Ultracold Atoms, Mixtures, and Molecules

Subhadeep Gupta

UW Physics 528, 25 Feb 2011

Ultracold AtomsHigh sensitivity (large signal to noise, long interrogation times in a well

known atomic system)Precision measurements (spectroscopy, fund sym, clocks)Sensing (accelerations, gravity gradients)

Many-body aspectsQuantum FluidsCondensed Matter PhysicsNuclear Physics

Quantum Engineering (Potentials and Interactions)Quantum Information ScienceQuantum SimulationUltracold Molecules (through mixtures)

Formation of a Bose-Einstein condensate (BEC)

Ultracold Atoms and Molecules at UW

A dual-species experiment (Li-Yb) formaking and studying ultracold polar molecules

and for probing strongly interacting Fermi gases

Development of precision BEC interferometry (Yb) for fine structure constant a and test of QED

Quantum Degeneracy in a gas of atoms1 atom per quantum state

Number of atoms =(available position space) (available momentum space)

h3

n(m kB T)3/2

h3 ~ 1

Dilute metastable gases n ~ 1014/cm3

Tc ~ 1mK !! Ultracold !!

Air n ~ 1019/cm3, Tc ~ 1mKStuff n ~ 1022/cm3, Tc ~ 0.1KEverything (except He) is solid

and ~ non-interacting

N atomsV volumeT temperature

(Dx)3 ~ V (Dp)3 ~ (m kB T)3/2

Quantum PhaseSpace Density

(n=N/V)

room temp.liquid Nitrogen

3He-4He dilution

BCS superconductors, 4He superfluid, CMB fractional quantum hall effect

dilute B E C

surface of sun

Joule-Thompson expansion

103

10-3

100

10-6

superfluid 3He

adiabatic demagnetization

Lase

r coo

ling

Evap

orat

ive

cool

ing

Dept

h of

ato

m tr

aps

atom

ic in

tera

ctio

ns

dilute Bp-wave threshold

ABSOLUTE TEMPERATURE (log Kelvin scale)

Zero point energy10-9

Degenerate Gas

emabs

=> COOLING !

S

P

Laser Cooling

(Need a 2 level system)

Magneto-Optical Trap (MOT)“Workhorse” of laser cooling

x

y

z

s+

s+

s+

s-

s-s-

I

I

Atom Source ~ 600 K; UHV environment

Evaporative Cooling in a Conservative Trap

V2 -1/2 V

V 0

pre collisionpost collision

mK

Optical Dipole TrapL << res

Depth ~ I/D; Heating Rate ~ I/D2

V2 -1/2 V

V 0

pre collisionpost collision

Optical Dipole TrapL << res

Depth ~ I/D; Heating Rate ~ I/D2

mK

Evaporative Cooling in a Conservative Trap

Imaging the Atoms

(Formation of a Rb BEC, UC Berkeley 2005)

Landmark achievements in ultracold atomic physics

Bose-Einstein condensation (JILA, MIT, Rice….)

Macroscopic coherence (Ketterle)

Superfluidity / observation and study of a vortex lattice (Dalibard, Ketterle, Cornell)

Superfluid to Mott-insulator quantum phase

transition (Hansch)

Degenerate Fermi gas

Molecular Bose-Einstein condensate

Superfluidity of Fermi pairs

(Jin, Hulet, Thomas, Ketterle, Grimm)

Landmark achievements in ultracold atomic physics

Ultracold Polar Molecules

Realization of new quantum gases based on precise particle manipulation and strong, long-range, anisotropic, interparticle dipole-dipole interactions (1/r3 vs 1/r6 “contact” potential)

Quantum Computing and Simulation

Enhanced electric dipole moment sensitivity for tests of fundamental symmetries

Precision Molecular Spectroscopy for clocks, time variations of fundamental constants, others.

Cold and ultracold controlled Chemistry

Polar (diatomic) Molecules from Ultracold Atoms

Unequal sharing of electronsPolarizable at relatively low field

New degrees of freedom bring with them scientific advantages New degrees of freedom bring with them technical issues

Polar (diatomic) Molecules from Ultracold Atoms

Ultracold Atom Menu

Very different mass, very different electronic structure strong dipole moment

Selected Molecular Constituents

Various species-selective manipulation methods

Interaction tuning through Feshbach resonances

Yb as an impurity probe of the Li Fermi superfluid.

Large mass difference mixture of interacting fermions

Photo- and magneto-association as starting point for making diatomics

Several Fermi/Bose isotopes available

Selected Molecular Constituents

Large difference in electronic structure and mass Large electric dipole moment and strong, anisotropic interactions

Bosonic or Fermionic Molecules can be formed

Paramagnetic + Diamagnetic Atom Molecular ground state will also have magnetic moment. Can be magnetically manipulated and trapped.

Paramagnetic ground state, heavy component candidate for electron EDM search

Quantum computing candidate

Dual Species Apparatus

Li Oven

Li ZeemanSlower

Yb Oven

Yb ZeemanSlower

Main Chamber

PumpArea

Ultrahigh Vacuum(UHV < 10-10 Torr)

AGENDACool and trap Li and Yb

Study interacting mixtureInduce controlled interactions

Form molecules

2-species Magneto-Optic trap

Sequential LoadingYtterbium – green, Lithium - red

The 2 MOTs are optimized at different parameters ofmagnetic field gradient and also exhibit inelastic interactions

Absorption image of optically trapped lithium atomstogether with uncaptured atoms released from a MOT

Optical Dipole Trap

Evaporative Cooling of Bosonic 174Yb

With forced evaporative cooling, cancool towards quantum degeneracys-wave scattering length = 5.5nm

20

20 )(2)()( tt

mkTtrtr

Temperature evaluated from size of absorption imaged atoms after variable time of flight

Strong Interactions in Fermionic Lithium

“Feshbach molecule” formationFeshbach resonancebetween |1> and |2>

Collisional Properties of the LiYb Mixture

Elastic interactions dominate forming a stable mixture. From the timescale of the thermalization process, we extract the interspecies collisional cross-section and s-wave scattering lengthmagnitude of 13 bohrs. Establishment of a new two-species mixture.

YtterbiumLithium

Sympathetic Cooling of Lithium by Ytterbium

T/TF = 0.7

With improvements to coolingprocedures (in progress), double degeneracy should be possible

Ultracold Molecules through PhotoAssociation

Scan

Trap loss measurement in the 6Li system

Provides information on excited potentialFirst step towards ground LiYb molecule

Future Li-Yb Work

UltracoldStable Mixture Simultaneous degeneracy

Yb as probe of Feshbach moleculesYb as probe of Fermi superfluidInduce strong Li-Yb interactions

Scan

2 photon PA +2 photon Raman

Ground state polar molecules through multiple2-photon processes,

Photoassociation in LiYb system

-250 -200 -150 -100 -50 0 50 100 150 200

– 1 in ppb

atomic physics route

a98-1a -1æ

èç

ac Josephson effect

neutron interferometryh/mn

Quantum Hall effect

g-2 of electron + QED

2 1137

ec

a

Cross-field comparisonsPrecision test of QED

Precision Measurement of Fine Structure Constant a

(2000)

(2008)

motivation2

222 2 2

ee

e R hc c m

hcR

MM

ma æ

ç è

QED-free Atomic Physics Route to a

0.008 ppb: hydrogen spectroscopy, (Udem et al.,1997; Schwob et al.,1999)

0.7 ppb: penning trap mass spectr. (Beier et al., 2002)

2rec

12 m

k

Penning trap mass spectroscopy

Cs (Berkeley) Rb (Paris)

-

BEC is a bright coherentsource for atom interferometry

Frequency comb

principle2Contrast Interferometry with a BEC

( ( ( ( 2r

2 1ec e2 c

3r

( ) ( )sin ( ), , , 82

sin 4S T t C T t C T tt t t T t æ ç

è D

The phase of the matter wave grating is encoded in oscillating contrast.

The phase of the contrast signal for various T gives rec.

Advantages:

• no sensitivity to mirror vibrations, ac stark shift, rotation, magnetic field gradients

• quadratic enhancement with additional momenta (x N2)

• single shot acquisition of interference pattern

principle2

( ( ( ( 2r

2 1ec e2 c

3r

( ) ( )sin ( ), , , 82

sin 4S T t C T t C T tt t t T t æ ç

è D

The phase of the matter wave grating is encoded in oscillating contrast.

The phase of the contrast signal for various T gives rec.

Na BEC2002

Contrast Interferometry with a BEC

principle2Na BEC

2002

With Na BEC experiment7ppm precision achieved,but inaccuracy at 200ppm,

attributed to mean field.

Contrast Interferometry with a BEC

Contrast Interferometry with a BEC

Scale Up

Increase sensitivity by increasing momentaof 1 and 3 by 20-fold and increasing T

We then expect sub-ppb precision in less than a day of data.Use of a Yb BEC – no B field sensitivity and multiple isotopes for systematic checks

At this level, have to very carefully account for atomic interactions and the meanfield shift. Theoretical analysis performed in collaboration with Nathan Kutz (AMath).Gearing up for the experiment - expect the next generation result in 2-3 years.

$$$ - NSF, Sloan Foundation, UW RRF, NIST

UW Ultracold Atoms Team

Undergrad Students: Jason Grad, Jiawen Pi, Eric Lee-WongGrad Students: Anders Hansen, Alex Khramov, Will Dowd, Alan JamisonPost-doc: Vladyslav Ivanov

One- and Two- Electron Atoms

6Lithium

Yb (Crossed)Beam Spectroscopy

Freq [MHz]

Freq [MHz]

Fluo

r [ar

b]

Fluo

r [ar

b]

Magneto-Optical Trapping of Li

few x 108 atoms loaded in a few seconds

Load time (s)

Fluo

r (ar

b)

1.2

1.1

1.0

0.9

8006004002000

Expansion time (ms)

Siz

e (m

m)

Temp ~ 400mK

Ultracold Mixtures of Lithium and Ytterbium Atoms

Preliminary Evidenceof Elastic Interactions

Ultracold AtomsHigh sensitivity (large signal to noise, long interrogation times in a well

known atomic system)Precision measurements (spectroscopy, fund sym, clocks)Sensing (accelerations, gravity gradients)

Many-body aspectsQuantum FluidsCondensed Matter PhysicsNuclear Physics

Quantum Engineering (Potentials and Interactions)Quantum Information ScienceQuantum SimulationUltracold Molecules (through mixtures)

Some major achievements in ultracold atomic physics

Bose-Einstein condensation (JILA, MIT, Rice….)

Macroscopic coherence

Superfluidity / observation and study of a vortex lattice Degenerate Fermi

gas

Evaporative Cooling of Fermionic Lithium

vacuum limitedlifetime ~ 40 secs

Evaporative Cooling of Bosonic 174Yb

With forced evaporation, can reach sub-microKelvinHere, temp X 2 above degeneracy for 50,000 atoms

Side view ofreleased YbN ~ 50,000

Plain evaporation at B=0N ~ up to 106 (here ~ 3 x 105)

1 inch

Dual Species Apparatus

Optical Dipole Trap

Shallow angle (10 degrees)crossed beam dipole trap1064nm, up to 50 Watts

Peng Zhang, ITAMP Harvard

LiYb Molecule: Theory

~ 0.4 DebyeElectric dipole moment

Svetlana Kotochigova,Temple Univ/ NIST preliminary results

DF

Fermi Degeneracy in 6Li

Size gets clamped by the Fermi Diameter

Top view ofTrapped Li With evaporation

at 300 G, can achieve T/TF ~ 0.5

TF T (calc)

T (meas)