Upload
others
View
1
Download
0
Embed Size (px)
Citation preview
Massimo InguscioMassimo Inguscio
Dipartimento di Fisica e Astronomia &Dipartimento di Fisica e Astronomia &
LENS LENS –– Università di FirenzeUniversità di Firenze
INOINO--CNRCNR
SIF Napoli, 20 SIF Napoli, 20 settembresettembre 20122012
Fasi quantistiche della materiaFasi quantistiche della materia
vverso lo zero assolutoerso lo zero assoluto
Verso lo zero assoluto...Verso lo zero assoluto...
sun surface water boiling water freezing liquid nitrogen
quantum degenerate dilute gases
(Bose / Fermi)
cosmic background radiation superfluidity, superconductivity
laser cooling
tem
pera
ture
BOSONS FERMIONS
EF
Ultracold quantum gases
87Rb 2001 SCIENCE
41K boson
87Rb
40K fermion
2002 PRL
Raffreddamento laser
atom
photon
p mv
p k
conservation of momentum in the
collision between atoms and photons
hottest atoms removal
+
collisions between trapped atoms
cooling
Raffreddamento evaporativoRaffreddamento evaporativo Evaporative cooling
http://www.colorado.edu/physics/2000/bec/
Ettore Majorana (1906-1938?)
E. Majorana, Nuovo Cimento 9, 43 (1932)
Majorana spin-flip
Quadrupole trap
magnetic field = 0
spin flips
Majorana spin-flip
Ioffe-Pritchard trap
magnetic field > 0
no spin flips
Bose-Einstein condensation
Bose-Einstein condensation:
atom number: N ~ 105
size: L ~ 10 mm
density: n ~ 1014 cm-3
temperature: T ~ 10-100 nK
T > TC
T < TC
T ~ TC
BEC
“thermal”
100 mm
Principio di indeterminazione di Heisenberg
caduta libera di un condensato di 87Rb
Intrappolare con la luceIntrappolare con la luce
Trappole fatte di luce
The atomic induced electric
dipole then interacts with the e.m.
wave
Optical trapping
U(r) E(r)p
Far off resonance light induces
an electric dipole
p E
“red” traps
“blue” traps
optical dipole potential detuning
Optical traps
Single-beam trap
1 mm
1 mm
Crossed-beam trap
Optical trapping
P. Pedri et al., Phys. Rev. Lett. 87, 220401 (2001)
2001 Centenario del Premio Nobel
Josephson effect:
The oscillation frequency w*
depends on the effective mass m*
F. S. Cataliotti et al.,
Josephson junctions arrays with BECs
Science 293, 843 (2001)
depending on the tunnelling energy J:
Dipole oscillations and Josephson effect
**
m
mw w
2
2
2*m m
J
Disruption of superfluidityDisruption of superfluidity
Anderson
interaction
Mott Superfluid
Subtle interplay between disorder and interactions
(granular superconductors, superfluid He in porous media, high-Tc, …)
Disordered interacting bosons
Bose glass...
T. Giamarchi and H. J. Schultz,
Anderson localization and interactions in one-dimensional metals
Phys. Rev. B 37, 325 - 340 (1988)
Phase diagram of disordered interacting bosons
Disruption of superfluidityDisruption of superfluidity
wwith interactionsith interactions
Optical lattices
Ultracold atoms in optical lattices
atoms in optical lattices electrons in a solid
Electrons vs atoms
Transizione all’isolante di Mott
LENS, 2005
quantum phase transition induced by repulsive interactions
superfluid
Mott insulator
Light scattering
Scattering (light, neutrons, etc...) provides information on the structure of matter:
excitations
Bragg scattering
Bragg scattering as stimulated inelastic scattering of light
absorption
1st beam
stimulated emission
2nd beam
cold atoms
Physical Review Letters 102, 155301 (2009)
Bragg spectra across the SF-MI transition
MI region ~ U
low energy, superfluid
In the MI regime different contributions to the spectrum can be distinguished:
SF MI / SF
Physical Review Letters 102, 155301 (2009)
Bragg spectroscopy of quantum lattice gases
L. Fallani and M. Inguscio
Controlling cold-atom conductivity
Science 322 (5th December 2008)
…Can physics be simulated by a universal computer? Richard P. Feynman, Int. J. Theor. Phys 21, 467 (1982)
Richard P. Feynman realized that certain phenomena in
Quantum Field Theory are well imitated by certain
Condensed Matter systems…
He thought that there should be a certain class of quantum
mechanical systems which would simulate any other system, a
UNIVERSAL QUANTUM SIMULATOR
that could serve as a quantum laboratory where the validity
of several theoretical models may be tested.
Disruption of superfluidityDisruption of superfluidity
wwith disorderith disorder
Anderson
interaction
Mott Superfluid
Subtle interplay between disorder and interactions
(granular superconductors, superfluid He in porous media, high-Tc, …)
Disordered interacting bosons
Bose glass...
T. Giamarchi and H. J. Schultz,
Anderson localization and interactions in one-dimensional metals
Phys. Rev. B 37, 325 - 340 (1988)
Phase diagram of disordered interacting bosons
Disorder
Effects of disorder are quite difficult to treat theoretically
Perturbative methods generally fail: small changes in the disorder strength
may result in dramatic changes in the transport properties of the system
Transition from a conductor to an insulator predicted by P. W. Anderson in 1958
Disorder is intrinsically present in all real materials:
impurities
vacancies
interstitials
dislocations
...
340 350 360 370 380 390 400 410
-400
0
400
-100
0
100
bin
din
g e
nerg
y (M
Hz)
scatt
erin
g length
(a0)
magnetic field (G)
348 350 352
-1
0
1
340 350 360 370 380 390 400 410
-400
0
400
-100
0
100
bin
din
g e
nerg
y (M
Hz)
scatt
erin
g length
(a0)
magnetic field (G)
.
Da = 0.06 a0
K3 = 1.3(5)10-29 cm6s-1
Interferometry: Fattori et al., PRL 100, 080405 (2008) Dipolar effects: Fattori et al., PRL 101, 190405 (2008)
39K BEC with tunable interactions G. Roati et al., PRL 99, 010403 (2007)
Dynamics under a costant force
g
Momentum distribution of
ultracold 40K fermions in a
vertical optical lattice:
w, k
w, k
1at B
rillo
uin
zone
Costant force gravity
Bloch oscillations with period
Bloch oscillations (non-interacting fermions) G. Roati et al., PRL 92, 230402 (2004)
Constant external force
Bloch oscillations
Accurate probe of forces
Bloch oscillations (BEC with tunable interactions) M. Fattori et al., PRL 100, 080405 (2008)
Adding interactions...
Schrödinger equation Gross-Pitaevskii equation
tunable with Feshbach!
Beginning of the story: BEC in a disordered potential (Florence, Orsay, Hannover, Rice, Illinois...)
BEC in disordered potentials Physical Review Letters 95, 070401 (2005)
bichromatic lattice speckle pattern
L.Fallani, C.Fort, M.Inguscio
Bose-Einstein condensates in disordered potentials
Advances Atomic, Molecular and Optical Physics vol 56, pp 119-160
edited by E.Arimondo, P.Berman, C.Lin (Academic Press 2008)
How to produce disorder
Anderson localization P. W. Anderson, Phys. Rev. 109, 1492 (1958)
Anderson model
quantum particles hopping in a disordered lattice
Anderson localization
one electron in a periodic lattice TRANSPORT
Anderson localization
introducing disorder in the lattice
Anderson localization
one electron in a disordered lattice LOCALIZATION
Localized states
Diffusion stops because the eigenstates are localized!
Periodic: wavefunction is delocalized on the whole system size
Disordered: eigenstates are localized in a finite region of space
exponentially decaying amplitude of wavefunction
Incommensurate bichromatic lattice
Tight-binding model with quasi-periodic on-site energies
The second lattice controls the site energies
Dan Shechtman Nobel Prize Chemistry 2011
no periodicity long-range order
Quasicrystals
The bichromatic lattice
The physics of bi-periodic systems interpolates between periodic systems and
disordered systems, as the degree of incommensurability is changed.
• Quasi-crystals
The physics of quasicrystals, ed. P. J. Steinhardt
and S. Ostlund (World Scientific, 1987)
• Fractal critical behavior
D. R. Hofstadter, Phys. Rev. B 14, 2239 (1976).
• Energy bands
M. Modugno, New J. Phys. 11, 033023 (2009).
• Localization transition in 1D
S. Aubry, G. André, Ann. Isr. Phys. Soc. 3, 133 (1980)
M. Modugno, New J. Phys. 11, 033023 (2009).
Extended and localized states
Localization transition in 1D incommensurate bichromatic lattice
S. Aubry and G. André, Ann. Israel Phys. Soc. 3, 133 (1980)
localized states:
extended states:
Disordered people
L. Fallani
G. Modugno
C. D’Errico
C. Fort
G. Roati M. Fattori
M. Modugno
M. Zaccanti
Probing the transport properties
A noninteracting 39K BEC is initially confined in a harmonic trap and then left
free to expand in the bichromatic lattice
39K BEC
Feshbach coils
Optical waveguide Bichromatic lattice
BEC
D=0
DJ=1
DJ=7
Ballistic expansion
with reduced velocity
Absence of diffusion:
Ballistic expansion:
Absence of diffusion G. Roati et al., Nature 453, 895 (2008)
Expansion in the bichromatic lattice
D/J = 0 D/J = 1.8 D/J = 4.2 D/J = 7
0 ms
750 ms
tim
e
expansion
expansion at
reduced speed
localization
G. Roati et al., Nature 453, 895 (2008)
Expansion in the bichromatic lattice G. Roati et al., Nature 453, 895 (2008)
Size of the condensate after 750 ms expansion in the bichromatic lattice:
Observing the localized states
momentum distribution with
narrow peaks
broad momentum
distribution
FT imaging of the
atomic state via time-of-flight
Observing the localized states
experiment theory
G. Roati et al., Nature 453, 895 (2008)
AUBRY- ANDRE’
Hamiltonian
P. W. Anderson, Nobel lecture (1977)
… about the role of interactions
A second reason why I felt discouraged in the early days was that
I couldn’t fathom how to reinsert interactions, and I was afraid
they, too, would delocalize.
The realization that, of course, the Mott insulator localizes without
randomness, because of interactions, was my liberation on this:
one can see easily that Mott and Anderson effects supplement,
not destroy, each other…
The present excitement of the field for me is that a theory of
localization with interactions is beginning to appear…
It is remarkable that in almost all cases interactions play a vital
role, yet many results are not changed too seriously by them.
39K BEC with tunable interactions G. Roati et al., PRL 99, 010403 (2007)
fine control on the
atom-atom interactions!
Feshbach resonance
Single localized state Anderson glass Fragmented BEC Bose-Einstein condensate
Interaction-induced delocalization
Delocalization scenario for increasing atom-atom repulsion
B. Deissler et al., Nature Physics 6, 354 (2010)
Single localized state Independent single-particle localized states
Creation of superfluid fragments Coherent macroscopic wavefunction
Diessler et al
Nature Physics 6, 354 (2010)
Momentum distributions for D/J=15
0 0.6a0 1.2a0 3a0 4.5a0 7a0 12a0 20a0 40a0 190a0
scattering length
Momentum distribution with interactions Nature Physics 6, 354 (2010)
Interaction-induced delocalization
Width of the central peak: Momentum distribution:
experiment theory
inte
ractio
ns
Repulsive interactions shift the
localization transition towards larger D!
Nature Physics 6, 354 (2010)
Ultracold quantum gases in Florence
FundingFunding byby ERC, EU FP7, IIT, MIUR, CNR, …ERC, EU FP7, IIT, MIUR, CNR, …
Ultracold quantum gases in Florence
Giacomo Giacomo RoatiRoati (Li)(Li)
2D Fermi 2D Fermi gasesgases
FundingFunding byby ERC, EU FP7, IIT, MIUR, CNR, …ERC, EU FP7, IIT, MIUR, CNR, …
K. Alexander Müller
Nobel Prize in Physics 1987 (with J. Georg Bednorz)
"for their important break-through in the discovery
of superconductivity in ceramic materials"
Relevant for high-Tc superconductors
Repulsive/attractive Fermi-Hubbard model
...tra 25 anni?
The coldest side of Florence
http://quantumgases.lens.unifi.it