the gas phase and near chip surfaces

Transcription

1 RQI Winter School Obergurgl, 12 February 2013 Manipulating Rydberg atoms and molecules in the gas phase and near chip surfaces Ch. Seiler, P. Allmendinger, S. D. Hogan, H. Saßmannshausen, J. Deiglmayr and F. Merkt Physical chemistry, ETH Zürich with T. Thiele, M. Stammeier, S. Filip and A. Wallraff Quantum Device Lab, ETH Zürich I. Rydberg state and cold molecule primer II. Manipulating Rydberg atoms/molecules in the gas phase III. Manipulating Rydberg atoms near surfaces

2 Atoms and Molecule as quantum systems in the gas phase Alkali-metal atoms (alkaline earth ions) have a simple electronic structure have easily accessible excited states can be laser cooled can be further cooled to quantum degeneracy have well-known properties exquisite control with electric and magnetic fields possible Molecules have a complex electronic structure rarely have easily accessible excited states cannot in general be laser cooled --- many properties only poorly known control with electric and magnetic fields possible in principle

3 But molecules have: - Rotational and vibrational degrees of freedom - Permanent electric dipole moments - Complex potential energy surfaces giving rise to coupled electronic and nuclear degrees of freedom tunneling geometric phase effects large-amplitude motion of nuclei - Almost infinite number of atom combinations Challenge: Develop general methods to produce (ultra)cold molecular gases

4 Cold atoms and molecules I. Introduction Cold molecules in chemistry and physics: High-resolution spectroscopy (Molecular structure and dynamics) Reaction dynamics at very low temperature Metrology Molecular quantum gases Methods of production: By insertion in a helium nanodroplet From ultracold (alkali metal) atoms by: - photoassociation - association through Feshbach resonances By cooling molecules: - buffer gas cooling (general) - Deceleration of supersonic beams Stark deceleration (polar molecules) Stark deceleration by optical fields (general) Zeeman deceleration (radicals) Rydberg-Stark deceleration (general) By selecting slow (cold) molecules from a thermal ensemble (polar molecules) By billiard collisions...

5 Cold molecules by deceleration of supersonic beams H? 2 CH4 N2 C H 6 6 S.D. Hogan, M. Motsch and FM, Phys. Chem. Chem. Phys.13, (2011)

6 Rydberg states as quantum systems Balmer, 1885 Rydberg 1890 Balmer formula = m = 3,4,5,... m 2 A m 2-22 Rydberg formula ~ R = IP - 2 (n - ) Energy n=52 n=51 n=50 Z. Phys. Chem. 5, 227 (1890)

7 Rydberg states of atoms and molecules + e Ionisationcontinuum n = + v =0 + N =0 + v =0 + N =1 + v =0 + N =2 Energy n=53 =0-52 n=52 =0-51 n=51 =0-50 n=53 n=52 n=51 n=50 =0-49 H-Atom =0 =1 =3,...,n-1 =2 Other Atoms n=50 Molecules

8 Properties of Rydberg states -4 n Property n-dependence n=25 n=100 Classical radius 2 a 0 n 310 Å 5000 Å Binding energy -2 -R n 21 mev cm 1.3 mev cm Ionisation field (V/cm) 1450 V/cm 4 V/cm Radiative lifetime > 10 s > 100 s Max. Induced dipole moment ea n Debye 25 kdebye Further properties: Transition moments n 4 Polarisability: n 7 Van der Waals interaction: n 11

9 Factors governing the n-dependence of the properties of Rydberg states: 1) The amplitude of the Rydberg elecron wave function near the ion core (R<R ) ~ n nlm core -3/2 2) The size of the Rydberg orbit (geometric interpretation of cross-sections and transition moments) <R> ~ n 2 <nl n l > ~ n 2 ~ (for n = n) 3) The energetic distance to the nearest zero-order state (perturbation theory) E(0) ~ n -3

10 Transition moments and polarizabilities Attenuation of the millimeter waves Control of stray fields Argon: n=71 p[5/2](j=2) n=64 s[3/2](j=1) mw/cm 30 MHz Krypton F(mV/cm) n=91 f[5/2](j=2) n=77 d[3/2](j=1) 2 E=1/2 F 0.03 mw/cm 5 MHz 2 F stray =-646(20) V/cm < 1 W/cm 1 MHz 2

11 Preparation, manipulation and detection of Rydberg states Experimental procedure: 1) Excitation from the neutral ground state: Broadly tunable, narrow bandwidth UV/VUV laser radiation 2) Transitions between neighbouring Rydberg states: Millimetre waves and microwaves 3) Deceleration/deflection and trapping Inhomogeneous electric fields 4) Detection of Rydberg states Electric field ionisation

12 The VUV laser spectrum of argon in a supersonic beam n 300 Ionensignal PFI signal (arb. Units) Argon Ionensignal (willk. Einheiten) n= Wave number/cm Wellenzahl / cm

13 The millimeter wave spectrum of Cs in a MOT 20 khz (from 93P ) 3/2 230s 231s 232s H. Saßmannshausen, J. Deiglmayr

14 II. Manipulating Rydberg atoms/molecules in the gas phase Oxford (Softley group): H 2: Procter et al., Chem. Phys. Lett. 374, 667 (2003) Yamakita et al., J. Chem. Phys. 121, 1419 (2004) Zurich: Ar, H, H : 2 Vliegen et al., Phys. Rev. Lett. 92, (2004) Vliegen et al., Phys. Rev. Lett. 97, (2006) Hogan et al., Phys. Rev. Lett. 103, (2009) `

15 The Stark effect in the hydrogen atom n=8 Energy a 0 + n= Electric field / (10 a.u.)

16 The Stark effect in other atoms and deceleration H atom Classical ionisation limit Other atoms Energy Energy -5 F/(10 a.u.) -5 F/(10 a.u.) 2 Induced dipole moment µ elec ~ n (>2000 Debye at n=30) Hydrogen 2 n=30, grad( E Stark = -µ elec F, F) = 50 kv/cm :. 8 2 a = 2 10 m/s f = grad( ) Stark µ elec F

17 Rydberg-Stark deceleration The simplest experiment Energy n+1 n -5 Electric field / (10 a.u.) Photoexcitation +V -V Gas nozzle Skimmer Deceleration region Microchannel CCD Camera plate with phosphor screen Vliegen et al., Phys. Rev. Lett., 92, (2004) Procter et al., Chem. Phys. Lett., 374, 667 (2003)

18 Deceleration of argon atoms Argon, n=22 Stark shift: cm at 320 V/cm (n=22) V z= 560 m/s, T tr = 1 K E kin = 530 cm (1 10 J) E = 9 cm ( J) kin Intensity (Arb. Units) Y X 1200 Debye Debye Intensity (Arb. Units) Y X Time of flight (:s) Vliegen et al., Phys. Rev. Lett., 92, (2004)

19 Deceleration experiments with time-dependent fields Deceleration configuration Trapping configuration Field-ionisation configuration V 0 V 0 V V

20 3D-Rydberg atom trap Camera Hogan et al., Phys. Rev. Lett. 100, (2008)

21 H atoms n=30, k=21 Stopping distance: 1.8 mm Stopping time: 5 s Acceleration: m/s2 Temperature = 100 mk Density 107 cm-3 =135 s Hogan et al., Phys. Rev. Lett. 100, (2008)

22 Trap-loss mechanisms Avoiding collisional effects by off-axis trapping

23 -15 V 15 V -15 V I. Photoexcitation phase t=0-50 ns dc field of 35 V/c 15 V -15 V 15 V

24 -600 V 600 V -15 V II. On-axis deceleration phase t=50 ns - 5 s 15 V -15 V 15 V

25 -600 V 30 V -15 V III. Turning phase t = 5-8 s -15 V 30 V 15 V

26 -600 V 15 V -15 V o IV. 90 straight phase t = 8-12 s 600 V -15 V 15 V

27 -15 V 15 V -600 V V. Final deceleration phase t= s 15 V -15 V 600 V

28 -15 V 15 V -15 V VI. Trap t > 15 s 15 V -15 V 15 V

29 On-axis vs. off-axis trap decay H atom 298 K, n=30 Seiler et al. Phys. Rev. Lett. 106, (2011)

30 Measurements of trap decay at low temperatures n=30 Copper housing for experiments at lower temperature

31 Radiative decay rates at n=30, T= 125 K lying electronic states blackbody radiation 4000 s s -1 at 125 K IP IP n-redistribution 3000 s -1 at 125 K n n n + 2 n + 1 n Energy n = 3 n = 2 n = 3 n = 2 n 1 n 2 n = 1 n = 1

32 Rydberg-Stark deceleration of molecules Difficulties: The Rydberg states of molecules have shorter lifetimes because of predissociation Many more ionization channels The crossings between Stark states in a Stark map are avoided Solution: Carry out the experiments with nonpenetrating Rydberg states

33 Hydrogenic Stark states of H below the H + X g (v=0,n=0) level

34 Excitation of nfm=3 Rydberg states of H 2 and of m=3 Stark states

35 Trapping molecular hydrogen at n=22 V 1,3 = 100 V V 2,4 = -100 V V decel= +/- 1.7 kv T = 50 s ion Temperature: 150 mk Density: atoms/cm 3 Trapping time: 35 s V 1,3 = 100 V V 2,4 = -100 V V decel= 0 kv T = 3 s ion V 1,3 = 0 V V 2,4 = 0 V V decel= 0 kv T = 3 s ion Hogan et al., Phys. Rev. Lett. 103, (2009) Seiler et al., PCCP 13, (2011)

36 Conclusions (Part II) H molecules and H atoms can be decelerated from 500 m/s to 0 m/s 2 over 2 mm and in less than 10 s The acceleration is large: a = m/s 8 2 H-atom-like behavior can be achieved in molecules by preparing m=3 Stark states Rydberg molecules in selected Rydberg states can be trapped (T = 150 mk, densities cm -3 ) Trapping times limited by radiative processes and collisions o Deflection of a supersonic beam by 90 is achievable in 15 s

37 III. Manipulating Rydberg atoms near chip surfaces Rydberg-Stark deceleration on a chip Similar method for polar molecules: S. A. Meek, H. L. Bethlem, H. Conrad, and G. Meijer, Phys. Rev. Lett. 100, (2008) Photolysis laser z Pulsed valve Electrode stack PCB y Source chamber MCP detector CCD camera x z Rydberg atom excitation region (uv and vuv lasers) Deceleration region (PCB) Detection region

38 Moving surface-electrode trap Rydberg atom beam 40 Electrode 2 Electrode 6 Electrical potential (V) Electrode 4 Electrode 1-40 Electrode 5 Electrode Time( s) Hogan et al. PRL 108, (2012)

39 Normalized integrated H + ion signal (arb. unit) Experiment n = H atom time-of-flight ( s) Normalized integrated H + ion signal (arb. unit) Undecelerated 0.5 v = 765 m/s Simulation v = 200 m/s f v = 300 m/s f v = 450 m/s f v = 600 m/s f v = 1000 m/s f v = 1200 m/s f H atom time-of-flight ( s)

40 n-acceptance of the decelerator

41 Trapping Rydberg atoms on a chip 1) Decelerate to zero velocity 2) Trap 3) Reaccelerate to extract out of the trap Normalized integrated H + ion signal (arb. unit) (a) (b) (c) = 10 s trap ô = 20 s trap = 40 s trap H atom time-of-flight ( s)

42 Interaction of Rydberg atoms with coplanar on-chip waveguides collaboration with T. Thiele, S. Filipp and A. Wallraff, Quantum Device Lab., ETH Zürich Objectives: Strongly couple Rydberg atoms to superconducting qubits via transmission-line resonator Quantum optics at a vacuum solid-state interface Wallraff et al., Nature (2004) Raimond, Brune and Haroche, Rev. Mod. Phys. (2004)

43 Beam of metastable 1 helium (1s2s S ) 0 Compatible with cryogenic environment Low-l quantum defects allow studies of surface fields Possibility of manipulating translational motion - laser cooling - Stark acceleration Laser Rydberg atom beam

44 Microwave transitions Laser excitation of 33p state Microwave pulse length Ät = 1 ìs Field ionization of 33s / 32d states Natural linewidth Äí ( FWHM ) 2 MHz Inhomogeneous broadening by surface stray fields

45 n dependence of line shapes F(d=0.5 mm) = 1 V/cm

46 Power Broadening Normalized integrated electron signal (arb. unit) 7 P = 32 mw ì P = 3.2 mw ì P = 320 ìw ì P = 32 ìw ì P = 3.2 ìw ì P = 320 nw ì P = 32 nw ì Frequency (GHz) 33p 33s 32 Microwave pulse: Ätì = 1 ìs Unsaturated linewidth: ÄíFWHM = 80 MHz

47 Rabi oscillations: 33p 33s Simulation of Rabi oscillations Spatial distribution of atoms Microwave field distribution Stray electric field Stark effect Distribution of oscillation frequencies: - stray electric field - microwave field distribution Decoherence time: 250 ns - limited by motion of atoms (1800 m/s) Hogan et al., PRL 108, (2012)

48 Conclusions (Part III) Rydberg-atom/molecule beams can be decelerated and trapped using a surface-electrode decelerator Transitions between Rydberg states can be induced by microwaves emanating from a microwave transmission line (Rabi oscillations observed)

49 Financial support: Swiss National Science Foundation ERC advanced grant program ETH Zürich