Les Houches 2009: Metastable Helium Atom Laser

Transcription

1 Les Houches 2009: Metastable Helium Atom Laser Les Houches, Chamonix, February 2005

2 Australian Research Council Centre of Excellence for Quantum-Atom Optics UQ Brisbane SUT Melbourne ANU Canberra

3 Snowy Mountains, Australia - September 2006

4 Metastable Helium Atom Optics He 2 3 S 1 (He*) is the longest lived (~8000s) atomic metastable state yet measured (SEAN S TALK). Metastable helium: Atom optics with nano-grenades, K.G.H. Baldwin, Contemporary Physics 46, 105 (2005). We first developed a bright He* beam line for cold atom studies Electron - He* collisions Atom lithography Atom guiding in hollow fibres We now have a He* BEC apparatus for ultracold atom and BEC studies Energy singlets 3 1 S o 2 1 S o 3 3 D P S 1 triplets 3 3 P nm trap laser (metastable) 389 nm laser Atom laser studies Atomic physics - He* lifetimes Quantum statistical effects 1 1 S o 19.8 ev electron excitation

5 He* BEC Facility LVIS ~ 2 x He* / s Vel. ~ 30 m / s Trap ~ 5 x 10 8 He* at ~ 1 mk He* atom source Low Velocity Intense Source (LVIS) BEC trap chamber (UHV) Load ~ 2x10 10 He*/s Vel. ~ m/s Trap ~ 3x10 9 He* Laser collimation Laser trap Slowing laser

6 Magnetic Trap 1 cm 1 cm 1 cm Equivalent coil configurations BEC ~ 3x10 6 He* at ~ 1μK Highly stable magnetic field trapping potential ~10-5 gauss BiQUIC magnetic trap coils

7 BEC chamber He* in Magnetic trap BEC 1 cm 1st MOT 2nd MOT Re-entrant window (top view)

8 He* BEC Experiments: 2D spatial profile MCP and phosphor 2-D detector T > T c T < T c T ~ 0.3T c

9 BEC vs. Atom Laser Slowly cool to increase db and occupy the ground state Further cool and force other atoms into ground state via stimulated Bosonic emission de Broglie waves in the lowest mode of the trap ( cavity ) form a single quantum state with the same wavelength and the same phase Only need to output couple to form an atom laser beam

10 RF Outcoupling Atom Laser 25 m f = RF coupling rf1 m f = 0 g MCP Detector

11 Atom Laser Noise Without stabilisation With stabilisation Atom laser output Power spectrum 100 times suppression

12 Atom laser spatial profile - less than ideal Reason: Initial theory by Th. Busch, M. Kohl, T. Esslinger and K. Mølmer, PRA 65, (2002) the chemical potential gradient not only spreads the atom laser beam => but atom trajectories with same final position yield quantum interference

13 Rb c.f. He* atom laser experiments Riou et al. (Orsay) theory and expt. PRL 96, (2006) Our He* experimental results Opt. Exp. 26, (2007)

14 Rb vs. He*: out-coupling surfaces Rb atoms experience a large sag - almost flat outcoupling surface He* atoms experience little sag - spherical shells

15 Fountain Effect

16 Simulated atom laser transverse spatial profiles Strong trap (460 Hz - small sag) output - coupled from near the trap centre Weak trap (50 Hz - large sag) Low frequency (horizontal) interference fringes High frequency (vertical) interference fringes Distance on MCP (a.u.)

17 Atom Laser Profile Dip in shadow of BEC Structure? Twin peaked structure

18 First observation of fringes

19 High output-coupling fringes

20 Profiles for two radial frequencies rf = 10 khz f r = 460 Hz f r = 113 Hz rf = 6 khz rf = 4 khz rf = 1 khz 3.1 mm rf = 3 khz 10 mm rf = 0.5 khz

21 He* Atom Laser: Conclusions Measured spatial profile of a He* atom laser Observed predicted interference fringes for the first time Atom laser beam not ideal - highly multimode transverse spatial profile

22 Q: How do we fix the horrible spatial profile of the He* atom laser? A: By matter wave guiding in an optical dipole potential.

23 Guerin et al. PRL (2006) - Orsay Hybrid trap - a magnetic trap crossed with an optical dipole guiding beam - and RF output coupled into the dipole guide Mode occupancy inferred from the transverse energy spread, with ~14% in lowest order mode

24 Couvert et al. EPL (2008) - ENS Crossed dipole trap which is outcoupled horizontally by an inhomogeneous magnetic field m F = -1 m F = 0 Mode occupancy inferred from the transverse energy spread, with ~50% in lowest order mode

25 ANU optical atom waveguide He* atoms cooled to ~1 μk in a magnetic trap BEC Atoms transferred to an optical dipole trap (1500 x 1075 x 23 Hz) and condensed to BEC Laser intensity reduced from 41 to 17 mw over 100 ms Far-detuned optical dipole potential from a 50 mw, 1550nm laser focused to 30 μm 200 mm Atoms no longer trapped and fall under gravity onto MCP 2D spatial detector (MCP)

26 Waveguide results BEC dropped onto MCP Atom laser profile Nearly single mode guided matter waves

27 Stern-Gerlach separation m F = -1 m F = 0 m F = +1

28 Guided image Expanded view of guided image: 1 pixel = 50 μm

29 Dual Gaussian Fit Close to theoretically predicted width Total profile Lowest order mode > 70% Additional modes

30 Multimode vs. single mode Single mode guiding (same pixel scale) Multimode guiding - note speckle pattern

31 Averaged multimode 1 3 multimode guided images average of Single mode (same scale)

32 Diffracted image Guided atoms diffracted through Quantifoil R2/2 (2 micron holes separated by 2 microns in carbon film) Single mode Multimode

33 3D - Diffracted image (B) (A)

34 Conclusions Optically trapped He* BEC is coupled into a far-detuned dipole potential waveguide Output of waveguide is mostly single mode (> 70%) Multimode excitation produces a speckle pattern Speckle pattern averages to a smooth profile much larger than the single mode guided image Significantly improved beam quality c.f. atom laser beam Potential applications to guided atom interferometry

35 He* BEC experiment Oscar Turazza Robert Dall Lesa Byron Sean Hodgman Andrew Truscott

36 Blue Lake, Snowy Mountains, Australia, 2004 Thank you for your attention!

37 Magnetic Field Nuller Separate feedback circuits Feedback Amplifier Helmholtz Coils Sensors Trap Centre x - axis Magnetic flux gate + pickup coils (16Hz crossover) C.J. Dedman, R.G. Dall, L.J. Byron, and A.G. Truscott, Review of Scientific Instruments 78, (2007)

38 DC magnetic noise Feedback Enabled

39 Atom Laser Coherence (2 frequency outcoupling) Above T c I. Bloch, T. W. Hänsch and T. Esslinger. Nature 403, (13 January 2000) Below T c