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