Transcript Towards a light-atom quantum interface with a Rb BEC

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
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
Use a high finesse cavity!
or
Use many atoms/photons!
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
Rb F=1 ensembles and polarized
light
Local atom light interaction
phase shift
level shift
polarization rotation
Larmor precession
birefringence
Raman coupling
Reduction to forward scattering
1. Transverse light propagating along z-direction
2. 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....
Application to Quantum memory
1. Quantum memory
Negative feedback:
(back-action cancellation)
in both quadratures
Single-pass
Optimized geometry
(Tune bV to zero)
halfstep output response
Step response at output
1.5
1
Output light for coherent
state input in the quantum
mode: oscillating response
0.8
0.6
0.4
0.2
1
0.5
0
-0.5
0
-1
-0.2
-1.5
-0.4
0
1
2
3
scaled time
4
5
0
0.5
1
1.5
2
scaled time
Feedback during propagation leads to spatial structure: ”Spin waves”
2.5
3
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
Important parameter for collective coupling
2


  

   N A 
n ph    0  1
2
A
 A 

2
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!!
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
Radial frequency  116 Hz
Aspect ratio  12
Atom number  6  105
Slope  -3
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
Super-radiant emission
• runaway dynamics until depletion sets in
Ordinary spontaneous emission
R.H.Dicke, Phys.Rev. 93, 99 (1954)
Sample shape and mode structure
High gain in directions of high optical depth
L
2w
Diffraction angle:
Geometric angle:
F<1 : single mode dominant
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
Detect reflected light to observe dynamics directly
Backscattered light for
different pump powers
Simulated pulse shape
from modified rate
equation model
Setup for reflected light detection
Reasonable but not yet
satisfactory agreement
• balanced detector
• shot-noise sensitive at 105 photons
• focused pump beam
Refined model needed…
Comparison experiment and model
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
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
Counting atoms and photons...the hard work
recoiling atoms
passive atoms
with atoms
Need to reduce
noise level in
atom detection
without atoms
Need to improve
background
reduction in light
detection
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…
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…
Under construction: Optical dipole trap
• state insensitive trapping potential
• matched aspect ratio for easier transfer
Achromat lens
f=60mm
Trap beam
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
Collaboration with Marco Koschorreck (ICFO)
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