Martin Zwierlein

TOPS, MIT, Cambridge, June 24th, 2009

Pairs and Loners inUltracold Fermi Gases

Massachusetts Institute of Technology

Center for Ultracold Atoms at MIT and Harvard

$$$: NSF, AFOSR- MURI, Sloan Foundation

EF

Bosons vs Fermions

e.g.: 1H, 23Na, 6Li2 e.g.: e-, 3He, 6Li, 40K

CT T

CT T

0T

Degenerate gases

Want lifetime > 1s Ultradilute

Ultracold

de Broglie wavelength ~ Interparticle spacing

Good news: Bosons condense at

1/3n1/3

dB n

15 310 cmn 2

2/3 1 KFB

T nk m

C FT T

How to measure temperature?

Gas Effusive atomic beam

How to measure temperature?

Gas Effusive atomic beam

Atom cloudLens

CCDCamera

Laser beam

Observation of the atom cloud

Shadow imageof the cloud

TrappedExpanded

1 mm

BEC @ MIT, 1995 (Sodium)

BEC @ JILA, Juni ‘95(Rubidium)

BEC @ MIT, Sept. ‘95 (Sodium)

Superfluidity in Bosonic Gases

• BEC 1995All atoms occupy same

macroscopic wavefunction

MIT

• Phase coherence 1997

JILA

ENS

MIT• Superfluidity 1999/2000Frictionless flow,

quantized vorticity

Fermions – The Building Blocks of Matter

Harvard-Smithsonian Center for Astrophysics

Lithium-6

Can we have superfluidityin a Fermi gas?

1911: Discovery of Superconductors

HeikeKamerlingh-Onnes

• Discovery of Superconductivity in Metals

Res

ista

nce

TemperatureNobel prize 1913

• No energy loss• persistent flow• Doesn’t want to rotate

• No energy loss• persistent currents• expels magnetic fields

Flow without friction Current without resistance

Onnes 1908,Kapitza, Allen & Misener 1938

Onnes 1911Müller & 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 electronsPairs 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 pairsFrictionless flow Resistance-less current

Condensation of Fermion Pairs

John Bardeen Leon N. Cooper John R. Schrieffer

High-temperature Superconductors

J. Georg Bednorz K. 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 likeelectrons in a superconductor

• Size of pairs isfreely controllable

• The gas becomes superfluid

How can you distinguish a superfluid from a normal one?

Rotating bucket

Normal Super

Fluid 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 certainunits (“Quanta”), carried by the Vortices

Look from top into the bucket

Abrikosov lattice (honeycomb lattice)

Look from top into the bucket

Aleksei A.Abrikosov

Nobel prize2003

Vortex latticesVortex latticesin bosonic gases/fluidsin bosonic gases/fluids

ENSENS(J. Dalibard, 2000)(J. Dalibard, 2000)

Rubidium BECRubidium BEC

BerkeleyBerkeley(R.E. Packard, 1979)(R.E. Packard, 1979)

Helium-4Helium-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

2F mv

F qv B

Demonstration of superfluidity in a Fermi gas

Ultracold gas

Vortex lattices

M.W. Zwierlein, J.R. Abo-Shaeer, A. Schirotzek, C.H. Schunck, W. Ketterle,Nature 435, 1047-1051 (2005)

- 0.7

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 gaswould become superfluid far above room temperature

What if there are too many singles?

Fermionic Superfluidity withImbalanced Spin Populations

94%90%56%30%22%12%6%

Fermionic Superfluidity with Imbalanced Spin Populations

|2>

0%

|1>

What is the Nature of theImbalanced State?

Cooling Down

Direct observation of the density difference

Y. Shin, M.W. Zwierlein, C.H. Schunck, A. Schirotzek, W. Ketterle,PRL 97, 030401 (2006)

SuperfluidNormal

Reconstruction of 3D density profile

Only assumption: cylindrical symmetry

Phase Separation !

= 0.6

Fermionic Superfluidity does not tolerate loners

Atomic Bose-EinsteinCondensates (Sodium)

Molecular Bose-EinsteinCondensates (6Li2)

Pairs of fermionic atoms(6Li)

Gallery of superfluid Gases

Ultracold Atoms

As Model systems:• How does matter work?

new quantum states, development of new materialsQuantum 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 standardbasis of all GPS-systems, more accurate positioning, faster telecommunication requires accurate clocks

The teamBEC 1:

Andre SchirotzekAriel Sommer

Fermi 1:Cheng-Hsun Wu

Ibon SantiagoDr. Peyman Ahmadi

Undergraduates:Caroline FiggattJacob SharpeSara CampbellKevin Fischer

39K 40K 6Li