Interaction of a BEC with Dipole

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15 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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Interaction of a BEC with Dipole
Barriers

Mirco Siercke, Chris Ellenor, Matt Partlow,

Fan Wang, Jan Henneberger, Aephraim Steinberg

Department of Physics, University of Toronto

Motivation


Our first experiments will study the time a tunneling
particle spends in a dipole barrier


Later experiments will further probe the interaction of
coherent atoms with detuned laser light


The creation of non
-
classical momentum components during
collisions with barriers, and even the complete suppression of
central momentum components.


Long range laser induced dipole
-
dipole interactions (LIDDI)



Possible experiments in quantum hydrodynamics



Upper MOT

Lower MOT

Quadrupole coils

TOP coils

push beam

Absorption

probe

Not Drawn:

Compensation
coils along 3 axes

20mm

Ø=10cm, 295 turns

max gradient:


470 G/cm@30A

Ø=6.3cm

40G max

5.4 kHz

Inner Diameter 1.7cm

Optical pumping beam

Experimental Details


Loading of lower MOT with push beam: 7s


MOT beams


3.2 mW/cm
2


2 cm diameter


Trap lifetime: 100s


Heating without RF shield: 300nK/s


Atom loss at 52nK with RF shield: < 13%/s


Trap frequencies


48, 68, 96 Hz (compressed)


20 micron resolution, 12
-
bit CCD array


Field turnoff 1ms (TOP field), 100

s (quadrupole field)


Single loop RF coil driven by Agilent 33250A arbitrary waveform
generator


Experiment Stages


Upper MOT


400 million atoms, 10
-
9

Torr


Lower MOT


Push beam loading for 7seconds


1 billion atoms, < 10
-
11

Torr


Molasses


2ms at 20MHz detuning 3ms at at 28MHz detuning and 3ms at 36MHz detuning


Optically pump on the F=2 to F=3 transition for 4ms


TOP Trap


initial loading parameters: 40 G TOP field, 71 G/cm gradient


Compensate for gravitational sag with an additional field in the vertical direction


Load 300 million atoms at 80

K


Collision rate 5 Hz, Phase Space Density 2.8

10
-
7


Quadrupole compression


Compress from 71 G/cm (weak) to 155 G/cm in 0.5s


Wait 15s for evaporation, compress to 235 G/cm in 0.5s, wait 15 more seconds


Lowering of the Bias Field


Effects further compression and brings in circle of death for initial evaporation


Lowered from 40G to 20G in 14.5s


48

K, 40 million atoms, Collision rate of 22 Hz, Phase Space Density 3.4

10
-
6


RF ramp


Ramp from 24MHz to ~14MHz in 54s


Optimal ramp calculated, and recalculated when cloud deviated from prediction

The Lower Chamber

Expansion after RF Evaporation

Absorption Images

8ms Expansion of a 52nK cloud

20ms Expansion of a 52nK cloud

1mm

Next steps


Further compression of trap during RF ramp by
lowering bias field


Raise transition temperature


Imaging


Improve resolution to

better than 10

m to resolve
aspect ratios we expect to be on the order of 80%, and
expanded condensates we expect to be 20
-
40

m


Improve absorption imaging


Implement phase contrast imaging


Stabilize atom number

How Long Does a Tunneling
Particle Spend in the Barrier?

After almost 70 years of discussion, no consensus has yet emerged on the answer
to this simple question. This question is not only of fundamental, but also of
technological interest. BEC provides an excellent tool to study this issue
experimentally. Dipole tunneling barriers can be created with a size comparable
to the DeBroglie wavelength of the atoms, allowing for significant tunneling
probabilities of atoms which can then be imaged relatively easily. The internal
(hyperfine, Zeeman) structure of these atoms also offers possibilities for the
study of interaction time.


A Proposed Geometry


A dipole beam traps
atoms in a separate
well, and acts as a
barrier as atoms
tunnel into the
magnetic trap


Raman beams are
overlapped with the
barrier and weakly
couple atoms into a
different hyperfine
state

atoms

Raman
beam(s)

dipole
barrier
beam

magnetic
trapping
potential

Other Things to Look at…


Büttiker and Landauer imagine a barrier
whose height is modulated at some frequency

. The frequency is raised to a critical
frequency

c

at which modulation in
transmission is no longer seen, and this
frequency serves to define a traversal time
known as the
Büttiker
-
Landauer time.

D. Boosé

and
F. Bardou
,
Europ
hys. Lett.
53
,
1

(2001
)


A. M. Steinberg, Journal of the Korean Physical Society 35 (3), 122 (1999)


C


A probe beam which could
in principle

be
used to image a tunneling atom could suffice
to enhance transmission probability without
necessarily attempting to perform the imaging


Work by Bardou suggests that significantly
enhanced transmission may be achieved by
applying a small momentum transfer to a
particle interacting with a steep potential

Büttiker and Landauer, PRL 49, 1739 (1982)

Collisional Transitory
Enhancement of the
High Momentum
Components of a
Quantum Wave Packet




Collisions are usually considered only in the asymptotic
regime, but the full quantum mechanical treatment of a
collision with a potential barrier reveals the transitory
population of classically forbidden momentum states.


By quickly switching off the barrier during the collision,
we will observe these states in the free expansion of our
condensate

S. Brouard and J. G. Muga, Phys. Rev. Lett.
81
, 2621
(
1998
)


Transient Interference of
Transmission and Incidence


Extending the previous work,
a similar effect has been
described where interference
completely suppresses the
central momentum of the wave
packet


The interference and resultant
momentum distribution is a
result of the barrier shape,
over which we have splendid
control

A. L. Perez Prieto,

S. Brouard and J. G. Muga, Phys. Rev.
A

64
,
012710

(2001
)


Laser Induced Dipole
-
Dipole
Interactions (LIDDI)


LIDDI is a long range (~

3
) interaction induced between
atoms by an incident, propagating field


A product of forward photon scattering


Possible roton dip?

Borrowed from a talk by Duncan O’Dell at: http://www.quacs.u
-
psud.fr/Workshop/Presentation%20DEICS/ODell.ppt

DHJ O’Dell, S Giovanazzi and G Kurizki, PRL
90
(2003) 110402

Conclusions / Future


We are VERY close!


T=52nK, PSD=2.8???


Currently developing beams for dipole
barriers and Raman probes


Always interested in potential
collaborations







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And You

And if all else fails…

Upper Chamber

Lower Chamber