Super-radiant light scattering with BEC’s – a resource for useful atom light entaglement? 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 Jörg Helge Müller, Quantop NBI Copenhagen

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

Coupling at the microscopic level...plain dipole scattering In free space this coupling is small 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 Use a high finesse cavity! or Use many atoms/photons! Our strategy

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

Reduction to forward scattering 1.Transverse light propagating along z-direction 2.Atoms prepared initially in m F = -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....

Application to Quantum memory 1.Quantum memory Negative feedback: (back-action cancellation) in both quadratures (Tune b V to zero) Single-pass Optimized geometry Output light for coherent state input in the quantum mode: oscillating response Feedback during propagation leads to spatial structure: ”Spin waves”

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, (2006) Nunn et al., quant-ph/ Gorshkov et al., quant-ph/ Mishina et al., Phys.Rev. A 75, (2007) Efficient optimization of memory performance by tailored drive pulses possible

Important parameter for collective coupling On-resonance optical depth of the sampleSingle 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!!

Sample preparation: BEC setup

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

Evaporation and trap performance Slope  1.3Slope  -3 Radial frequency  116 Hz Aspect ratio  12 Atom number  6  10 5

First light: Super-radiance revisited 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 3-level system with total inversion initially Build-up of coherence enhances scattering Ordinary spontaneous emission R.H.Dicke, Phys.Rev. 93, 99 (1954) Super-radiant emission

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

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

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

End-pumped superradiant scattering (first experiments) in-trap illumination GHz detuning from F=1  F’=1 2 · photons/s through BEC cross section immediate release after pump pulse Rayleigh scattering dominant for these parameters! Threshold expected after 10 3 incoherent events Dynamics slower than Dicke model prediction Possible reasons: collisions, longitudinal structure, photon depletion, misalignment,…

Dynamics in experiment and simple models: the light side Setup for reflected light detection balanced detector shot-noise sensitive at 10 5 photons focused pump beam Detect reflected light to observe dynamics directly Backscattered light for different pump powers Comparison experiment and model Simulated pulse shape from modified rate equation model Reasonable but not yet satisfactory agreement Refined model needed…

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

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 N 1/2 count recoiling atoms reliably keep atom-atom collisions during expansion low quantitative modeling of the dynamics high Q.E. CCD detector implemented pump geometry changed to avoid stray light background Photo-detection Atom-detection Cross calibration with different methods more atoms than initially estimated

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

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

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

Achromat lens f=60mm Probe beam Trap beam state insensitive trapping potential matched aspect ratio for easier transfer 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 Collaboration with Marco Koschorreck (ICFO) Under construction: Optical dipole trap

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