1859-6 Summer School on Novel Quantum Phases and Non...
Transcript of 1859-6 Summer School on Novel Quantum Phases and Non...
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1859-6
Summer School on Novel Quantum Phases and Non-EquilibriumPhenomena in Cold Atomic Gases
Immanuel Bloch
27 August - 7 September, 2007
University of Mainz
Introduction to experiments in optical lattices - Part I
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Ultracold Atoms in Optical Lattices
Simon Fölling, Stefan Trotzky, Patrick Cheinet,Artur Widera, Fabrice Gerbier, Michael Feld, Ute Schnorrberger, Robert Saers,Torben Müller,
Thorsten Best, Ulrich Schneider,Sebastian Will, Dries van Oosten, Lucia Hackermüller, Martin Zwierlein, Christoph Clausen,
Stefan Kuhr, Jacob Sherson,Christof Weitenberg, Manuel Endres, Oliver Loesdau
Marionna Morena, Belén Paredes
Markus Greiner, Olaf Mandel,
I.B.
Johannes Gutenberg-Universität, Mainz
funding by DFG, European Union,$ AFOSR
www.quantum.physik.uni-mainz.de
Many inspiring discussions:E. Demler, M.D. Lukin,
A. Polkovnikov, A.-M. Rey, V. Gritsev
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Neutral Atoms in Optical Lattices
Part 1
•Introduction
•Experimental Setup
•Loading a BEC into an Optical Lattice
•From a Superfluid to a Mott Insulator
•Coherence, Atom Number Statistics,
•Shell Structure, Finite Temperature Effects, ...
Part 2
•Tonks-Girardeau Gas
•Multi Orbital Mott Insulator Physics
•Spin Dependent Lattice Potentials
•Generation of Multiparticle-Entangled States
•Novel Quantum Information Schemes for Ultracold Atoms in Optical Lattices
•Generating Nonclassical Field States
Part 3
•Quantum Noise Correlations
•Optical Superlattices
•Correlated Atom Pair Tunnelling
•Detection & Control of Superexchange Interactions
•Outlook
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Experimental Setup:
Vacuum System
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Ato
mqu
elle
n
Rb & K
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Double Species MOT
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Quadrupole trapMagnetic transport
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QUIC-trapEvaporation &
symp. Cooling
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Dipole trap +
Optical lattices
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Atomic Sources
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LasersystemLaser Setup 780nm for Rb und 767nm for K
3x
300mW
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How do we detect these quantum gases ?
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Udip = −d ·E
d = αE
Udip ∝ −α(ω)I(r)
Trapping Atoms in Light Field -
Optical Dipole Potentials
Energy of a dipole in an electric field:
An electric field induces a dipole moment:
Red detuning:
Atoms are trappedin the intensity maxima
Blue detuning:
Atoms are repelled from the intensity maxima
See R. Grimm et al., Adv. At. Mol. Opt. Phys. 42, 95-170 (2000).
Pioneering work by Steven Chu
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Optical Lattice Potential
/2= 425 nm
Laser Laser
optical standing wave
Effectively: Harmonic Oscillators Coupled via Quantum Mechanical Tunneling
J J J
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…and in Higher Dimensions
Tunnel Coupling Tunable!
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…and in Higher Dimensions
Tuning the Dimensionality
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Ψ(x) =∑
i
A(xj) · w(x − xj) · eiφ(xj)
Macroscopic Wave Function of a BEC
in an Optical Lattice
Number of atoms onjth lattice site
Phase of wave function on jth
lattice site
Localized wave function onJth lattice site
If there is a constant phase shift between lattice sites, the state is an eigenstate (Bloch wavefunction) of the lattice potential!
Quantum number characterizing these Bloch waves:
Crystal (Quasi-) momentum q =2
λ∆φ
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Time of flight interference pattern
• Interference between all waves coherently
emitted from each lattice site
Tim
e of
flig
ht
Periodicity of the reciprocal lattice
20ms
Wannierenvelope
Grating-likeinterference
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Momentum Distributions – 1D
Momentum distribution can be obtained by Fouriertransformation of the macroscopic wave function.
Ψ(x) =∑
i
A(xj) · w(x − xj) · eiφ(xj)
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Hφ (n)q (x) = E(n)
q φ (n)q (x) with H =
12m
p2 +V (x)
φ (n)q (x) = eiqx ·u(n)
q (x)
HB u(n)q (x) = E(n)
q u(n)q (x) with HB =
12m
(p+q)2 +Vlat(x)
Single Particle in a Periodic Potential - Band Structure (1)
Solved by Bloch waves (periodic functions in lattice period)
q = Crystal Momentum or Quasi-Momentumn = Band index
Plugging this into Schrödinger Equation, gives:
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V (x) = ∑r
Vrei2rkx and u(n)q (x) = ∑
lc(n,q)
l ei2lkx
V (x)u(n)q (x) = ∑
l∑r
Vrei2(r+l)kxc(n,q)l
(p+q)2
2mu(n)
q (x) = ∑l
(2hkl +q)2
2mc(n,q)
l ei2lkx.
V (x) = Vlat sin2(kx) = −14
(e2ikx + e−2ikx
)+ c.c.
Single Particle in a Periodic Potential - Band Structure (2)
Use Fourier expansion
yields for the potential energy term
and the kinetic energy term
In the experiment standing wave interference pattern gives
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∑l
Hl,l′ · c(n,q)l = E(n)
q c(n,q)l with Hl,l′ =
⎧⎨⎩
(2l +q/hk)2Er if l = l′−1/4 ·V0 if |l − l′| = 10 else
⎛⎜⎜⎜⎜⎜⎜⎜⎝
(q/hk)2Er − 14V0 0 0 . . .
− 14V0 (2+q/hk)2Er − 1
4V0 00 − 1
4V0 (4+q/hk)2Er − 14V0
− 14V0
. . .
⎞⎟⎟⎟⎟⎟⎟⎟⎠
⎛⎜⎜⎜⎜⎜⎜⎜⎜⎝
c(n,q)0
c(n,q)1
c(n,q)2
...
⎞⎟⎟⎟⎟⎟⎟⎟⎟⎠
= E(n)q
⎛⎜⎜⎜⎜⎜⎜⎜⎜⎝
c(n,q)0
c(n,q)1
c(n,q)2
...
⎞⎟⎟⎟⎟⎟⎟⎟⎟⎠
Single Particle in a Periodic Potential - Band Structure (3)
Use Fourier expansion
Diagonalization gives us Eigenvalues and Eigenvectors!
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Bandstructure - Blochwaves
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wn(x− xi) = N −1/2 ∑q
e−iqxiφ (n)q (x)
Wannier Functions
An alternative basis set to the Bloch waves can be constructed through localized wave-functions: Wannier Functions!
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∆φj = (V ′λ/2) ∆t
∆φ = 0 ∆φ = π
Preparing Arbitrary Phase Differences
Between Neighbouring Lattice Sites
Phase difference between neighboring lattice sites
(cp. Bloch-Oscillations)
But: dephasing if gradient is left on for long times !
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Mapping the Population of the Energy Bands
onto the Brillouin Zones
Crystal momentum
Free particlemomentum
Population of nth band is mapped onto nth Brillouin zone !
Crystal momentum is conserved while lowering the lattice depth adiabatically !
A. Kastberg et al. PRL 74, 1542 (1995)M. Greiner et al. PRL 87, 160405 (2001)
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Experimental Results
Pie
t M
on
dri
an
Brillouin Zones in 2DMomentum distribution of a dephased condensate after turning off the lattice potential adiabtically
2D
3D
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Populating Higher Energy Bands
Stimulated Raman transitions between vibrational levels are used to populate higher energy bands.
Single lattice site Energy bands
Measured Momentum Distribution !
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From a Conductor to a Band Insulator
Fermi Surfaces become directly visible!
M. Köhl et al. PRL (2005)
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Bose-Hubbard Hamiltonian
Expanding the field operator in the Wannier basis of localized wave functions on each lattice site, yields :
Bose-Hubbard Hamiltonian
Tunnelmatrix element/Hopping element Onsite interaction matrix element
M.P.A. Fisher et al., PRB 40, 546 (1989); D. Jaksch et al., PRL 81, 3108 (1998)Mott Insulators now at: NIST, ETHZ, MIT, Innsbruck, Florence, Garching...
J = −∫
d3xw(x − xi)(−
2
2m∆ + Vlat(x)
)w(x − xj) U =
4π2a
m
∫d3x|w(x)|4
H = −J ∑〈i, j〉
a†i a j +∑
iεini +
12
U ∑i
ni(ni −1)
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a = ψ +∆a
a†i a j = 〈a†
i 〉〈a j〉+ 〈a†i 〉∆a j +∆a†
i 〈a j〉= 〈a†
i 〉a j + a†i 〈a j〉−〈a†
i 〉〈a j〉
〈ai〉 =√
ni = ψ
H = −J ∑〈i, j〉
a†i a j +
12
U ∑i
ni(ni −1)−µ ∑i
ni
Describing the Phase Transition (1)
Usual Bogoliubov replacement does NOT capture SF-MI transition!(However can describe Quantum Depletion due to interactions)
Self consistent mean field approximation (decoupling approx.)
K. Sheshadri et al., EPL 22, 257 (1993)D. van Oosten, P. van der Straten & H. Stoof, PRA 63, 053601 (2001)
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H = −zJψ ∑i
(a†
i + ai
)+ ztψ2Ns +
12 ∑ ni(ni −1)−µ ∑
ini
Hi =12
U ni(ni −1)− µ ni −ψ(
a†i + ai
)+ψ2 U = U/zJ
µ = µ/zJ
H = H(0) +ψV
H(0) =12
U n(n−1)− µ n+ψ2
V = −(a† + a)
Describing the Phase Transition (2)
Is diagonal in site index i, so we can use an effective on-site Hamiltonian
Can diagonalize Hamiltonian in occupation number basis!or use perturbation theory with tunnelling term to find phase diagram analytically....
D. van Oosten, P. van der Straten & H. Stoof, PRA 63, 053601 (2001)
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E(2)n =
nU(n−1)− µ
+n+1
µ −Un
E(2)n = ψ2 ∑
n′ =n
|〈n|V |n′〉|2E(0)
n −E(0)n′
Eg(ψ) = a0 +a2ψ2 +O(ψ4)
a2 > 0 → ψ = 0a2 < 0 → ψ = 0
Describing the Phase Transition (3)
D. van Oosten, P. van der Straten & H. Stoof, PRA 63, 053601 (2001)
For our initial state (with fixed particle number), only second order perturbation gives a first correction.
a2 = 0 U/zJ ≈ n×5.83Phase transition for
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Superfluid – Mott-Insulator Phase Diagram
For an inhomogeneous system an effective local chemical potential can be introduced
Jaksch et al. PRL 81, 3108 (1998)
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Superfluid Limit
Atoms are delocalized over the entire lattice !Macroscopic wave function describes this state very well.
Poissonian atom number distribution per lattice site
n=1
Atom number distribution after a measurement
〈ai〉i = 0|ΨSF 〉U=0 =
(M∑i=1
a†i
)N
|0〉
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“Atomic Limit“ of a Mott-Insulator
n=1
Atoms are completely localized to lattice sites !
Fock states with a vanishing atom number fluctuation are formed.
Atom number distribution after a measurement
|ΨMott〉J=0 =M∏i=1
(a†
i
)n
|0〉 〈ai〉i = 0
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+
The Simplest Possible “Lattice“:
2 Atoms in a Double Well
0.5 x
Superfluid State MI State
0.25 x
0.25 x
Average atomnumber per site:
Average onsite Interaction per site:
<n> = 1 <n> = 1
<Eint> = U <Eint> = 0
+
+
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Quantum Phase Transition (QPT) from a
Superfluid to a Mott-Insulator
At the critical point gc the
system will undergo a phase transition from a superfluid to an insulator !
This phase transition occurs even at T=0 and is driven by quantum fluctuations !
Characteristic for a QPT
•Excitation spectrum is dramatically modified at the critical point.
•U/J < gc (Superfluid regime)
Excitation spectrum is gapless
•U/J > gc (Mott-Insulator regime)
Excitation spectrum is gapped U/J = z 5.8
Critical ratio for:
see Subir Sachdev, Quantum Phase Transitions,Cambridge University Press
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Ground State of an Inhomogeneous System
From Jaksch et al. PRL 81, 3108 (1998)
From M. Niemeyer and H. Monien (private communication)
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Momentum Distribution for Different Potential Depths
0 Erecoil
22 Erecoil
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Phase coherence of a Mott insulator
Does a Mott insulator produce an interference
pattern ?
F. Gerbier et al., PRL (2005)
Theory : V. N. Kashurnikov et al., PRA 66, 031601 (2002). R. Roth & K. Burnett, PRA 67, 031602 (2003).
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Quantitative Analysis of Interference Pattern
Visibilitymeasurescoherence
nmaxnmin
Visibility decaysslowly withincreasing
latticedepth!
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Excitations in the zero tunneling limit
Perfect Mott insulator ground state
• Low energy excitations :
• Particle/hole pairs couples to the ground state :
Energy E0
Energy E0+U, separated from the ground state
by an interaction gap U
n0: filling factorHere n0=1
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Ground state for J=0 :
``perfect´´ Mott insulator
Ground state for finite J<<U :
treat the hopping term Hhop in 1st order perturbation
=
Coherent admixture of particle/holes at finite J/U
Deviations from the perfect Mott Insulator
J
U
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Predictions for the visibility
Perfect MI
MI with
particle/hole pairs
Perturbation approach predicts a finite visibility, scaling as (U/J)-1
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Comparison with experiments
Average slope measured to be -0.97(7)
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Many-body calculation for the homogeneous case
• 1st order calculation : admixture of particle/hole pairs to the MI bound to neighboring lattice sites
• Higher order in J/U : particle/holes excitations become mobile
Dispersion relation of the excitations is still characterized by an interaction gap.
One can obtain analytically the interference pattern (momentum distribution) for a given n0.
A more careful theory
More details in :
D. van Oosten et al. PRA 63, 053601 (2001) and following papersD. Gangardt et al., cond-mat/0408437 (2004)K. Sengupta and N. Dupuis, PRA 71, 033629 (2005)
F. Gerbier et al., PRA 72, 53606 (2005)
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From a Superfluid to a Mott Insulator
Localized particles Mott Insulator State
Delocalized particles Superfluid State
Phase coherence Number statistics Density Distribution
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How can we Probe the Number Statistics?
We want to know:
1) How many sites with 1 atom
2) How many sites with 2 atoms
3) How many sites with 3 atoms
4) …
For a weakly interacting BEC, one would obtain Poissonian
type number distribution (e.g. coherent states on each
lattice site)
F. Gerbier et al., PRL 96, 090401 (2006) G.Campbell et al, Science 313, 649 (2006)
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Spin Changing Collisions
Spin-dependent case
m2
m3
m4Spin-dependent interaction strength
s-wave collisions
Spin-independent case
m1
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|0, 0〉 ↔ (|↑, ↓〉 + |↓, ↑〉) /√
2
Spin Changing Collisions in an Optical Lattice
Collisionally induced „Rabi-Type“ Oscillations
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AC-“Stark“ shift control of the resonance frequency
Detuning 0 is present even at
zero magnetic field
Spin-1 two-level system at zero magnetic field
0
M
Energy shift due to microwave field can bring levels into resonance.
f=2
f=1
H. Pu and P. Meystre PRL 2000 andDuan, Sorensen, Cirac, Zoller PRL 2000
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Energy shift can be tuned by power of
the microwave and detuning
0+(B2)
AC-Stark shift control of the resonance frequency
∆E ∝Ω2
M4∆
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F. Gerbier et al., PRA 73, 041602 (2006)
B=0.4G
=0+(B2)=0
AC-Stark shift control of the resonance frequency
N+1 +N−1
Ntot∝
(Ω0
Ω′
)2
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Amplitude decrease due to single site spectators
Sensitive and non-destructive detectorfor doubly occupied
lattice sites
On shell-structure see:
Jaksch et al., PRL (1998)
Batrouni et al. PRL (2000);
Kashurnikov, Prokof´ev, Svistunov, PRA (2002)
Alet et al., PRA (2004), recent work P. Denteneer
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Quantum Spin Oscillations as
Non-Destructive Probe of Atom Number Statistics
Classical field (mean field) limit (continuous frequencies)
Quantum limit (discrete frequencies)
Leads to quantum dynamics beyond mean field!Collapse & Revivals, Cat states, etc.
Cf. Work of L. You, J. Ho,…
Amplitude of Spin-Changing
Oscillations at Freq Nat
Number of sites withNat atoms
Resembles exp. in Cavity QED to reveal photon number statistics (Haroche, Walther)see also work of G. Campbell et al. (MIT)
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U
What is the atom number distribution in a lattice?
Mott-Insulator state
Creation of Mott-shells
On shell-structure see:
Jaksch et al., PRL (1998)
Batrouni et al. PRL (2000);
Kashurnikov, Prokof´ev, Svistunov, PRA (2002)
Alet et al., PRA (2004)
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U
Mott insulator
What is the number distribution in a lattice??
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Mott insulator
Strong suppression of n=2 sites for low atom numbers
What is the number distribution in a lattice??
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Mott insulator
Formation of Mott-Shells
What is the number distribution in a lattice??
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Mott insulator
Formation of Mott-ShellsPoissonian number
distribution
Superfluid
Can be measured with our non-destructive
detector for sites with two atoms.
What is the number distribution in a lattice??
Related scheme proposed: D.C. Roberts, K. Burnett, PRL (2003)
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Prepare the system at a certain lattice depth and atom number:
Quickly increase lattice depth in orderto preserve atom number statistics:
Amplitude of coherent spin-
oscillations
Measure of sites with two or more
atoms per site
What is the number distribution in a lattice??
F. Gerbier et al., PRL (2006)
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Atom number statistics…N=2 sites vs Total Atom Number
n=1
n=1
n=2
F. Gerbier et al., PRL 96, 090401 (2006)
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Probing the density distribution
B=152.95 G
B=152.96 G
B=152.97 G
B=152.98 G
B=152.99 G
B=153.00 G
B=153.01 G
9 G6739.190 MHz
Atoms on the line of resonance are transferred to another hyperfine state!
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Dissecting a Mott Insulator
High spatial resolution of up to 1 µm can be achieved!
S. Fölling, PRL 97, 060403 2006
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What to expect…
BEC
Mott insulator
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Density Profile in the SF Regime
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In Trap Atom Number Resolved Profiles - MI
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Mott Insulator Shell Radii
Good agreement with ab-initio T=0 theory!
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In Trap Observation of the Transition
from a Compressible SF to an Incompressible MI
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nh(µ,T ) sh(µ,T )
µloc(r) = µ −VT (r)
U0n−µ µ −U(n0 −1)
Estimating Finite Temperature Effects
Let us consider isolated wells (J=0):
Particle & Entropy densities
Work in local density approximation
Lowest lying excited states within Mott domains
Free energy cost foradding a particle
Free energy cost forremoving a particle
n0 −1,n0,n0 +1Higher lying excitations cost at least energy U and are suppressed by !e−βU
Restrict States to
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z0 = ∑n
e−β (E(n)−µn)
E(n) =12
Un(n−1)
n0 ≈ n0 +(
B(+) −B(−))
/z0
Var(n)0 ≈(
B(+) +B(−))
/z20
B(−) = eβ (U(n0−1)−µ)B(+) = eβ (µ−Un0)
z0 = 1+B(+) +B(−)
Onsite Thermodynamics
Onsite Partition Function:
with
We obtain
with
being the Boltzmann factors for the addition/subtraction of a particle from the„background“ value n0.
F. Gerbier arXiv:0705.3956
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T ∗ ≈ 0.2U/kB
Thermal Effects on Shell Structure
F. Gerbier arXiv:0705.3956
Shell structure is completelydestroyed by
thermal defects for:
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kBTc ≈ zJ(n0 +1)/2
Tc T ∗
Thermal Effects for Finite J
Superfluid shells will turn normal for
F. Gerbier arXiv:0705.3956,see also: T.-L. Ho cond-mat/0703169
with
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What can we hope to reach?
For perfect adiabatic loading, entropy in BEC equals entropy in MI state!
F. Gerbier arXiv:0705.3956
ω f = 2π ×70Hz
UU
Load into 20 Er lattice.
ω f = 2π ×70HzFor fixed trap frequency
For fixed initial temperatureTi=0.3 Tc0
UU
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Finite Temperature Effects (1)
Integrated Profiles Radial Profiles
kBT=0.01U
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Finite Temperature Effects (2)
Integrated Profiles Radial Profiles
kBT=0.1U
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Finite Temperature Effects (3)
Integrated Profiles Radial Profiles
kBT=0.5U
Comparing with our measured integrated profiles, we find that
kBTexp<0.1U
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What Happens to the Relative Phase of
two Quantum Liquids over Time ?
Start with a singleBose-Einstein condensate
Split it into two BECs with a constant relative phase
BEC 1 BEC 2
Fundamental question arises:
What happens to the relative phase between the two condensates over time ?
What happens to the individual wave functions of the two BECs over time ?
Relative phase
M. Greiner, O. Mandel, T. W. Hänsch and I. Bloch Nature, 419 (6901), 2002
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Dynamical Evolution of a Many Atom State
due to Cold Collision
How do collisions affect the many body state in time ?
Phase evolution of the quantum state of two interacting atoms:
Collisional phase
•Phase shift is coherent !
•Can be used for quantum computation(see Jaksch, Briegel, Cirac, Zoller schemes)
•Leads to dramatic effects beyond mean-field theories !
Collisional phase of n-atoms in a trap:
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Time Evolution of a Coherent State due to Cold Collisions
+
+
+
Coherent state in each lattice site !
=
1. Here = amplitude of the
coherent state
2. Here ||2 = average
number of atoms per lattice site
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Freezing Out Atom Number Fluctuations
A
BRamp up lattice fast from the superfluid regime (A) to the MI regime (B), such that atoms do not have time to tunnel !
Atom number fluctuations at (A) are “frozen“ !
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Collapse and Revival of the
Matter Wave Field due to Cold Collisions
Quantum state in each lattice site (e.g. for a coherent state)
Matter wave field on the ith lattice site
1. Matter wave field collapses but revives after times multiple times of h/U !
2. Collapse time depends on the variance of the atom number
distribution !
Yurke & Stoler, 1986, F. Sols 1994; Wright et al. 1997; Imamoglu, Lewenstein & You et al. 1997, Castin & Dalibard 1997, E. Altman & A. Auerbach 2002, G.-B. Jo et al 2006
Similiar to Collapse and Revival of Rabi-Oscillations in Cavity QED !
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Dynamical Evolution of a Coherent State
due to Cold Collisions
The dynamical evolution can be visualized through the Q-function
Characterizes overlap of our input state with an arbitrary coherent state
Re()
Im(
)
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Dynamical Evolution of a Coherent State
due to Cold Collisions
time
Re()
Im(
)
0
0
-4
4
-4 4
G.J. Milburn & C.A. Holmes PRL 56, 2237 (1986);B. Yurke & D. Stoler PRL 57, 13 (1986)
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Dynamical Evolution of the Interference Pattern
t=600 µst=450 µst=400 µs
t=300 µst=200 µst=150 µst=50 µs
After a potential jump from VA=8Er to VB=22Er.
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Collapse and Revival Ncoh/Ntot
Up to 5 revivals are visible !
Oscillations after lattice potential jump from 8 Erecoil to 22 Erecoil
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Revival Frequency vs. Lattice Potential Depth
h/U from theory
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Influence of the Atom Number Statistics on the
Collapse Time
Final potential depth VB=22Er
VA=11Er
VA=4Er
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tc/trev for Different Initial Potential Depths
Atom Number Statistics
n=1, U/J0
Independent proof of sub-Poissonianatom number statistics for finite U/J !
Atom Number Statistics
n=1, U/J=17
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The End.......for today...