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=1/2 I=1/2 I=0 I=0 The Experiment: Ultracold Helium Helium-3 / Helium-4 mixture is excited to the 2 3 S 1 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 3 He* is sympathetically cooled with 4 He* +1 +1 0 0 -1 -1 +3/2 +3/2 -1/2 -1/2 -3/2 -3/2 3 He* He* 4 He* He* 8.4 MHz 8.4 MHz +1/2 +1/2 5.6 5.6 MHz MHz • 2 3 S 1 state (He*): = 8000 s, • 20 eV internal energy: single He* atom detection • Penning ionization: He + ( He* + He* → He + He + + e ─ ) • 3 He* fermion and 4 He* boson • Scattering lengths large and positive: a BB =+7.5105(25) nm (exp. ENS Paris) a FB =+27.1 (5) nm (theory Warsaw) 4 He* 3 He * + 4 He * 3 He* 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 T F . In the lower figure the dashed- dotted line shows the BEC contribution to the signal and the dashed line the DFG contribution. ground state: 1s 2 1 S 0 metastable state: 1s2s 3 S 1 ( 4 He*) F= 1/2,3/2 ( 3 He*) Magnetic sublevels for both isotopes as a function of position in the cloverleaf trap after compression N c >10 7 N=2×10 6 T/T F =0.45 The setup Boson – Fermion Quantum Phase Separation Optical Dipole Trap Feshbach Resonances in situ spatial separation of fermions and bosons depends on fermion-boson and boson-boson scattering length a 44 and especially a 34 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 m F 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 |M F |<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 > 0 rotating molecules a → ±∞ interacting pairs a < 0 cooper pairs B (G) E (GHz) 3 He density profile for N F = N B = 10 6 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 4 He: (J,M J )=(1,-1) 3 He: (F,M F )=(3/2,-1/2) n F / 10 12 cm -3 z / μm r / μm

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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

-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 (

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.