Ultracold Helium Research Roel Rozendaal Rob van Rooij Wim Vassen.

Ultracold Helium Research

Roel Rozendaal

Rob van Rooij

Wim Vassen

Outline

• Reminder on ultracold Helium

• S-wave scattering length

• Optical Dipole trap

Ultracold Helium

• Cooling down atoms using– Lasers– Evaporative cooling

• Bosons and fermions– Bose-Einstein Condensate (BEC)– Degenerate Fermi-gas (DFG)

• Study atom properties– Ultra-cold interactions– Quantum statistics– QED tests

Helium reminder - Scattering length - Dipole trap

Helium reminder

• Metastable Helium– ~ 8000 s– 19.8 eV internal energy– Cooling at 1083 nm

• Bosonic: 4He

• Fermionic: 3He


Cooling down

Magneto-Optical Trap:T ~ 0.8 mKN ~ 109


Cooling further

• Transfer to magnetic trap (MT)

• Laser cooling

• RF-induced evaporative cooling– BEC / DFG


Detection

• MCP – time-of-flight

• CCD camera– in-situ– time-of-flight– QE ~ 1% @ 1083 nm

• Photodiode array camera– InGaAs– QE ~ 70% @ 1083 nm


s-wave scattering length

Cold collisions

• Collision in central scattering potential

• Long distance behaviour

• Scattering cross-section • Low-energy

– Spherical wave expansion– Only a few terms contribute


Ultra-cold collisions

• only s-wave scattering– Incoming plane wave, outgoing plane wave

with a phase shift

• = 8 a2 (identical particles)

• = 4 a2 (non-identical particles)


Relevant a’s

• boson-boson– a44 (measured accurately, +7.5 nm)

• boson-fermion– a34 (unknown, theory: +27 nm)

• fermion-fermion– a33 = zero


Measuring a

• Photo-association spectroscopy

• Damped modes

• Thermalization– After disturbing equilibrium


Disturbing the cloud

• Radial confinement off / on

Too rough.


t (ms)

He* count (mV)


• Raising temperature in one direction– Slowly increase, quickly restore confinement

in one direction



• Raising temperature in one direction


Obtaining a

• Thermalization after 2.65 collisions (/atom)

• rel = -1 n v


Obtaining a


• rel = -1 n v

± 5%


Obtaining a


• rel = -1 n v

± 5%

± 20%


Obtaining a


• rel = -1 n v

± 5%

± 20%

± 40%


Obtaining a


• rel = -1 n v

± 5%

± 20%

± 40%± 50%


Obtaining a


• rel = -1 n v

± 5%

± 20%

± 40%± 50%

→ a ± 34%


First measurement

• a44 = 4 ± 2 nm

– T ~ 250 K: too hot

t (ms)

/ <>


3He

• Different 3He/4He ratios

• 4He needed for cooling

• Fewer atoms– Imaging more challenging


Optical Dipole Trap

Overview

• Electric dipole interaction– Red-detuned: trapped in intensity maximum

• 1557 nm, 2W trapping laser– Recoil: ~ 1 K– ~40 m focus (diameter)

• Crossed-dipole configuration– ~ 20º


Trap characteristics

• Scattering rate


r (w0)

(s-1)


• Depth & harmonicity – radial (7.2 kHz)


Depth (K)

r (w0)


• Depth & harmonicity – axial (1.5 kHz)


r (w0)

Depth (K)

Projects

• 1557 nm forbidden transition– Loss detection?

• Feshbach resonances

• Phase-separation

• Very high trap frequencies (>100 kHz)– Suppression of Penning ionization?


Ultracold Helium Research Roel Rozendaal Rob van Rooij Wim Vassen.

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