ultracold molecules - a new frontier for quantum & chemical … · 2015-04-24 · ultracold...
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![Page 1: Ultracold molecules - a new frontier for quantum & chemical … · 2015-04-24 · Ultracold atomic matter Precise control of a quantum system The most precise measurements, e.g.,](https://reader034.vdocuments.net/reader034/viewer/2022042407/5f227c005a884b759d199e21/html5/thumbnails/1.jpg)
Jun Ye
JILA, NIST & CU, Boulder
Debbie Jin
University of Virginia April 24, 2015
NIST, NSF, AFOSR, ARO
Ultracold molecules - a new frontier for quantum & chemical physics
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Ultracold atomic matter Precise control of a quantum system
The most precise measurements, e.g., clock
Quantum information
Quantum sensors
Control: A tool for understanding complexity
Strongly correlated many-body quantum systems
• Superfluidity & Superconductivity
• Quantum magnetism • Quantum chemistry
Control atomic interactions Regulate atomic motions
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Atomic Interactions
0 ∞ interaction strength
We can understand & control them!
Short-range collisions
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• Extend capability to control complex quantum systems
Quantum gas of polar molecules
E
Exotic
quantum
matter
well-understood microscopics
tunable, long-range interactions
non-equilibrium quantum dynamics
• Study frontier problems in strongly correlated quantum
material, with
E
Quantum metrology, Correlated material, Chemistry
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Ultracold molecules: The challenge
energ
y
E/kB=6000 K
two atoms
molecule
KRb
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Molecules are complex!
energ
y
two atoms
molecule
100 K
vibration
Ultracold molecules: The challenge
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Molecules are complex!
energ
y
0.1 K
rotation
Ultracold molecules: The challenge
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Molecules are complex!
energ
y
Ultracold molecules: The challenge
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Molecules are complex!
energ
y
38 mK
nuclear spin
Ultracold molecules: The challenge
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log 1
0(d
en
sity
[cm
-3])
log10(T [K]) -9 -6 -3 0
3
6
9
12
Stark, magnetic,
optical deceleration
Buffer-gas cooling
Photo-association
Coherent state transfer
Quantum degeneracy
Technology for making cold molecules
Quantum degeneracy
~ kBT Towards
quantum regime
Carr, DeMille, Krems, Ye, New. J. Phys. 2009.
Laser cooling Evaporative cooling Sympathetic cooling
Quantum gas
of molecules
• High resolution collisions/reactions
• Precision test
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energ
y
two atoms
molecule
Associate ultracold atoms into molecules
6000 K
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Large, floppy molecules
>
V(R)
Ebinding
Magnetic field B
R R R R
Energy
Make Feshbach molecules
Zirbel et al., Phys. Rev. Lett. 100, 143201 (2008).
Magnetic field B
Scattering length a
0 ∞
Start with ultracold atoms
40K 87Rb
Interaction tuned by scattering resonance
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Weakly bound molecules
energ
y
6000 K
• no dipole moment • losses
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Coherent two-photon transfer – the absolute ground state
Beat note 125 THz
Laser 2 Laser 1
frequency
6000 K
970 nm 690 nm
(entropy-less chemistry)
Fully coherent, >90% efficiency
36 nuclear spin states: We populate & control single state S. Ospelkaus et al., Phys. Rev. Lett. 104, 030402 (2010).
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Polar molecules in the quantum regime
87Rb Bosons
K.-K. Ni et al., Science 322, 231 (2008). KRb molecules
(Dipole ~0.5 Debye)
Temperature ~ 100 nK
Density ~1012/cm3
T/TF ~ 1.3
104 times colder, 106 times more dense than other cold molecule gases
40K Fermions
A. Chotia et al., Phys. Rev. Lett. 108, 080405 (2012).
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T = 200 nK > 1 s lifetime
Density (
10
12
cm
-3) Two-body loss
Time (s)
Chemistry near absolute zero Trapped molecules in the lowest energy state (electronic, vibrational, rotational, hyperfine)
2)()( tntn
KRb + KRb K2 + Rb2
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Cold collisions between identical Fermions (1) Particles behave like waves
(2) Angular momentum is quantized
(3) Quantum statistics matter
0110 Fermions c L = 1, p-wave collisions
s p d
0 1ħ 2ħ
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Ultracold chemistry
At low T, the quantum statistics of fermionic molecules suppresses chemical reaction!
Energ
y
distance between the molecules
deBroglie wavelength
Ospelkaus et al., Science 327, 853 (2010).
Rate proportional to T
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Distinguishable molecules do not enjoy the suppression rate is x 100 higher !
Energ
y
distance between the molecules
deBroglie wavelength
Ospelkaus et al., Science 327, 853 (2010).
Ultracold chemistry
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Anisotropic dipolar collisions
mL = +1, -1
mL = 0
p-wave barrier
K.-K. Ni et al., Nature 464, 1324 (2010).
Collisions under a single partial wave (L = 1).
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0.00 0.05 0.10 0.15 0.20
10-12
10-11
10-10
10-9
3D
D
D(
cm
3s
-1)
Dipole moment (D)
E M. de Miranda, et al., “Controlling the quantum stereodynamics of ultracold bimolecular reactions,” Nature Phys. 7, 502 (2011).
Theory: Büchler, Zoller, Bohn, Julienne
2D geometry – loss suppression
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3D optical lattice – suppressing chemical reaction with quantum Zeno effect
τ = 25(2) seconds
A. Chotia et al., Phys. Rev. Lett. 108, 080405 (2012).
Lifetime ~ 20 s Filling ~ 5%
B. Zhu et al., Phys. Rev. Lett. 112, 070404 (2014).
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Spin exchange in a lattice of molecules Barnett et al., Phys. Rev. Lett. 96, 190401 (2006). Micheli et al., Nature Phys. 2, 341 (2006). Gorshkov et al., Phys. Rev. Lett. 107, 115301 (2011).
Temperature and entropy requirements less stringent for molecules
Long-range dipolar interactions for direct (~kHz) spin
exchanges - motion & spin decoupled
Fully tunable with electromagnetic fields
2.23 GHz
70 kHz
270 kHz
Rotation Spin
Dipole moment
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0,1
1,1
1,1
0,0
2.2GHz
A good system to study many-body quantum localization ?
D. Huse, G. Shlyapnikov, M. Lukin, E. Demler, …
Molecules (material) are physically pinned down, but spins (excitations) can be exchanged and mobile !
Energy flow in a macro-molecule !
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Interaction strength for quantum magnetism
Flip-flop term
The oscillation frequency for a pair of molecules is 𝐽⊥/2ℎ.
104 Hz
208 Hz
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A Dipolar Spin-Lattice Model
|N=0>
|N=1 >
• Start with N=0.
2
1• Drive a coherent spin superposition.
0 20 40 60 80 1000.0
0.2
0.4
0.6
0.8
1.0
T (ms)
Co
ntr
ast
N=1.77x104
N=5x103
T (ms)
• Probe spin coherence at T. (Ramsey spectroscopy)
B. Yan et al., Nature 501, 521 (2013).
Nu
mb
er (
10
3)
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Oscillations due to dipolar interactions
0 20 40 60 80 1000.0
0.2
0.4
0.6
0.8
1.0
T (ms)
Contr
ast
N=1.77x104
N=5x103
Oscillation frequencies & decay time both depend on rotational states
|𝟏, 𝟎 ↔ |𝟎, 𝟎
|𝟏, −𝟏 ↔ |𝟎, 𝟎
Co
her
ent
tim
e
1/N scaling
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Control dipolar interaction
𝐽⊥
2~ 102 Hz
for |0,0> to |1,0>
( 𝐽⊥
2~ 51 Hz for|0,0> to |1,-1>)
|1,-1
|1,0
|1,1
|0,0
Theory (MACE) Experiment
Co
ntrast
Co
ntrast
1
0
0.5
1
0.5
0
|1,-1
|1,0
|1,-1
|1,0
Dark squares: |0,0> -> |1,0> Red circles: |0,0> -> |1,-1> (time rescaled by ½).
One fitting parameter (filling 5-10%) reproduces the experiment
f = 106 Hz, f/√2, f/2
K. Hazzard et al., Phys. Rev. Lett. 113, 195302 (2014).
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Highly filled optical lattice
~5% filling in a 3D lattice
Goal: A near zero entropy 3D lattice
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Creating molecules in a 3D lattice
1. Rb MOTT insulator
2. Add lots of K atoms (tune Rb – K interaction energy)
3. Magnetic association & Raman transfer
Challenges: • Fermions have a lot of entropy • Size of the N=1 MI is small (need
to keep Rb density low)
Seeing effects of high Rb density in the optical trap
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Rb Mott insulator imaged in-situ A superfluid BEC phase transition to a MOTT insulator
Lattice depth
12 Erec 18 Erec 24 Erec
1 10 1000.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Rb (103)
filli
ng
50,000 Rb
2000 Rb
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Imaging of K in momentum & real space
104
0.4
0.5
0.6
0.7
0.8
0.9
1.0K Filling vs Number
K Number
Pe
ak F
illin
g F
ractio
n
105
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-300 -200 -100 0 100 2000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
aK-Rb
(a0)
Rb fill
ing
• Vary aKRb before loading lattice
• At aKRb = 0, Rb MI unaffected by K
• Filling is sensitive to Rb BEC fraction
Overlap of Rb and K
5000 Rb 105 K
Rb K
Rb & K overlap
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• Load the lattice at aKRB = 0
• Jumping across the
resonance dilutes the filling (populating higher bands)
• Flip to a noninteracting spin state (|9/2,-7/2>) to avoid resonance when ramping B
• Flip back to |9/2,-9/2>, then proceed with Feshbach association
Pairing up K-Rb in lattice without heating
K: |F =9/2,mF =−9/2> + Rb: |F =1,mF =1>
B
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A low entropy lattice of molecules Convert > 60% of Rb MOTT Insulator to KRb
• For BEC of a few 103 Rb, the
peak filling of Rb ~ 1
• To measure KRb, dissociate the molecules & count the numbers of K = Rb
0.1 1 100.0
0.2
0.4
0.6
0.8
Rb (104)
KR
bF
esh
ba
ch /
Rb
Image of KRb
Ground state KRb, ~ 40% filling in 3D lattice (entropy/molecule ~ 1.7 kB)
~5% filling
~5% filling
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Special Thanks (KRb team):
Theory collaborations: J. L. Bohn, K. Hazzard, P. S. Julienne, S. Kotochigova, M. Lukin, A. M. Rey, P. Zoller
Former members:
Brian Neyenhuis
Amodsen Chotia
Marcio de Miranda
Dajun Wang
Silke Ospelkaus
Kang-Kuen Ni
Avi Pe’er
Josh Zirbel
Bryce Gadway
Bo Yan
Jacob Covey
Steven Moses