coherence and correlations in an atomic mott insulator quantum optics vi, krynicka, poland, 13-18...
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Coherence and correlations in an atomic Mott insulator
Quantum Optics VI, Krynicka, Poland, 13-18 June 2005
Fabrice GerbierArtur WideraSimon FöllingOlaf Mandel
Tatjana GerickeImmanuel Bloch
Johannes Gutenberg Universität Mainz
www.physik.uni-mainz.de/quantum
Back rowImmanuel BlochSimon FöllingThomas BergTim Rom
Middle rowFGTatjana GerickeThorsten Best
Front rowArtur WideraOlaf MandelSusanne Kreim
Not on picture Dries van OostenUlrich SchneiderHerwig Ott
Outline
Optical lattices and the superfluid to Mott insulator transition
Reviews : I. Bloch, J. Phys. B 38, S629 (2005)D. Jaksch and P. Zoller, Annals of Physics 315, 52 (2005).W. Zwerger, J. Opt. B 5, 89 (2003)
Phase coherence of a Mott insulatorwhat does the interference pattern tell us about the nature of the ground state ?
Spatial correlations in expanding cloudstwo-particle correlations to probe phase-uncoherent
samples
1D optical lattice
Potential:
Natural scales :
• Recoil energy :
ER/h ~ 3.2 kHz (150 nK)@ L=850 nm
• Lattice spacing :
alat=L/2 = 425nm
3D Optical Lattices
• Three pairs of counter-propagating laser beams produce a simple cubic lattice
• Typically 20-60 sites occupied in each direction
• Mean atom number per site (filling factor) between 1 and 3
• Spontaneous emission rate ~ 1 Hz
• Produce a 87Rb Bose-Einstein condensate in a purely magnetic QUIC trap
• Expand the condensate to reduce its density (and avoid losses)
• Ramp up slowly lattice beams intensity
• Switch off the trap, expand and take an absorption image
Loading a BEC in the lattice
0
10
Latt
ice d
eoth
(E
R)
160 msHold time
Time of flight interference pattern
• Interference between all waves coherently
emitted from each lattice site
Tim
e o
f flig
ht
Periodicity of the reciprocal lattice
20 ms
Wannierenveloppe
Grating-likeinterference
Reversible loss of coherence
in deep lattices
0 Erecoil 22 Erecoil12 Erecoil16Erecoil
Phase coherence disappears with increasing lattice depth. This is reversible:
Generalization to a general matter wave:
Correlation functiondetermines the visibility
M. Greiner et al., Nature 415, 39 (2002)
see also :C. Orzel et al., Science 291, 2386 (2001) Z. Hadzibabic et al., PRL 93, 180403 (2004)
before 0.1 ms 1 ms 4ms 14 ms
ramping
down
Describes interacting Bose gas in a lattice, in the tight-binding limit
Compétition between tunneling and on-site interactions :
Interactions matter:Bose-Hubbard model
M.P.A. Fisher et al., PRB 40, 546 (1989)D. Jaksch et al., PRL 81, 3108 (1998)
Lattice depth
Ground state in the zero tunneling limit
The system try to form an atom distribution as regular as possible to minimize locally the interaction
energy
Mott insulator ground state
Integer number of atoms per siteZero fluctuations
Survives at finite temperature << U
Intermediate regime
Superfluid ground state, J << U
Mott insulator ground stateU>> J
Gapless excitations: compressibleLong-range phase coherence Superfluid currents
Gapped excitations: incompressibleNo off-diagonal long range order or superfluid currents
Phase coherence of a Mott insulator
Does a Mott insulator produce an interference pattern ?
F. Gerbier et al., cond-mat/0503452, accepted in PRL.
Theory : V. N. Kashurnikov et al., PRA 66, 031601 (2002).R. Roth & K. Burnett, PRA 67, 031602 (2003).
Visibility of the interference pattern
minmax
minmax
nn
nnV
SF to MI transition
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
Ground state for t 0 :
``perfect´´ Mott insulator
Ground state for finite t<<U :
treat the hopping term Hhop in 1st order perturbation
=
Coherent admixture of particle/holes at finite t/U
Deviations from the perfect Mott Insulator
J
U J
U
Predictions for the visibility
Perfect MI
MI with
particle/hole pairs
0V
0
41
3
zJV n
U
Perturbation approach predicts a finite visibility, scaling as (U/J)-1
Comparison with experiments
Average slope measured to be -0.97(7)
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 papers
D. Gangardt et al., cond-mat/0408437 (2004)
K. Sengupta and N. Dupuis, PRA 71, 033629 (2005)
Shell structure of a trapped MI
D. Jaksch et al. PRL 81, 3108 (1998)
Smooth ``external´´ potential present on top of the lattice potential (combination of magnetic trap +optical potential due to Gaussian profile)
Consequence: alternating MI/superfluid shells present at the same time
Figures courtesy of M. Niemeyer and H. Monien (Bonn)
Comparison with experiments
• Simplify shell structure :
ignore superfluid rings
No adjustable parameters
Extends to trapped system using the Local Density Approximation
F. Gerbier et al., in preparation
18 ER
Kinks in the visibility curve : evidence for n>1 Mott shell formation ?
Experiment:
Kink #1 14.1 (8) ErKink #2 16.6 (9) Er
Theory:
n=2 Mott shell 14.7 Er
n=3 Mott shell 15.9 Er
Reproduced in munerical
calculations by the GSI Darmstadt
group (R. Roth et al, unpublished)
Spatial correlations in expanding atom clouds
Experiment : S. Fölling et al., Nature 434, 481 (2005).
Theory : E. Altman, E. Demler & M. Lukin, PRA 70, 013603 (2004).
Related work : Z. Hadzibabic et al., PRL 93, 180403 (2004) . M. Greiner et al. , Phys. Rev. Lett. 94, 110401 (2005).
J. Grondalski et al., Opt. Exp. 5, 249 (1999)A. Kolovski, EPL 68, 330 (2005).
R. Bach and K. Rzazewski, PRA 70 (2005).
Hanbury Brown Twiss experiment
0,00
1,00
2,00
3,00
4,00
5,00
6,00
7,00
8,00
detectorssuperimposed
detectorsseparated
Experiment
Theory
Seminal experiment by Hanbury-Brown and Twiss in 1952
Joint detection probability twice as large for superimposed detectors
Second order coherence function :
g(2) = 1 : uncorrelated particles
g(2) > 1 : bunching, typical for Bose statistics
g(2)
2 6
1
Intensity interferometry
via the Hanbury Brown and Twiss effect
Bunching as consequence of Bose statistics
(Quantum-statistical) Noise analysis as a sensitive probe of the source properties, with a wide range of applications :
• Quantum optics• Nuclear and particle physics (angular correlations)• Condensed matter physics (electron antibunching, mesoscopics, …)
Emerging field in cold atom physics Masuda & Shimizu, PRL (1996); Orsay (2005) also pursued in optical cavity: Münich, Heidelberg, Zürich, Berkeley,
…
Hidden information in expanding atom clouds
For a cloud deep in the Mott state (here V0=50 ER), the interference pattern is unobservable.
Can we still extract information from such a picture ?
The answer is yes, if we use noise analysis.
Correlated fluctuations in time of fight images
Correlation function(normalized)
Bunching effectfor relative distances equal to a reciprocallattice vector
dx
dy
Hanbury Brown-Twiss Effect for Atoms (1)
Detector 1 Detector 2
Hanbury Brown-Twiss Effect for Atoms (2)
Detector 1 Detector 2
There‘s another way ...
Hanbury Brown-Twiss Effect for Atoms (3)
Detector 1 Detector 2
Cannot fundamentally distinguish between both paths...
Two Particle
Detection probability
ie 2
Relative phase accumulated When propagating from sourceto detector
Hanbury Brown-Twiss Effect for Atoms (4)
Interference in Two-Particle Detection Probability
Detector 1 Detector 2
ie 2
depends on source separation alat
alat
Multiple Wave Hanbury Brown-Twiss Effect
Interference in Two-Particle Detection Probability
Detector 1 Detector 2
alat
Calculation for Ns=6 sites
Detection system
Atom density is in fact integrated over a column parallel to the probe.
In each bin, Nbin>>1 atoms are counted.
Bin geometry :
HBT theory predicts a factor of 2 enhancement of fluctuations, or
In the experiment, the enhancement varies between 10-4 and 10-3 !(note the noise floor ~ 10-4)
: imaging resolution
w : cloud size
Coherence length : also ideal peak width
Great spatial resolution : fringe spacing l >> Lcoh >> res.
Poor spatial resolution : res. >> fringe spacing l >> Lcoh
Intermediate spatial resolution : fringe spacing l >> res. >> Lcoh
How large are the correlations ?
Probe direction : w >> l
Imaging plane:l >> > Lcoh
w
Scaling of correlations
Comparison of the results to a more sophisticated model, taking shell structure into account :
Scaling of the correlation amplitude with 1/N and t2 approximately verified However correlation amplitude is too small by 40 %
Noise floor
2
Applications to the detection of magnetic phases
Antiferromagnet (Bose/Fermi)
Spin waves (Bose/Fermi)
Charge density wave (predicted in Bosons/Fermions mixtures)
E. Altman, E. Demler & M. Lukin, PRA 70, 013603 (2004).
Conclusion and perspectives
• Fundamental deviations from a perfect
Mott state can be observed in the visibility
Signature for particle/hole pairs
Evidence for n>1 shell formation ?
Implications for the fidelity of entanglement
schemes in a lattice
• Spatial correlations of density fluctuations
In expanding clouds
signature of lattice ordering
Applications to the study of magnetic systems;
also works for fermions
Other directions
• Visibility in a 2D lattice D. Gangardt et al., cond-mat (2004) :possible signature of correlations
in each tube (Tonks-Girardeau)
• Dynamical studies
• Resolve shell structure (microwave or rf spectroscopy)
• Detection of magnetic ordering
Adiabatic or diabatic loading
How fast can we go to stay close to the ground state ?
• Produce a 87Rb Bose-Einstein condensate in a purely magnetic QUIC trap
• Expand the condensate to reduce its density (and avoid losses)
• Ramp up slowly lattice beams intensity
• Switch off the trap, expand and take an absorption image
Loading a BEC in the lattice
0
10
Latt
ice d
eoth
(E
R)
160 msHold time
Adiabatic loading in the lattice ?
0
10
Latt
ice d
epth
(E
R)Ramp time Hold time
Smooth profile to ramp upthe intensity of the lattice beams
Typically ramp time = 160 ms
Adiabaticity wrt the band structure : easy to fulfill (s time scale)
J. Hecker-Denschlag et al., J. Phys. B 35, 3095 (2002)
Adiabaticity wrt many body dynamics ?
S. Sklarz et al., PRA 66, 053620 (2002).S. Clark and D. Jaksch, PRA 70, 043612 (2004).J. Zakrzewski, PRA 71, 043601 (2005).
Influence of ramp time(SF regime)
Fix :
• Lattice depth V0 = 10 ER
• Hold time thold = 300 ms
Vary ramp time
1
20 100 200Ramp time (ms)
0
0.5
Vis
ibili
ty160 ms
Time constant ~ 100 msMuch longer than microscopic time scales
Lattice depth V0 = 10 ER
N=2.2 105
N=3.6 105N=4.3 105N=5.9 105
Adiabaticity in the MI state
• Superfluid regime :Time constant ~ 100 ms, much longer than tunneling time, trap frequencies, …
Long-lived collective excitations involved
• MI regime :Breakdown of adiabaticity for lattice depth such that the tunneling time is
comparable to ramp time Single particle redistribution
Compare calculated to measured visibility in the deep MI state
Breakdown around V0~25 ER, whereTunneling time ~ 200 ms