Exploring Quantum Matter with Ultracold...
Transcript of Exploring Quantum Matter with Ultracold...
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Exploring Quantum Matter with Ultracold Atoms
Les Houches 2011
Max-Planck-Institut für Quantenoptik, GarchingLudwig-Maximilians Universität, München
www.quantum-munich.de
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• Introduction to UCG
• Interactions (Scattering, Feshbach, ...)
• Introduction to Optical Lattices
• Detection Techniques
• Many-Body Physics in Optical Lattices
• Bose Hubbard Model
• Fermi Hubbard Model
• Controlling Few Body Physics
Repulsively Bound Pairs, Correlated Atom Tunneling
Superexchange Interactions
Creating & Probing Entangled Atom States
Minimal Versions of Topologically Ordered States(RVB, d-Wave,...)
Non-Equilibrium Dynamics
• Outlook
• Polar Molecules, Rydberg Atoms
Course Outline
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The Challenge of Quantum Many Body Systems
• Understand and Design Quantum Materials - one of the biggest challenge of Quantum Physics in the 21st Century
• Technological Relevance
High-Tc Superconductivity (Power Delivery)
Magnetism (Storage, Spintronics...)
Novel Quantum Sensors (Precision Detectors)
Quantum Computing
Many cases: lack of basic understanding of underlying processesDifficulty to separate effects: probe impurities, complex interplay, masking of effects...Many cases: even simple models “not solvable”Need to synthesize new material to analyze effect of parameter change
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Strongly Correlated Electronic Systems
In strongly correlated electron system spin-spin
interactions exist.
Underlying many solid state & material science problems:Magnets, High-Tc Superconductors, Spintronics ....
H = �J �⌅i, j⇧,�
c†i,� c j,� +U �
ini,⇥ni,⇤ +V0 �
i,�R2
i ni,�
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Roadrunner – Los Alamos
2300 estimated number of protons in the universe
each doubling allows for one more spin 1/2 only
State of the art: < 40 spins (240x 240) (what does it take to simulate 300 spins ?)
1.1 Petaflops/s2000 t
3.9 MW
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Introduction
• Controlling Single Quantum Systems
• New challenges ahead: control, engineer and understand complex quantum system quantum computers, quantum simulators, novel (states of) quantum matter, advanced materials, multi-particle entanglement
R. P. Feynman‘s Vision
A Quantum Simulator to study the quantum behaviour
of another system.
Single Atoms and Ions Photons Quantum Dots
R.P. Feyman, Int. J. Theo. Phys. (1982)R.P. Feynman, Found. Phys (1986)
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From a Classical Gas to a Bose-Einstein-Condensate
Classical Gas
CoherentMatter Wave
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Why is it Difficult to Reach BEC?
Condition for BEC:
e.g. Water
For a typical density of water nH20 one obtains Tc=1K
Problem: Water is Ice @ 1K
Solution: Reduced densities by several orders of magnitude, such that the solid is only formed very slowly!
Even Lower Temperaturesare Necessary
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The Path to Bose-Einstein Condensation
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Laserkühlung am Werk
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Magnetic Traps for Neutral Atoms
Energy of an atom inan external magnetic field
Force on an atom inan inhomogeneous field
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Evaporative Cooling
With the help of RF-transitions between neighbouring magnetic sublevels, the hottest atoms can be selectively removed from the trap.
Elastic collisions rethermalize the atoms resulting in a cooler and denser atomic distribution.
Phase space density is increased
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The Path to Bose-Einstein-Condensation
1. Magneto Optical Trap (MOT) (109 atoms)
2. Compressed MOT to increase density of atom cloud
3. Optical molasses mooling
4. Optical pumping to spin polarize atoms
5. Magnetic trapping
6. Evaporative cooling
7. Bose-Einstein condensation (105-106 atoms) around temperatures of 1µK and densitied of 1014 cm-3
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From a Bose Gas without Interactions to a Strongly Correlated Bose System
No Interactions
Weak Interactions
Strongly Correlated System
Many-Body State
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Ato
mqu
elle
n
Rb & K
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Double Species MOT
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Dipole trap + Optical lattices
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Our Starting Point – Ultracold Quantum Gases
Parameters: Densities: 1015 cm-3
Temperatures: Nano KelvinAtom Numbers 106
Bose-Einstein Condensates e.g. 87Rb
Degenerate Fermi Gasese.g. 40K
Ground States at T=0
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Introduction
• Controlling Single Quantum Systems
• New challenges ahead: control, engineer and understand complex quantum system quantum computers, quantum simulators, novel (states of) quantum matter, advanced materials, multi-particle entanglement
R. P. Feynman‘s Vision
A Quantum Simulator to study the quantum dynamics
of another system.
Single Atoms and Ions Photons Quantum Dots
R.P. Feyman, Int. J. Theo. Phys. (1982)R.P. Feynman, Found. Phys (1986)
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From Artificial Quantum Matter to Real Materials
e.g. High-Tc Superconductors (YBCO)
•Densities: 1024-1025/cm3
•Temperatures: mK – several hundred K
•Crystal Structures and Material Parameters given by Material(Tuning possible via e.g. external parameters like e.g. pressure, B-fields or via synthesis)
Real MaterialsUltracold Quantum Gases in Optical Lattices
•Densities: 1014/cm3
(100000 times thinner than air)
•Temperatures: few nK(100 millionen times lower than outer space)
•Crystal Structures and Material Parameters canbe changed dynamically and in-situ.
New tunable model systems for many body systems!
Low densities require us to work at even lower
temperaturesbut
we gain the control & manipulations techniques of the atomic physics
toolbox
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Atomic Interactions
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Scattering Theory
Schrödinger Equation of Scattering Problem
H0 +U(r)|yki= E|yki
y+k = eikr + f (k,k0)
eikr
r
Wave functionin far-field (outsideregion of scattering potential)
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Scattering Cross Section
Differential Scattering Cross Section
dsdW
=Rate of particles scattered into solid angle dW
incident particle flux
Particle flux
j = hm
Im{Y⇤—Y}
we obtain
dsdW
= | f (q)|2
total scattering cross section
s =Z ds
dWdW
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Partial Wave Expansion
For spherically symmetric scattering potential we can write (partial wave decomposition)
yk = Âl=0
AlPl(cosq)Rl(r)
For every angular momentum l, we obtain radial wave equation
✓h2
2m
⇢� d2
dr2 � 2r
ddr
+l(l +1)
r2
�+U(r)
◆Rl(r) = ERl(r)
For free particle motion (U=0), this corresponds to the differential equation of the spherical Bessel functions.
Rl(r) µ cosdl jl(kr)+ sindlnl(kr)
Rl(r) µr!•
1kr
sin(kr+dl � lp2)
This yields in the far field limit:
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Scattering Phase Shift & Scattering Amplitude
We can relate the scattering phase shift to the scattering amplitude, via:
f (q) = 1
k
•
Âl=0
(2l +1)eidlsindlPl(cosq)
dl
s =4pk2
•
Âl=0
(2l +1)sin2 dl
sl 4pk2 (2l +1)
scattering cross section
unitarity limit for partial wave cross sections
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s-Wave Scattering
R0(r) µr!•
1kr
sin(kr+d0)
For r0 < r < 1/k we can approximate the above to
a =k!0
�d0
k
R0 ⇡ 1+d0
kr= 1� a
r
Scattering Length
Far field radial wave function
f (k) =1
k cotd0
(k)� ik!� a
1�arek2/2+ ika
Scattering amplitude (including effective range)
tand0 'k!0
�ka
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Scattering from Attractive Square Well Potential
for r < r0
for r > r0
Ansatz:
Wave vector in inner region of potential
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Scattering Wave Functions
a<0
a>0
a=0
for
no bound state
one bound state
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Scattering Length (Box Potential)
Scattering length
Resonances for:
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Weakly Bound “Halo” States
Very extended “Halo states” are formed close to a Feshbach Resonance for a>0.These correspond to weakly bound states that enter the potential well.
Binding energy of Halo state
Wave function of Halo state
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Some s-wave Scattering Spheres
Multiple scattering spheres become between different momentum components become visible!
Images shown after time of flight period.
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Scattering of Bosons
s-wave (l=0) d-wave (l=2,m=0)
from. Ch. Buggle (thesis UoA 2005)
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Pseudopotential
For ultracold collisions, scattering between particles is characterized by a single parameter - the scattering length.
We can replace the molecular scattering potential with alternative potential thatgives same scattering length!
e.g. Pseudopotential
For regular functions at the origin, this latter derivative may be omitted:
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Identical Particle Scattering
For scattering of identical particles, the scattering wave-function has to obey the rightsymmetry under particle exchange!
+ for Bosons, - for Fermions
Leads to constructive or destructive interference in partial wave amplitudes!
Identical Boson: s,d,f... wave scattering (even partial waves)Identical Fermions: p,g,h... wave scattering (odd partial waves)
s-wave scattering
distinguishable particles
indistinguishable particles
Consequence: no s-wave scattering for identical fermions!
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Feshbach Resonance
Potential curves of open andclosed scattering channels.
Scattering length and bindingenergy of weakly bound stateacross Feshbach resonance.
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Feshbach Resonances - Experiment
S. Inouye et al. Nature S. Cornish et al. PRL
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Converting Atoms Pairs into Bound Molecules
Adiabatic Feshbach Ramp
RF Association
Three-Body Recombination
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B
a
Creating a MBEC out of a Fermi Gas
atoms
Ebinding
EF
molecules BEC
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Molecular Bose-Einstein Condensates
S. Jochim et al., Science, 2003(Innsbruck)
M. Greiner, C. Regal and D. Jin Nature, 2003(JILA)
M. W. Zwierlein et al.,Phys. Rev. Lett, 2003(MIT)see also Ch. Salomon (ENS) and J. Thomas (Duke)
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www.quantum-munich.de
Atoms in Periodic Potentials
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Optical Lattice Potential – Perfect Artificial Crystals
λ/2= 425 nm
Laser Laser
optical standing wave
Periodic intensity pattern creates 1D,2D or 3D light crystals for atoms (Here shown for small polystyrol particles).
Perfect model systems for a fundamental understanding of quantum many body systems
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1D, 2D & 3D Lattices
2D LatticesArray of one-dimensional quantum systems
3D LatticesArray of quantum dots
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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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H� (n)q (x) = E(n)
q � (n)q (x) with H =
12m
p2 +V (x)
� (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�iqxi� (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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Dispersion Relation in a Square Lattice
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Measuring Momentum Distributions
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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
20 ms
Wannierenvelope
Grating-likeinterference
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Momentum Distributions – 1D
Momentum distribution can be obtained by Fourier transformation of the macroscopic wave function.
�(x) =�
i
A(xj) · w(x� xj) · ei�(xj)
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�⇥j = (V ��/2) �t
�� = 0 �⇥ = �
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
Piet
Mon
dria
n
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. Physical Review Letters (2005)
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