Post on 25-Feb-2016
description
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)