D. Jin JILA, NIST and the University of Colorado $ NIST, NSF Using a Fermi gas to create Bose-Einstein condensates Outline I.Intro and motivation a)A little quantum physics b)Basics of the experiment II.Interactions - An amazing new knob a)Experimental demonstration b)Implications (more motivation) III.Condensates of correlated fermion pairs Outline I.Intro and motivation a)A little quantum physics b)Basics of the experiment II.Interactions - An amazing new knob a)Experimental demonstration b)Implications (more motivation) III.Condensates of correlated fermion pairs Quantum Gases high T low T deBroglie d classical behaviorquantum behavior 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: 87 Rb, 23 Na, 7 Li, H, 39 K, 4 He*, 85 Rb, 133 Cs fermions: 40 K, 6 Li Quantum Particles Bosons and Fermions half-integer spin other fermions: protons, electrons, neutrons, liquid 3 He integer spin T = 0 Atoms in a harmonic potential. Bose-Einstein condensation 1995 other bosons: photons, liquid 4 He Fermi sea of atoms 1999 E F = k b T F (two spin states) Ultracold atomic gases low density n ~ cm -3 N~10 6 ultralow T ~ 100 n K 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. Excitons, biexcitons in semiconductors Cooper pairs of electrons in superconductors 4 He atoms in superfluid liquid He 3 He atom pairs in superfluid 3 He-A,B Neutron pairs, proton pairs in nuclei and neutron stars Mesons in neutron star matter Alkali atoms in ultracold atom gases Condensates with Fermions? Condensation requires bosons. Material bosons are composite particles, made up of fermions. Starting with a gas of bosonic atoms, you can only explore the behavior of bosons. 87 Rb, 23 Na, By starting with a gas of fermionic atoms we can explore the behavior of fermions AND BOSONS. 40 K, 6 Li, Cooling a gas of atoms 1. Laser cooling and trapping 2.Magnetic trapping and evaporative cooling 300 K to 1 mK 10 9 atoms 1 mK to 1 K 10 8 10 6 atoms spin 1 spin 2 3.Optical trapping and evaporative cooling 4.Probing the atoms Cooling a gas of atoms 1 K to 50 nK 10 6 10 5 atoms can confine any spin-state can apply arbitrary B-field Quantum degeneracy velocity distributions T/T F =0.77 T/T F =0.27 T/T F =0.11 Fermi sea of atoms EFEF EFEF n 0 = 0.28 n 0 = n 0 = Outline I.Intro and motivation a)A little quantum physics b)Basics of the experiment II.Interactions - An amazing new knob a)Experimental demonstration b)Implications (more motivation) III.Condensates of correlated fermion pairs 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 C. A. Regal and D. S. Jin, PRL 90, (2003) repulsive attractive spectroscopic measurement of the mean-field energy shift Magnetic-field Feshbach resonance R V(R) R R R a0, repulsive Magnetic-field Feshbach resonance R V(R) R R R a0, repulsive molecules BB > atoms Turning atoms into molecules Ramp across Feshbach resonance from high to low B The atoms reappear if we sweep back to high B energy B up to 90% conversion to molecules! molecules are extremely weakly bound ! molecules can survive many collisions ! Bosonic molecules Interesting regime Theory: D.S. Petrov et al., cond-mat/ , Expts: Rice, ENS, Innsbruck, JILA rf photodissociation C. Regal et al. Nature 424, 47 (2003) BEC of diatomic molecules BCS superconductivity/superfluidity Something in between? Making condensates with fermions 1. Bind fermions together. 2. BEC Condensation of Cooper pairs of atoms (pairing in momentum space, near the Fermi surface) EFEF spin spin BCS-BEC crossover (generalized Cooper pairs) BCS-BEC landscape energy to break fermion pair transition temperature BEC BCS superfluid 4 He alkali atom BEC high T c superconductors superfluid 3 He superconductors M. Holland et al., PRL 87, (2001) interactions Outline I.Intro and motivation a)A little quantum physics b)Basics of the experiment II.Interactions - An amazing new knob a)Experimental demonstration b)Implications (more motivation) III.Condensates of correlated fermion pairs Magnetic-field Feshbach resonance molecules attractive repulsive BB > free atoms repulsive Changing the interaction strength in real time molecules attractive BB > EFEF 2 s/G : FAST Changing the interaction strength in real time: SLOW molecules attractive BB > EFEF 40 s/G Changing the interaction strength in real time: SLOWER molecules attractive BB > EFEF 4000 s/G Cubizolles et al., PRL 91, (2003); L. Carr et al., cond-mat/ Molecular Condensate M. Greiner, C.A. Regal, and D.S. Jin, Nature 426, 537 (2003). Time of flight absorption image initial T/T F : repulsive 40 s/G Observing a Fermi condensate attractive BB > EFEF ? 4000 s/G ? Condensates w/o a two-body bound state C. Regal, M. Greiner, and D. S. Jin, PRL 92, (2004) Dissociation of molecules at low density B = 0.12 G B = 0.25 G B=0.55 G T/T F =0.08 B (gauss) Fermionic condensate Clearly see condensation on the atom-side of the resonance! T/T F =0.08 molecules atoms two-body molecules pairing due to many-body effects BCS-BEC Crossover BCS (atoms) BEC (molecules) N 0 /N 0 C. Regal, M. Greiner, and D. S. Jin, PRL 92, (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