Interaction of a BEC with Dipole Barriers

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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

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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

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push beam
Upper MOT
Absorption probe
Ø6.3cm 40G max 5.4 kHz
Lower MOT
Quadrupole coils
Ø10cm, 295 turns max gradient 470 G/cm_at_30A
Inner Diameter 1.7cm
Not Drawn Compensation coils along 3 axes
20mm
TOP coils
Optical pumping beam
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Experimental Details

• Loading of lower MOT with push beam 7s
• MOT beams
• 3.2 mW/cm2
• 2 cm diameter
• Trap lifetime 100s
• Heating without RF shield 300nK/s
• Atom loss at 52nK with RF shield lt 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

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Experiment Stages

• Upper MOT
• 400 million atoms, 10-9 Torr
• Lower MOT
• Push beam loading for 7seconds
• 1 billion atoms, lt 10-11 Torr
• Molasses
• 2ms at 20MHz detuning 3ms at at 28MHz detuning
  and 3ms at 36MHz detuning
• Optically pump on the F2 to F3 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 80mK
• 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

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The Lower Chamber
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Expansion after RF Evaporation
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Absorption Images
1mm
8ms Expansion of a 52nK cloud
20ms Expansion of a 52nK cloud
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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

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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.
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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

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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.

Büttiker and Landauer, PRL 49, 1739 (1982)

• 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

D. Boosé and F. Bardou, Europhys. Lett. 53, 1
(2001)
A. M. Steinberg, Journal of the Korean Physical
Society 35 (3), 122 (1999)
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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)
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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)
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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 ODell at
http//www.quacs.u-psud.fr/Workshop/Presentation2
0DEICS/ODell.ppt
DHJ ODell, S Giovanazzi and G Kurizki, PRL 90
(2003) 110402
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Conclusions / Future

• We are VERY close!
• T52nK, PSD2.8???
• Currently developing beams for dipole barriers
  and Raman probes
• Always interested in potential collaborations

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SUPPORT
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SUPPORT
And You
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And if all else fails
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Upper Chamber
Lower Chamber