Martin Zwierlein TOPS, MIT, Cambridge, June 24 th, 2009 Pairs and Loners in Ultracold Fermi Gases Massachusetts Institute of Technology Center for Ultracold Atoms at MIT and Harvard $$$: NSF, AFOSR- MURI, Sloan Foundation

EFEF Bosons vs Fermions e.g.: 1 H, 23 Na, 6 Li 2 e.g.: e -, 3 He, 6 Li, 40 K

Degenerate gases Want lifetime > 1sUltradilute Ultracold de Broglie wavelength ~ Interparticle spacing Good news: Bosons condense at

Effusive beam How to measure temperature?

Effusive beam How to measure temperature?

Atom cloud Lens CCD Camera Laser beam Observation of the atom cloud Shadow image of the cloud TrappedExpanded 1 mm

BEC phase transition MIT, 1995 (Sodium)

JILA, Juni ‘95 (Rubidium) MIT, Sept. ‘95 (Sodium) Superfluidity in Bosonic Gases BEC 1995 All atoms occupy same macroscopic wavefunction MIT Phase coherence 1997 JILA ENS MIT Superfluidity 1999/2000 Frictionless flow, quantized vorticity

Fermions – The Building Blocks of Matter Harvard-Smithsonian Center for Astrophysics Lithium-6

Can we have superfluidity in a Fermi gas?

1911: Discovery of Superconductors Heike Kamerlingh-Onnes Discovery of Superconductivity in Metals Resistance Temperature Nobel prize 1913

No energy loss persistent flow Doesn’t want to rotate No energy loss persistent currents expels magnetic fields Flow without frictionCurrent without resistance Onnes 1908, Kapitza, Allen & Misener 1938 Onnes 1911 Müller & Bednorz 1987 SuperconductorsSuperfluids

What are superconductors? Apparently the electrical current flows without friction But: Carrier of electrical current are Electrons  Electrons are Fermions

What are superconductors? Apparently the electrical current flows without friction But: Carrier of electrical current are Electrons  Electrons are Fermions L. Cooper (1956) (45 years after Onnes) : Pairing of electrons Pairs are Bosons Superconductivity: Condensation of Electron Pairs J. Bardeen, L. Cooper, R. Schrieffer (BCS), 1957, Nobel prize 1972

Fermionic Superfluidity Superconductors: Charged superfluids of electron pairs Frictionless flow  Resistance-less current Condensation of Fermion Pairs John BardeenLeon N. CooperJohn R. Schrieffer

High-temperature Superconductors J. Georg BednorzK. Alex Müller Nobel prize 1987 Critical temperature: 35 K above Absolute Zero (-238 °C) Record today: 138 K (-135 °C)

Room temperature superconductors? Today: ~5-10% energy loss only due to transport of energy The problem: High-temperature superconductivity not really understood Electrons interact so strongly that it’s hard to model The hope: Superconducting cables No resistance  No energy loss during transport We need: A model system for superconductors Ultracold atomic gases

Can we do this with atoms? YES! The ultracold Fermi gas at MIT: Lithium-6 (3p, 3n, 3e - ) is a fermion The atoms form pairs like electrons in a superconductor Size of pairs is freely controllable The gas becomes superfluid

How can you distinguish a superfluid from a normal one?

Rotating buckets Rotating bucket Normal Super Fluid

Rotating superfluid Superfluids are described by matter wave The wave has to close in itself (Example: Vibrating rubber band) Superfluid does not want to rotate Only possibility: Vortices, “Mini-Tornados”, “Quantum whirlpools” Only full wavelengths are allowed Circulation is only possible in certain units (“Quanta”), carried by the Vortices

Vortex structure Look from top into the bucket

Vortex structure Abrikosov lattice (honeycomb lattice) Look from top into the bucket Aleksei A. Abrikosov Nobel prize 2003

Vortex lattices in bosonic gases/fluids ENS (J. Dalibard, 2000) Rubidium BEC Rubidium BEC Berkeley (R.E. Packard, 1979) Helium-4

U. Essmann and H. Träuble, Physics Letters A, 24, 526 (1967) Rotation of a neutral Fluid Coriolis Force Superconductor in a magnetic field Lorentz Force

Demonstration of superfluidity in a Fermi gas Ultracold gas

Vortices in the BEC-BCS Crossover Vortex lattices M.W. Zwierlein, J.R. Abo-Shaeer, A. Schirotzek, C.H. Schunck, W. Ketterle, Nature 435, (2005) B Demonstration of superfluidity in a gas of atom pairs A high-temperature superfluid Pair size Scaled to the density of electrons in a metal, the gas would become superfluid far above room temperature

What if there are too many singles? Fermionic Superfluidity with Imbalanced Spin Populations

94%90%56%30%22%12%6% Fermionic Superfluidity with Imbalanced Spin Populations |2> 0% |1>

What is the Nature of the Imbalanced State?

Cooling Down Direct observation of the density difference Y. Shin, M.W. Zwierlein, C.H. Schunck, A. Schirotzek, W. Ketterle, PRL 97, (2006) SuperfluidNormal

Reconstruction of 3D density profile Only assumption: cylindrical symmetry Phase Separation !  = 0.6 Fermionic Superfluidity does not tolerate loners

Atomic Bose-Einstein Condensates (Sodium) Molecular Bose-Einstein Condensates ( 6 Li 2 ) Pairs of fermionic atoms ( 6 Li) Gallery of superfluid Gases

Ultracold Atoms As Model systems: How does matter work? new quantum states, development of new materials Quantum computer, Quantum simulators (Bose and Fermi gases) As measuring device: Development of highly sensitive sensors gravitational gradient sensors (important for mining, geophysics), sensors for navigation New highly accurate atomic clocks as time standard basis of all GPS-systems, more accurate positioning, faster telecommunication requires accurate clocks

The team BEC 1: Andre Schirotzek Ariel Sommer Fermi 1: Cheng-Hsun Wu Ibon Santiago Dr. Peyman Ahmadi Undergraduates: Caroline Figgatt Jacob Sharpe Sara Campbell Kevin Fischer 39 K 40 K 6 Li