prospects for ultracold metastable helium research: phase separation and bec of fermionic molecules...

Prospects for ultracold metastable helium research: phase separation and BEC of fermionic molecules

R. van Rooij, R.A. Rozendaal, I. Barmes & W. Vassen

Laser Centre Vrije Universiteit, Amsterdam, the Netherlands

I=1/2I=1/2 I=0I=0

The Experiment: Ultracold Helium

Helium-3 / Helium-4 mixture is excited to the 2 3S1 state using a dc discharge

Metastable helium beam is laser cooled and trapped in a MOT

Trapped helium is spin polarised The 1 mK atoms are transferred to a cloverleaf

magnetic trap 1D Doppler cooling to 0.1 mK is performed within

trap Evaporative cooling is performed down to ~1K 3He* is sympathetically cooled with 4He*

+1+1

00

-1-1

+3/2+3/2

-1/2-1/2

-3/2-3/2

33He*He*44He*He*

8.4 MHz8.4 MHz

+1/2+1/2

5.6 MHz5.6 MHz

• 2 3S1 state (He*): = 8000 s, • 20 eV internal energy: single He* atom detection• Penning ionization: He+ ( He* + He* → He + He+ + e─ )• 3He* fermion and 4He* boson

• Scattering lengths large and positive:aBB=+7.5105(25) nm (exp. ENS Paris)aFB=+27.1 (5) nm (theory Warsaw)

4He*

3He*+

4He*

3He*

Time-of-flight pictures of a BEC (upper figure), a DFG (middle figure) and a mixture (lower figure). The upper figure shows a thermal cloud above the BEC temperature, a mixture of BEC and thermal cloud, and a pure BEC with the typical inverted parabola shape. In the middle figure a fit to a Fermi-Dirac distribution is shown from which we extract a temperature T=0.45 TF. In the lower figure the dashed-dotted line shows the BEC contribution to the signal and the dashed line the DFG contribution.

ground state: 1s2 1S0

metastable state:1s2s 3S1 (

4He*)F= 1/2,3/2 (3He*)

Magnetic sublevelsfor both isotopes asa function of position in the cloverleaf trap after compression

Nc>107

N=2×106

T/TF=0.45

The setup

Boson – Fermion Quantum Phase Separation

Optical Dipole TrapFeshbach Resonances

in situ spatial separation of fermions and bosons

depends on fermion-boson and boson-boson scattering length

a44 and especially a34 are large: good for the Helium case (B=0)

proposed detection: in situ imaging in magnetic trap

Very low quantum efficiency of Si at 1083 nm

disturbances under investigation

Laser: Er-fiber 2 W 1557 nm

Trap depth 24 K

Never implemented for Helium

Access to Feshbach resonances

Allows fast trap switching: better absorption imaging

Starting point for optical lattice experiments

DC discharge

Collimation / Deflection

Tuning of scattering length by changing the magnetic field

Predictions are (being) made for boson-fermion resonances in Helium and fermion-fermion resonances (different mF states)

No resonances have (yet) been confirmed experimentally for Helium

Scattering length tuning changes phase-separation behaviour

Fermion – Fermion interactions

Not possible in the s-wave regime due to large PI losses when a two-state mixture is used

For spin-polarized gas possible when |MF|<3/2 in case of Feshbach resonance: possible observation of fermionic molecules and BEC of those molecules when a>0 and, when a<0, p-wave pairing.

Only accessible in optical trap

BEC BCS

a > 0rotating molecules

a → ±∞interacting pairs

a < 0cooper pairs

B (G)

E (

GH

z)

3He density profile for NF = NB = 106

Calculated by T. Tiecke (UvA) and S. Kokkelmans (TUe)

Calculated by finding the self-consistent solution to

With ,

the boson-boson and fermion-boson interaction strength, respectively.

Feshbach Resonance at 10G

4He: (J,MJ)=(1,-1)3He: (F,MF)=(3/2,-1/2)

nF

/ 1

01

2

cm-3

z / μm

r / μm