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: 4 He Fermionic: 3 He Helium reminder - Scattering length - Dipole trap

Cooling down Magneto-Optical Trap: T ~ 0.8 mK N ~ 10 9 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 ~ 1083 nm Photodiode array camera –InGaAs –QE ~ 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  a 2 (identical particles)  = 4  a 2 (non-identical particles) Helium reminder - Scattering length - Dipole trap

Relevant a’s boson-boson –a 44 (measured accurately, +7.5 nm) boson-fermion –a 34 (unknown, theory: +27 nm) fermion-fermion –a 33 = 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 a 44 = 4 ± 2 nm –T ~ 250  K: too hot t (ms)   / Helium reminder - Scattering length - Dipole trap

3 He Different 3 He/ 4 He ratios 4 He 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)