Sample preparation: BEC setup

skillfulbuyerUrban and Civil

Nov 16, 2013 (3 years and 8 months ago)

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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 shift

polarization 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....


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 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, 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

1
0
2
2
2
























ph
A
n
A
N
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 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




-

1.8 GHz detuning from F=1


F’=1




2

10
11

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/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 suppress

Super
-
radiance as a competing channel…

Forward scattering with state change

Achromat lens

f=60mm

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 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