COLLISIONS IN ULTRACOLD METASTABLE HELIUM GASES G. B. Partridge, J.-C. Jaskula, M. Bonneau, D. Boiron, C. I. Westbrook Laboratoire Charles Fabry de l’Institut d’Optique, Palaiseau France

2 Outline Methods, apparatus, He*. Motivation and Background - Optics, atomoptics, quantum optics, quantum atom optics… Optical Trapping and Relative Number Squeezing Experiments: 4-wave mixing of matter waves Spin Mixtures

3 Motivation, atom optics… Optics : Photons, waves… wave particle duality. Atomic physics  atom optics : i.e – slits, interferrometers, etc Bec  coherent atom optics: Atom Laser, fringes, + nonlinear atom optics (interactions): 4wm, solitons… Quantum atom optics? -ex’s correlations, squeezing, entanglement, teleportation… Use counting, single particles, statistics… -- Key is detection: metastable Helium (He*). T. Pfau (Stuttgart) L. Deng et al. (NIST) Strecker et al. (Rice) Andrews et al Science,275,637,1997

4 He* : What’s it hiding? The 2 3 S 1 state of He has a decay time ~ 8000 s !* *single atom ~ spin polarized This energy can kick off electrons & ionize atoms of surfaces that the atom meets. The stored energy of the metastable state is 19.8 eV/atom. Add in a potential, get an avalanche of electrons.  High gain amplifier = single atom sensitivity. e-e- + (So what?)

5 Trapping and Cooling He* Laser cooling helium? Behaves a lot like an alkali-metal. (Cycling Optical Transition, magnetically trappable ) I I

6 Single Atom Detection Use a micro-channel plate (many e- avalanche detectors in parallel) to give position information. Gather resulting electric pulses using crossed delay lines. Use relative arrival times to reconstruct atoms’ positions (time of flight) in 3D.

7 A new tool for He* Statistical measurements: 1000’s of repetitions. magnetic trap was not engineered for this… (although we try anyway)  Long term: favors Optical Trap magnetic optical TOF Also, better geometry: aligns long axis of potential ( short TOF, short correlation length) w/ high resolution direction, Z. Gives freedom to try spin mixtures… First step towards more complicated potentials for He* (lattices, disorder etc.)

8 BEC of He* in the optical trap N 0 = 10 5 r = 1.5 kHz, z = 8 Hz Transfer from magnetic trap after some pre-cooling: N = 5 x 10 6, T = 15  K Evaporate by reducing intensity of trap laser over ~ 4 sec. G. B. Partridge, J.-C. Jaskula, M. Bonneau, D. Boiron, C. I. Westbrook, Phys. Rev. A 81, (2010).

9 Quantum Optics: photon pairs

10 Matter Wave FWM: atom pairs S-wave interactions lead to spherical shell of scattered atoms at k=k S  spontaneous FWM k0k0 k0k0 kSkS kSkS k0k0 k0k0 Create an m = 0 condensate w/ raman pulse. Split BEC into two momentum components with Bragg pulse: +/- k 0

11 The “intuitive” result Scattered pairs are correlated… 0 Δt P(Δt) k0k0 k0k0 kSkS kSkS Like in photon pairs: “Enhanced coincidence rate when phase matching condition is met.”

12 Beyond Optics: smaller sphere |k S | < |k 0 | k0k0 k0k0 kSkS kSkS Energy Cost to put atom into scattered mode (still overlapped w/ condensate). k0k0 k0k0 kSkS kSkS (per atom) Energy gain from removal of atom from condensate mode < “energy balance” V. Krachmalnicoff, J.-C. Jaskula, M. Bonneau, V. Leung, G. B. Partridge, D. Boiron, C. I. Westbrook, P. Deuar, P. Zin, M. Trippenbach, K. Kheruntsyan, Phys. Rev. Lett. 104, (2010).

13 Plus, the sphere’s not a sphere After colliding, atoms still have to get out of the region of the condensates. i.e. they roll down the mean field hill: V = 2g  (r,t) But the hill is collapsing out from under them. Anisotropy of BEC’s leads to directional acceleration Lesson Learned: Do Q.O. experiments using atoms, but be careful about simple 1:1 intuition. There are differences, for better or worse… Analogy? ponderomotive force in high harmonic generation (Balcou et al PRA 1997) Phys. Rev. Lett. 104, (2010).

14 Intermediate Q.O.: Relative N Squeezing Heidmann et al. PRL (1987) A B Measurement of intensity noise between “twin” beams. Reduction in noise, 30% below the shot noise limit!

15 New Atom Pairs RF + Bragg pulse. Optical Trap BEC Back-to-Back Correlations: 3600 shots Collision along long axis + better repeatability gives improved S/N. Now what about squeezing?

16 Matter Wave N Squeezing Divide scattered halo into sections, compare number difference in geometrically opposing zones to that of non-opposing zones. (for uncorrelated N, i.e. shot noise) 1 M 16 zones

17 Details… Detail 1: Raw data ~ -0.5 dB squeezing Why isn’t it perfect? (partly b/c its an experiment) Specifically, the detector efficiency, , limits the measured variance. Perfect correlations:  M = (1-  )  = 0.6 (“open area”) : -3 dB  =.13 (best estimate): -13 dB Detail 2: Effect of of correlation length: ~Measurement bandwidth

18 What’s next?

19 But! Trapped He* gases are prone to loss due to Ionization-enhanced inelastic loss processes. Spin Polarization in the m J = 1 provides stabilization by ~5 orders of magnitude. What about other states and combinations of states? State specific loss constants unconfirmed experimentally (only m J = 1 is magnetically trappable) With optical trap, we can think about using different spin states (m J = +1,-1,0)  spin mixtures, spinor condensates … RF transfers: spin mixtures Alternate Future: spin mixtures

20 Loss Rates in a spin mixture Inelastic Loss Experiment 1: Put them all together and see what survives… “Small” loss rate:  01,  0-1,  11,  -1-1 “Large” loss rate:  00,  ±1 G. B. Partridge et al., Phys. Rev. A 81, (2010).

21 Quantitative Loss Rates  00 = 6.6(4) × 10 −10 cm 3 /s  ±1 = 7.4(10) × 10 −10 cm 3 /s.  Not necessarily prohibitive! (for certain things…) Inelastic Loss Experiment 2: Make careful measure of the dominant processes  00  ±1. G. B. Partridge et al., Phys. Rev. A 81, (2010).

22 Summary 1.Quantum Atom Optics: Spontaneous FWM of deBroglie matter waves. Don’t forget they’re atoms. 2.Relative Number Squeezing for correlated atom pairs. Atomic version of a Quantum Optics Classic. 3.Spin Mixtures in of He* ? Stay tuned…

23 Thanks! Questions?