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
Helium reminder - Scattering length - Dipole trap
Cooling down
Magneto-Optical Trap:T ~ 0.8 mKN ~ 109
Helium reminder - Scattering length - Dipole trap
Cooling further
• Transfer to magnetic trap (MT)
• Laser cooling
• RF-induced evaporative cooling– BEC / DFG
Helium reminder - Scattering length - Dipole trap
Detection
• MCP – time-of-flight
• CCD camera– in-situ– time-of-flight– QE ~ 1% @ 1083 nm
• Photodiode array camera– InGaAs– QE ~ 70% @ 1083 nm
Helium reminder - Scattering length - Dipole trap
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
Helium reminder - Scattering length - Dipole trap
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)
Helium reminder - Scattering length - Dipole trap
Relevant a’s
• boson-boson– a44 (measured accurately, +7.5 nm)
• boson-fermion– a34 (unknown, theory: +27 nm)
• fermion-fermion– a33 = zero
Helium reminder - Scattering length - Dipole trap
Measuring a
• Photo-association spectroscopy
• Damped modes
• Thermalization– After disturbing equilibrium
Helium reminder - Scattering length - Dipole trap
Disturbing the cloud
• Radial confinement off / on
Too rough.
Helium reminder - Scattering length - Dipole trap
t (ms)
He* count (mV)
Disturbing the cloud
• Raising temperature in one direction– Slowly increase, quickly restore confinement
in one direction
Helium reminder - Scattering length - Dipole trap
Disturbing the cloud
• Raising temperature in one direction
Helium reminder - Scattering length - Dipole trap
Obtaining a
• Thermalization after 2.65 collisions (/atom)
• rel = -1 n v
Helium reminder - Scattering length - Dipole trap
Obtaining a
• Thermalization after 2.65 collisions (/atom)
• rel = -1 n v
± 5%
Helium reminder - Scattering length - Dipole trap
Obtaining a
• Thermalization after 2.65 collisions (/atom)
• rel = -1 n v
± 5%
± 20%
Helium reminder - Scattering length - Dipole trap
Obtaining a
• Thermalization after 2.65 collisions (/atom)
• rel = -1 n v
± 5%
± 20%
± 40%
Helium reminder - Scattering length - Dipole trap
Obtaining a
• Thermalization after 2.65 collisions (/atom)
• rel = -1 n v
± 5%
± 20%
± 40%± 50%
Helium reminder - Scattering length - Dipole trap
Obtaining a
• Thermalization after 2.65 collisions (/atom)
• rel = -1 n v
± 5%
± 20%
± 40%± 50%
→ a ± 34%
Helium reminder - Scattering length - Dipole trap
First measurement
• a44 = 4 ± 2 nm
– T ~ 250 K: too hot
t (ms)
/ <>
Helium reminder - Scattering length - Dipole trap
3He
• Different 3He/4He ratios
• 4He needed for cooling
• Fewer atoms– Imaging more challenging
Helium reminder - Scattering length - Dipole trap
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
Helium reminder - Scattering length - Dipole trap
Trap characteristics
• Scattering rate
Helium reminder - Scattering length - Dipole trap
r (w0)
(s-1)
Trap characteristics
• Depth & harmonicity – radial (7.2 kHz)
Helium reminder - Scattering length - Dipole trap
Depth (K)
r (w0)
Trap characteristics
• Depth & harmonicity – axial (1.5 kHz)
Helium reminder - Scattering length - Dipole trap
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?
Helium reminder - Scattering length - Dipole trap
(end)