Using a Fermi gas to create Bose-Einstein condensates

Using a Fermi gas to create Bose-Einstein condensates D. Jin JILA, NIST and the University of Colorado $ NIST, NSF

Outline • Intro and motivation • A little quantum physics • Basics of the experiment • Interactions - An amazing new knob • Experimental demonstration • Implications (more motivation) • Condensates of correlated fermion pairs

Quantum Gases high T low T classical behavior quantum behavior ldeBroglie»d matter waves

There are two types of quantum particles found in nature - • bosons and fermions. • Bosons like to do the same thing. • Fermions are independent-minded. • Atoms, depending on their composition, can be either.bosons: 87Rb, 23Na, 7Li, H, 39K, 4He*, 85Rb, 133Cs fermions: 40K, 6Li Quantum Particles

Atoms in a harmonic potential. EF= kbTF Fermi sea of atoms Bose-Einstein condensation 1999 1995 Bosons and Fermions • half-integer spin • integer spin T = 0 (two spin states) other fermions: protons, electrons, neutrons, liquid 3He other bosons: photons, liquid 4He

Ultracold atomic gases • low density n ~ 1013 – 1014 cm-3 N~106 • ultralow T ~ 100 nK • amenable to theoretical analysis • unique experimental control • dramatic detection of condensation

Bose-Einstein condensation BEC shows up in condensed matter, nuclear physics, elementary particle physics, astrophysics, and atomic physics. Cooper pairs of electrons in superconductors 4He atoms in superfluid liquid He Excitons, biexcitons in semiconductors Alkali atoms in ultracold atom gases 3He atom pairs in superfluid 3He-A,B Neutron pairs, proton pairs in nuclei and neutron stars Mesons in neutron star matter

Starting with a gas of bosonic atoms, you can only explore the behavior of bosons. 87Rb, 23Na, … • By starting with a gas of fermionic atoms we can explore the behavior of fermions AND BOSONS. 40K, 6Li, … Condensates with Fermions? • Condensation requires bosons. • Material bosons are composite particles, made up of fermions.

Laser cooling and trapping • Magnetic trapping and evaporative cooling 1 mK to 1 mK 108→ 106 atoms Cooling a gas of atoms 300 K to 1 mK 109 atoms spin 2 spin 1

Optical trapping and evaporative cooling • Probing the atoms 1 mK to 50 nK 106→ 105 atoms Cooling a gas of atoms • can confine any spin-state • can apply arbitrary B-field

EF Quantum degeneracy velocity distributions EF T/TF=0.77 T/TF=0.27 T/TF=0.11 n0= 0.28 n0= 0.944 n0= 0.99984 Fermi sea of atoms

Interactions • Interactions are characterized by the • s-wave scattering length, a • In an ultracold atomic gas, we can control a! a > 0 repulsive, a < 0 attractive Large |a| → strong interactions  0 scattering length

Magnetic-field Feshbach resonance • spectroscopic measurement of the mean-field energy shift repulsive attractive C. A. Regal and D. S. Jin, PRL 90, 230404 (2003)

R R R R Magnetic-field Feshbach resonance V(R) a>0, repulsive a<0, attractive

R R R R Magnetic-field Feshbach resonance V(R) a>0, repulsive a<0, attractive atoms → ← DB > molecules

energy → ← B Turning atoms into molecules Ramp across Feshbach resonance from high to low B The atoms reappear if we sweep back to high B • up to 90% conversion to molecules!

Bosonic molecules • molecules are extremely weakly bound ! • molecules can survive many collisions ! Interesting regime rf photodissociation C. Regal et al. Nature 424, 47 (2003) Theory: D.S. Petrov et al., cond-mat/0309010, Expts: Rice, ENS, Innsbruck, JILA

EF spin  spin  Making condensates with fermions • BEC of diatomic molecules • BCS superconductivity/superfluidity • Something in between? 1. Bind fermions together. 2. BEC Condensation of Cooper pairs of atoms (pairing in momentum space, near the Fermi surface) BCS-BEC crossover (“generalized Cooper pairs”)

alkali atom BEC superfluid 4He high Tc superconductors superfluid 3He superconductors BCS-BEC landscape M. Holland et al., PRL 87, 120406 (2001) BEC transition temperature BCS interactions energy to break fermion pair

→ ← Magnetic-field Feshbach resonance repulsive free atoms DB > attractive molecules

EF Changing the interaction strength in real time : FAST repulsive 2 ms/G DB > attractive molecules

EF Changing the interaction strength in real time: SLOW 40 ms/G DB > attractive molecules

EF Changing the interaction strength in real time: SLOWER 4000 ms/G DB > attractive molecules Cubizolles et al., PRL 91, 240401 (2003); L. Carr et al., cond-mat/0308306

Molecular Condensate initial T/TF: 0.19 0.06 Time of flightabsorption image M. Greiner, C.A. Regal, and D.S. Jin, Nature 426, 537 (2003).

? 4000 ms/G EF ? Observing a Fermi condensate repulsive 40 ms/G DB > attractive

Condensates w/o a two-body bound state Dissociation of molecules at low density C. Regal, M. Greiner, and D. S. Jin, PRL 92, 040403 (2004) DB (gauss) DB = 0.12 G DB = 0.25 GDB=0.55 G T/TF=0.08

two-body molecules pairing due to many-body effects Fermionic condensate T/TF=0.08 molecules atoms  • Clearly see condensation on the “atom-side” of the resonance!

N0/N 0 BCS-BEC Crossover BCS (atoms) BEC (molecules)  C. Regal, M. Greiner, and D. S. Jin, PRL 92, 040403 (2004)

Conclusion • An atomic Fermi gas provides experimental access to the BCS-BEC crossover region. • Fermi gas ↔ molecular BEC interconversion has been explored. • Condensates of correlated fermionic atom pairs have been achieved ! • generalized “Cooper pairs” with strong interactions Many opportunities for further experimental and • theoretical work ... Next…

Current group members: M. Greiner J. Goldwin S. Inouye C. Regal J. Smith M. Olsen

Using a Fermi gas to create Bose-Einstein condensates