Transcript Quantum Polar Molecular Gases

Ultracold Atoms, Mixtures, and Molecules
Subhadeep Gupta
UW Physics 528, 25 Feb 2011
Ultracold Atoms
High 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 aspects
Quantum Fluids
Condensed Matter Physics
Nuclear Physics
Quantum Engineering (Potentials and Interactions)
Quantum Information Science
Quantum Simulation
Ultracold Molecules (through mixtures)
Formation of a BoseEinstein condensate (BEC)
Ultracold Atoms and Molecules at UW
A dual-species experiment (Li-Yb) for
making 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 atoms
1 atom per quantum state
N
V
T
atoms
volume
temperature
(Dp)3 ~ (m kB T)3/2
(Dx)3 ~ V
Number of atoms =
Quantum Phase
Space Density
(available position space) (available momentum space)
h3
n h3
(m kB T)3/2
Air n ~ 1019/cm3, Tc ~ 1mK
Stuff n ~ 1022/cm3, Tc ~ 0.1K
Everything (except He) is solid
~ 1
(n=N/V)
Dilute metastable gases n ~ 1014/cm3
Tc ~ 1mK !! Ultracold !!
and ~ non-interacting
3
10
0
room temp.
liquid Nitrogen
Evaporative cooling
3 He- 4 H e dilution
10
-3
10
-6
BCS superconductors, 4He superfluid, CMB
fractional quantum hall effect
s uperfluid 3 H e
dilute
B threshold
p-wave
adiabatic demagnetiz ation
10
-9
dilute
B E C Gas
Degenerate
Zero point energy
atomic interactions
J oule-T homps on expans ion
10
s urface of s un
Depth of atom traps
Laser cooling
ABSOLUTE TEMPERATURE (log Kelvin scale)
Laser Cooling
z
I
s-
P
ss+
x
s-
s+
y
s+
I
S
Magneto-Optical Trap (MOT)
“Workhorse” of laser cooling
Atom Source ~ 600 K; UHV environment
abs  em
=> COOLING !
(Need a 2 level system)
Evaporative Cooling in a Conservative Trap
V
2 -1/2 V
Optical Dipole Trap
L << res
Depth ~ I/D; Heating Rate ~ I/D2
mK
pre collision
post collision
V0
Evaporative Cooling in a Conservative Trap
V
2 -1/2 V
Optical Dipole Trap
L << res
Depth ~ I/D; Heating Rate ~ I/D2
mK
pre collision
post collision
V0
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)
Landmark achievements in ultracold atomic physics
Degenerate Fermi gas
Molecular Bose-Einstein condensate
(Jin, Hulet, Thomas, Ketterle, Grimm)
Superfluidity of Fermi pairs
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
Polar (diatomic) Molecules from Ultracold Atoms
New degrees of freedom bring with them scientific advantages
New degrees of freedom bring with them technical issues
Unequal sharing of electrons
Polarizable at relatively low field
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
Yb Oven
Yb Zeeman
Slower
Pump
Area
Ultrahigh Vacuum
(UHV < 10-10 Torr)
Main Chamber
Li Zeeman
Slower
Li Oven
AGENDA
Cool and trap Li and Yb
Study interacting mixture
Induce controlled interactions
Form molecules
2-species Magneto-Optic trap
Sequential Loading
Ytterbium – green, Lithium - red
The 2 MOTs are optimized at different parameters of
magnetic field gradient and also exhibit inelastic interactions
Optical Dipole Trap
Absorption image of optically trapped lithium atoms
together with uncaptured atoms released from a MOT
Evaporative Cooling of Bosonic 174Yb
r (t )  r (t0 ) 2 
2kT
(t  t0 ) 2
m
Temperature evaluated from
size of absorption imaged atoms
after variable time of flight
With forced evaporative cooling, can
cool towards quantum degeneracy
s-wave scattering length = 5.5nm
Strong Interactions in Fermionic Lithium
Feshbach resonance
between |1> and |2>
“Feshbach molecule” formation
Collisional Properties of the LiYb Mixture
Ytterbium
Lithium
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 length
magnitude of 13 bohrs. Establishment of a new two-species mixture.
Sympathetic Cooling of Lithium by Ytterbium
T/TF = 0.7
With improvements to cooling
procedures (in progress), double
degeneracy should be possible
Ultracold Molecules through PhotoAssociation
Scan
Trap loss measurement
in the 6Li system
Provides information on excited potential
First step towards ground LiYb molecule
Future Li-Yb Work
UltracoldStable Mixture
Simultaneous degeneracy
Yb as probe of Feshbach molecules
Yb as probe of Fermi superfluid
Induce strong Li-Yb interactions
Scan
2 photon PA +
2 photon Raman
Ground state polar molecules
through multiple2-photon processes,
Photoassociation in LiYb system
Precision Measurement of Fine Structure Constant a
neutron interferometry
h/mn
e2
1
a 
c 137
ac Josephson effect
Quantum Hall effect
g-2 of electron + QED
Cross-field comparisons
Precision test of QED
atomic physics route
-250
-200
(2000)
-150
-100
-50
0
50
a -1
 a -1 – 1 in ppb
98

100
150
200
(2008)
QED-free Atomic Physics Route to a
motivation2
0.008 ppb: hydrogen spectroscopy, (Udem et al.,1997; Schwob et al.,1999)
Penning trap mass spectroscopy
Frequency comb
2
 e2 
2 R h 2 R M h
a    

c me
c Me m
 c
2
 rec
1 - 2

k
2m
Cs (Berkeley)
Rb (Paris)
0.7 ppb: penning trap mass spectr.
(Beier et al., 2002)
BEC is a bright coherent
source for atom interferometry
Contrast Interferometry with a BEC
principle2
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
The phase of the matter wave grating is encoded in oscillating contrast.
  (t )   3 (t )

S (T , t   C (T , t  sin 2  1
  2 (t )   C (T , t  sin 2 ( 8 recT  4 rec Dt 
2


The phase of the contrast signal for various T gives rec.
Contrast Interferometry with a BEC
principle2
Na BEC
2002
The phase of the matter wave grating is encoded in oscillating contrast.
  (t )   3 (t )

S (T , t   C (T , t  sin 2  1
  2 (t )   C (T , t  sin 2 ( 8 recT  4 rec Dt 
2


The phase of the contrast signal for various T gives rec.
Contrast Interferometry with a BEC
principle2
Na BEC
2002
With Na BEC experiment
7ppm precision achieved,
but inaccuracy at 200ppm,
attributed to mean field.
Contrast Interferometry with a BEC
Scale Up
Increase sensitivity by increasing momenta
of 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 mean
field shift. Theoretical analysis performed in collaboration with Nathan Kutz (AMath).
Gearing up for the experiment - expect the next generation result in 2-3 years.
UW Ultracold Atoms Team
Undergrad Students: Jason Grad, Jiawen Pi, Eric Lee-Wong
Grad Students: Anders Hansen, Alex Khramov, Will Dowd, Alan Jamison
Post-doc: Vladyslav Ivanov
$$$ - NSF, Sloan Foundation, UW RRF, NIST
One- and Two- Electron Atoms
6Lithium
Fluor [arb]
Yb (Crossed)Beam Spectroscopy
Fluor [arb]
Freq [MHz]
Freq [MHz]
Magneto-Optical Trapping of Li
few x 108 atoms loaded
in a few seconds
Size (mm)
Fluor (arb)
1.2
1.1
1.0
Temp ~ 400mK
0.9
Load time (s)
0
200
400
600
Expansion time (ms)
800
Ultracold Mixtures of Lithium and Ytterbium Atoms
Preliminary Evidence
of Elastic Interactions
Ultracold Atoms
High 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 aspects
Quantum Fluids
Condensed Matter Physics
Nuclear Physics
Quantum Engineering (Potentials and Interactions)
Quantum Information Science
Quantum Simulation
Ultracold 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 limited
lifetime ~ 40 secs
Evaporative Cooling of Bosonic 174Yb
Plain evaporation at B=0
N ~ up to 106 (here ~ 3 x 105)
With forced evaporation, can reach sub-microKelvin
Here, temp X 2 above degeneracy for 50,000 atoms
Side view of
released Yb
N ~ 50,000
Dual Species Apparatus
1 inch
Optical Dipole Trap
Shallow angle (10 degrees)
crossed beam dipole trap
1064nm, up to 50 Watts
LiYb Molecule: Theory
~ 0.4 Debye
Electric dipole
moment
Peng Zhang, ITAMP Harvard
Svetlana Kotochigova,
Temple Univ/ NIST
preliminary results
Fermi Degeneracy in 6Li
TF
Top view of
Trapped Li
T (calc)
T (meas)
With evaporation
at 300 G, can
achieve T/TF ~ 0.5
Size gets clamped by
the Fermi Diameter
DF