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Super-radiant light scattering with BEC’s – a resource for useful atom light entaglement?

Super-radiant light scattering with BEC’s – a resource for useful atom light entaglement?. Jörg Helge Müller, Quantop NBI Copenhagen. Motivation – quantum state engineering Light-atom coupling in Rubidium Sample preparation: BEC setup First light: Superradiance revisited

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Super-radiant light scattering with BEC’s – a resource for useful atom light entaglement?

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  1. Super-radiant light scattering with BEC’s – a resource for useful atom light entaglement? Jörg Helge Müller, Quantop NBI Copenhagen • Motivation – quantum state engineering • Light-atom coupling in Rubidium • Sample preparation: BEC setup • First light: Superradiance revisited • Dynamics in simple models • Counting atoms and photons • Future directions QCCC Workshop, Burg Aschau, October 2007

  2. Light-Atom interaction seen from both sides Spectroscopy: light is modified by atoms (e.g. polarization rotation) Laser manipulation: Atoms are modified by light (laser trapping, optical pumping,...) Both things happen at the same time We want to study and exploit the regime where quantum effects matter to prepare interesting quantum states! Quantum State Engineering

  3. Coupling at the microscopic level ...plain dipole scattering Use a high finesse cavity! or Use many atoms/photons! In free space this coupling is small Our strategy • mix quantum modes with strong orthogonally polarized ”local oscillator” • light quadratures show up as polarization modulation • use ensemble of many polarized atoms  macroscopic spin/alignment • phase matched scattering into forward direction • polarization modulation modifies the macroscopic spin/alignment

  4. Rb F=1 ensembles and polarized light Local atom light interaction birefringence phase shift polarization rotation Raman coupling Larmor precession level shift

  5. Reduction to forward scattering • Transverse light propagating along z-direction • Atoms prepared initially in mF = -1 , +1 , (0) J : Bloch vector of the 2-level system (one classical, two for quantum storage) S : Stokes vector for light (one classical, two for the quantum mode) b coefficients can be tuned with the choice of laser frequency! Vector coefficient: Faraday interaction (single quadrature, QND coupling) Tensor coefficient: Raman coupling (two quadratures, back-action) Now we need to add propagation effects....

  6. Application to Quantum memory • Quantum memory Negative feedback: (back-action cancellation) in both quadratures Single-pass Optimized geometry (Tune bV to zero) Output light for coherent state input in the quantum mode: oscillating response Feedback during propagation leads to spatial structure: ”Spin waves”

  7. Application to light atom entanglement 2. Parametric Raman amplifier Positive feedback: (back-action amplification) EPR-type entanglement between light and atoms Super-radiant Raman scattering Our detour: Super-radiant Rayleigh scattering Input/Output relations can be calculated and decomposed into mode pairs for atom and light Wasilewski, Raymer, Phys.Rev. A 73, 063816 (2006) Nunn et al., quant-ph/0603268 Gorshkov et al., quant-ph/0604037 Mishina et al., Phys.Rev. A 75, 042326 (2007) Efficient optimization of memory performance by tailored drive pulses possible

  8. Important parameter for collective coupling On-resonance optical depth of the sample Single atom spontaneous scattering Coupling strength bigger than 1 (usually) means quantum noise of atoms becomes detectable on light and vice versa. Optical depth should be as high as possible!!

  9. Sample preparation: BEC setup

  10. BEC setup (2) QUIC trap (inspired by Austin group, good thermal stability) Ioffe coil with optical access Imaging along vertical direction Ioffe axis free for experiments

  11. Evaporation and trap performance Slope  1.3 Slope  -3 Radial frequency  116 Hz Aspect ratio  12 Atom number  6  105

  12. First light: Super-radiance revisited 3-level system with total inversion initially Build-up of coherence enhances scattering • Example: Coherence in momentum space • photons and recoiling atoms created in pairs • atom interference creates density grating • enhanced scattering off density modulation • runaway dynamics until depletion sets in Super-radiant emission Ordinary spontaneous emission R.H.Dicke, Phys.Rev. 93, 99 (1954)

  13. Sample shape and mode structure High gain in directions of high optical depth L 2w Diffraction angle: Geometric angle: F<1 : single mode dominant

  14. Modes and competition • Backreflected light and recoiling atoms • Forward scattering with state change State change constrained by dipole pattern

  15. Rayleigh scattering dominant • Favor Rayleigh scattering by choice of detuning and polarization • Backward reflected light and recoiling atoms • Forward scattering with state change suppressed First experiments in end-pumped geometry

  16. End-pumped superradiant scattering(first experiments) • in-trap illumination • - 1.8 GHz detuning from F=1  F’=1 • 2 · 1011 photons/s through BEC cross section • immediate release after pump pulse Rayleigh scattering dominant for these parameters! Threshold expected after 103 incoherent events Dynamics slower than Dicke model prediction Possible reasons: collisions, longitudinal structure, photon depletion, misalignment,…

  17. Dynamics in experiment and simple models: the light side Detect reflected light to observe dynamics directly Simulated pulse shape from modified rate equation model Backscattered light for different pump powers • Setup for reflected light detection • balanced detector • shot-noise sensitive at 105 photons • focused pump beam Reasonable but not yet satisfactory agreement Refined model needed… Comparison experiment and model

  18. Dynamics in experiment and simple models: the atom side • clearly observable but poorly understood structures in original and recoiling cloud • separation of the clouds does not match photon recoil • 3-D modeling of expansion urgently needed! • high population of scattering halo Modeling the role of collisions • decoherence • gain reduction

  19. Can we use it? • Backscattered photons and atoms should be fully correlated • (in fact, entangled) but we need to show it! • Challenges: • count backscattered photons to better than N1/2 • count recoiling atoms reliably • keep atom-atom collisions during expansion low • quantitative modeling of the dynamics Atom-detection Photo-detection • high Q.E. CCD detector implemented • pump geometry changed to avoid stray light background • Cross calibration with different methods • more atoms than initially estimated

  20. Counting atoms and photons...the hard work recoiling atoms passive atoms with atoms without atoms Need to improve background reduction in light detection Need to reduce noise level in atom detection

  21. What do other people do? arXiv/cond-mat/0707.1465v1 Atom-Atom entanglement by super-radiant light assisted collisions Also here the challenge is actually detecting the entanglement…

  22. Future directions:Quantum memory Access to internal atomic degrees of freedom Use of light polarization degree of freedom Forward scattering with state change Funnily enough, we might need to suppress Super-radiance as a competing channel…

  23. Under construction: Optical dipole trap • state insensitive trapping potential • matched aspect ratio for easier transfer Achromat lens f=60mm Trap beam • diode lasers at 827 nm (P = 100 mW) • shared optics with probe beam • stable confinement without magnetic fields • scattering into probe mode below 100 ph/s • compatible with magnetic bias field control • flexible trap geometry Probe beam Trap beam Collaboration with Marco Koschorreck (ICFO)

  24. Who did the actual work? Andrew Hilliard Franziska Kaminski Rodolphe Le Targat Marco Koschorreck Christina Olausson Patrick Windpassinger Niels Kjaergaard Eugene Polzik Funding by Danmarks Grundforsknings Fund, EU-projects QAP and EMALI

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