Corinna Kollathhelper.ipam.ucla.edu/publications/qs2009/qs2009_8055.pdf · 2009. 2. 5. · Quantum...
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Rather don´t use Power-Points title Page
Use my page one insteady p g
Quantum dynamics in ultracold atoms
Corinna KollathCorinna Kollath(Ecole Polytechnique Paris, France)
T. Giamarchi (University of Geneva)A Läuchli (MPI Dresden)A. Läuchli (MPI Dresden)
I. McCulloch (Queensland)A. Kleine, U. Schollwöck (RWTH Aachen)
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Quantum dynamicsQ yNanostructures Quantum processing
Delft Innsbruck
Spintronics Quantum gases
Sadler et al (2006)
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Preparing ultracold atoms
laser cooling T~100 μKevaporative cooling T~100nK
Boson (87Rb, F=2, mF=2)Fermion (40K, F=9/2, mF=9/2)
magnetic trapmagnetic trap
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Bose-Einstein condensate
Boulder
Dilute gases: n ≈ 1014 cm-3
Ultracold: Tdegeneracy ≈ 100 nK
Weak interactions: n1/3a << 1
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Strong interactions in quantum gases
2004 Fermi gases in optical lattices
g q g
2004 BEC- BCS crossoverFermi gases at unitarity
2004 Fermi gases in optical lattices
2003 low dimensional gases
2002 superfluid to Mott insulator transition
2001 Nobelprize BEC:
Boulder 1999 Quantum degenerate Fermi gases
Cornell, Ketterle, Wieman
1995 BEC created in dilute atomic gases
1924 predicted by Bose and Einstein
1995 BEC created in dilute atomic gases
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Optical lattices
standing wave laser field -> periodic potential for atoms
i t it f l > t th f t ti l
2D lattice:•intensity of laser -> strength of potential
•wavelength/2 -> lattice spacing
•different geometries possible•different geometries possible
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Bosons in an optical lattice
kinetic energy interactiongyenergy
∑∑ −++−=><
+
jjj
ijji nnUchbbJH )1(2/.).(
jj
U and J related to lattice height
Jaksch et al. (1998)
g
U tunable by Feshbach resonances
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Cold gases as quantum simulators
superfluid Mott-insulator
well tunable in time
well decoupled from environment
large kinetic energy large interaction
ti f fli ht i-> quantum dynamics in isolated system
time-of-flight images~ momentum distribution
Greiner et al (2001)
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Quantum dynamics in a closed system Hilbertspace
time evolution by Schrödinger equation
small time step:
space
state
h fl id M i l i i
∑ Δ−Δ− =≈Δ+n
ntEittHi ncetett n)()( )( ψψ
ex: quench across superfluid to Mott-insulator transition
methods: exact diagonalization and time-dependent DMRG
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The idea of DMRG: reduced Hilbert spaceS. White (1992)
problem: too large Hilbert space idea: construct effective Hilbert space
p
Variational method in matrix product state spacep p
∑≈ 111 ])[][( σσσσ LL llll AALexact sites
σσ•Schmidt decomposition of 'wanted' state
α σσλψ 121 ++∑= llll RLblock L block R
1+ll σσ
ααα
αψ 121 ++−∑ llll
•keep only M highest λ
new block L new block R∑=
+≈M
ll RL1
1α
αααλψ
f•sweep free sites
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idea: time-dependent DMRGp
static:
breakdown afterh i
enlarged:
numerically
adaptive:
numerically cheapshort time
(Cazalilla,Marston)
very expensive
(Luo,Xiang,Wang;S h itt k t)
long times
(Vidal;C S öSchmitteckert) Daley,CK,Schollwöck,Vidal;
White,Feiguin)
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Algorithm: time-stepg p
time-evolution (Schrödinger eq)effeff
ttt )()( Δ+→ ψψ
H(t) H(t Δt)→H(t)eff H(t+Δt)eff→
block L block R
exact sites
∏∏ ++≈ llll UUU 1,1,
Suzuki Trotter decomposition
block L block R
apply Ul,l+1
∈∈ evenloddl
)exp( 1, tihU lll Δ−≈ +
with
new block L new block R
block L block R,
repeat for all sites
new block L new block R
errors:
• Trotter-Suzuki error
• truncation error• truncation error
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Trotter error ~LΔtnSz
• dominating at short timedominating at short time• well controlled by Δt
Gobert, CK, et al PRE (2004)
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Truncation error
runaway time: crossover between Trotter error and truncation errorll t ll derrors well controlled
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Experiment: abrupt changefrom superfluid to Mott-insulator
fl id t tal
from superfluid to Mott-insulator
superfluid state
of
ce p
oten
tiahe
ight
la
ttic
time after quench
time-of-flight images
Greiner et al. Nature (2002)
g g~ momentum distribution
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Theoretical description
questions:
response on short times?
interaction strength
r
how do correlations build up/decay?
entanglement evolution?
Mott-insulator
speed?
if stationary state:
superfluid
time T?
if stationary state:
what are its properties?
subsystems ‘thermalized’?
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Total revival of the wave function
only interaction term:
time evolution operatorM tt
Utime evolution operator
12 ˆ ˆexp[ ( 1)]it
f j jj
U n n− −∑h
Mott-insulator Uf
integer value
all Fock states revive latest at T=h/U
superfluid
time
Ui
-> wave function evolves periodically in time
T h/UT=h/U
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Relaxation with finite hopping
Fouriertransformation <b+j(t)bj+1(t)>
U =2 J U =40 J> Ui =2 J, Uf=40 J
+ j(t)b
j+1(
t)
frequency ω
<b+ frequency ω
quasi-particle frequency bands -> beating -> relaxation ~1/(zJ)-> relaxation ~1/(zJ)
C. Kollath, A. Läuchli, E. Altman, PRL 98, 180601 (2007)
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Light-cone like evolution to quasi-steady state r
density-density correlations
)()( 00 tnntnn rr −
distance rdistance=velocity x time
)()( 00 rr
6
quasiampi
ng light cone like evolution in different models:
quasi-steady statefir
st d
a
1
Lieb and Robinson (1972)spin models
Igloi and Rieger
0 2 time/[h/J]D. Gobert, CK, U. Schollwoeck, G.
Schütz (2005)
Calabrese and Cardy (2006)conformal field theory
A. Läuchli and C. Kollath, J.Stat. Mech. (2008)
conformal field theory
specific exactly solvable models…
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Speed of correlations
sound velocityposition of dip in density-density correlation
velocity of fermion model
sound velocity
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Speed of correlations
sound velocity
velocity of particle-hole excitations
velocity of fermion model
sound velocity
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Entanglement evolution
bl k A
l
von Neuman entropy of block A
block A
logA A A AS Tr ρ ρ= −
saturation after different timest~ v l (open boundary conditions)
A. Läuchli and C. Kollath (2008) P. Calabrese and J.Cardy (2005) for CFT
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Summary: quench
distance r
6what determines speed of light-cone?
deviations from light-cone?am
ping
deviations from light cone?
general understanding of speed?
Long-time limit?quasi-steady state
first
da
1
0 2 time/[h/J]
S. Manmana et al. (2007)non-integrable fermionic model
specific exactly solvable models(Luttinger model, Ising model, …)M. Rigol et al. PRL 98, 50405 (2007),M. Cazalilla PRL (2007), P. Calabrese and Cardy PRL (2006), Barthel and Schollwöck (2008), Roux(2008),Flesch et al (2008)...
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Dynamic of local excitations
single particle excitations density perturbations
y
•characteristics of systems
•transport through nanostructures
•information transfer
here:here:spin-charge separation in real time
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Dynamics of single particle excitationsy g p
3D Fermi liquid
quasi-particlewith spin and charge
1D Luttinger liquid
separation of spin and charge k fwith spin and charge key feature
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Spin-charge separation: simple sketch
one-dimension two-dimensions
p g p pm
etim
are held together
holon spinon
move separatelymove separately
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Condensed matter physics
H = H +H +Hinteration
spectral function
p y
introducing charge: ρ(x) ~ ρ (x) + ρ (x) and spin: σ(x) ~ ρ (x) - ρ (x)
bosonization valid at low energyand spin: σ(x) ρ (x) ρ (x)
using bosonic (amplitude and phase) fields H = Hρ + Hσ
-q Vρ q Vσ q VρVoit (1993)
short times? ρ σ
no interaction!strong perturbations?
interfaces?
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Single particle excitation two component fermionsg p p
add single particle J/h
9.0
4.0
ncharge= n +n
nspin= n -n
C.Kollath, U. Schollwöck, and W. Zwerger PRL 95, 176401
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Single particle excitationg p
9.0
ncharge= n +n
4.0nspin= n -n
C.Kollath, U. Schollwöck, and W. Zwerger PRL 95, 176401
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Single particle excitation and entropy growthg p py g
two component bosons
ncharge= n +n
nspin= n -n • separation of spin and charge
• strong growth of entropy with time• strong growth of entropy with time
• contribution of spin and charge part
A. Kleine, CK, I. McCulloch, U. Schollwöck (2008)
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Density excitation and its entropy growthy py g
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Comparison of maximum entropy growthp py g
• separation of spin and charge
• strong growth of entropy with time for single particle excitation (numerically difficult)(numerically difficult)
• slow growth of entropy with time for density excitation
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Experimental observations
condensed matter:
p
cold atoms:• detection in real time
measure of density average over several lattice sites
Auslaender et al. (2005) • Raman spectroscopy
tunneling between parallel wires
p pyspectral function
Dao et al. PRL 98, 240402 (2007)Stewart et al. Nature 454 (2008)
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Spectral functionp
two component mixture of bosons in one-dimension
ncharge=0.63, u=3, u12=2.1
A. Kleiner, C.Kollath, I.McCulluoch, T. Giamarchi, U. Schollwöck, (2008)
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Applications of DMRG variantspp
•non-equilibrium situationsdynamics across quantum phase transition
•finite temperaturethermodynamics in spin-ladders
C. Rüegg et al. (2008)
A Laeuchli and CK (2008)
•local excitations & dynamic propertiesspin charge separation
A. Laeuchli and CK (2008)
hi h di ispin-charge separationspectral function
•higher dimensions•...
Kleine et al. (2008)
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Postdoc position available