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 matterto 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 shift polarization rotation birefringence

level shift Larmor precession Raman coupling

Reduction to forward scattering

1. Transverse light propagating along z-direction2. 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....

0 1 2 3 4 5-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

scaled time

Step response at output

Application to Quantum memory

1. Quantum memory

0 0.5 1 1.5 2 2.5 3-1.5

-1

-0.5

0

0.5

1

1.5

scaled time

halfstep output response

Negative feedback:(back-action cancellation)in both quadratures

(Tune bV to zero)

Single-passOptimized geometry

Output light for coherentstate input in the quantummode: 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 atomsSuper-radiant Raman scatteringOur 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

Important parameter for collective coupling

On-resonance optical depth of the sample

102

22

phA n

AN

A

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!!

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 1.3 Slope -3

Radial frequency 116 HzAspect ratio 12Atom number 6 105

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

• - 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,…

Dynamics in experiment and simple models: the light side

Setup for reflected light detection

• balanced detector • shot-noise sensitive at 105

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 N1/2

• 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-detectionAtom-detection

• Cross calibration with different methods• more atoms than initially estimated

Counting atoms and photons...the hard work

with atoms

recoiling atoms

without atomspassive 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/0707.1465v1

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 suppressSuper-radiance as a competing channel…

Forward scattering with state change

Achromat lens f=60mm

Probe beam

Trap 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 HilliardFranziska KaminskiRodolphe Le TargatMarco KoschorreckChristina OlaussonPatrick WindpassingerNiels KjaergaardEugene Polzik

Funding by Danmarks Grundforsknings Fund, EU-projects QAP and EMALI