Yong P. Chen Quantum Matter and Devices Lab Department of Physics and School of Electrical and Computer Engineering and Birck Nanotechnology Center Purdue University <[email protected]> Expanding Atomic Quantum Gases with tunable interaction and disorder ---emulating dynamics and fluctuations in superfluids … and nuclear matter? Brookhaven National Laboratory May 23, 2008 Nuclear Physics & RIKEN Theory Seminar
50
Embed
with tunable interaction and disorder ---emulating … › quantum › files › bnl2008.pdfdiffusive plate (rough glass, CNT film etc) IR (1030nm) CCD can image both disorder potential
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Yong P. ChenQuantum Matter and Devices Lab
Department of Physics andSchool of Electrical and Computer Engineering and
Advantages: Tunable system parameters (customer-designed Hamiltonians): You can watch the system (probe both density and phase) to evolve!
How can we design condensed matter experiments to emulate nuclear/high energy physics phenomena?(esp. those difficult to test in real NP/HEP experiments, i.e. accelerators)
Emulational physics?
Talk outline
• Brief review of cold atoms & BEC---examples of NP/HEP connection
• Tunable interaction and expanding BEC• Disordered BEC:
superfluid to insulator transitionevolution of quantum and density
• in-situ imaging of cloud-- probe ground state wavefunction
• free expansion: momentum space translates into real space---probe phase coherence/fluctuation
•(“restricted-fly”) evolution dynamics in selective potentials
• excite and collective mode
How to Experimentally Probe an Atomic Gas
“sit”
“fly”
“kick”
Just watch it!
A golden decade of quantum gases of cold atomic quantum gases --- some highlights• Bose-Einstein Condensate (BEC) [1995] now in most alkali, H, He* and Yb, Cr
• We measure global transport <different from local-insulator probe such as Greiner>
Apparently 3 different regimes of transport:• High VD : inhibition of transport (global insulator)
“pinning point” (0.7-1µ) not universal value (dep on disorder realizationand measurement type) --- probably sensitive to high peaks in disorder?
• Small VD damps dipole excitation dissipation or dephasing? “metallic” regime? (cf: on lattice, DeMarco’07)dissipation mechanism --- “phase slips”, vortices ...??where does the energy go?
• Medium VD : overdamped transport what’s the transport mechanism? tunneling assisted?fate of this regime at T=0? --- “semiconductor” regime?
Time of Flight (TOF) Free Expansion
372 75 248 5 124 25 0 124 25 248 5 372 75
disorder
trap
BEC
TOF free expansion time (τ) [few ms]
Time of Flight probes phase coherence
Time of Flight (TOF) Free Expansion
disorder:
off
on
off
on
optical trap:
off
TOF free expansion time (τ) [few ms]
imaging cloud
<~1s
τTOF
0ms
1ms
2ms
3ms
6ms
8ms
0 Hz
note: color scale all relative!
VD
VD ~0.3µ
VD ~µ
0ms
1ms
2ms
3ms
6ms
8ms
increasing disorder...700 Hz
chemical potential µ~1 kHzτTOF
0ms
1ms
2ms
3ms
6ms
8ms
0 Hz
note: color scale all relative!
VD 200 Hz
(Random) interference “stripes”Yong P. Chen et al., PRA(2008)
-500 0 500
tTOF = 8 mstTOF = 0 ms (in-situ)
z (μm) z (μm)-500 0 500
VD/μ~0.3
VD/μ~0.5
VD/μ~1.0
VD/μ=0
0.00 0.25 0.50 0.75 1.000.0
0.2
0.4
0.6
0.8
1.0
1.2
8 ms TOFIn-situ
VD / μ
Con
trast
Density Fluctuations: In-situ vs TOF
Column density(phase-contrast imaging)
Random butreproducible interference fringes
Phase coherence
1400
1200
1000
800
600
400
200
0
arb
unit
5004003002001000
pixel
TOF: 4.5 ms 6 ms 9 ms 0 ms
Vd=500Hz
•not correspond to insitu disorder(though does vary with disorder realization)
•can be reproduced from shot-shot!•not sensitive to “hold time”--unlikely from random quantumfluctuation (in phase)
Features of the interference pattern
BEC phase fluctuation at t=0 ----> density fluctuation (t=τ)
n (r ) = Ψ (r ) 2 :
φ :
t=0
∂∂x
eikx( )= ikeikx
(phase ~ velocity) n (a
rb. u
nit)
25002000150010005000x (µm)
Numerics (GPE) show can be derivedand evolved from initial density fluctuation & phase coherent ground state(Clement &Sanchez-Palencia’07)
Disorder-inducedPhase fluctuation in BEC?
n(r) = Ψ(r) 2 :
φ :
t=0 t=τ
free expansion
∂∂x
eikx( )= ikeikx
(phase ~ velocity)
phase fluctuations also observed in elongated interacting BEC at finite TS.Dettmer et al., (W.Ertmer, Hanover), PRL2001
in our case: (quantum)phase fluctuation due to disorder instead of heating!
Lφ = Lφ (T ,<Vd >, B[a])
(Reproducible) Random Interference : Matterwave Speckle
Note: in periodic potential (lattice), interference pattern (one-shot) occurs even w/o coherence
Random potential+reproducible “fluctuation” pattern=>coherence--mesoscopic physics
“interference” stripes <---coherence• reproducible• only BEC (not seen with thermal gas)• 1D modulation<--1D disorder
Reproducible TOF fringes in disordered 87Rb BEC: eg, D. Clement et al., arXiv. 0710.1984 (2007)
n (a
rb. u
nit)
25002000150010005000x (µm)
Digression: Speckle and Mescoscopic Physicsconductance through small conductors (wires, rings etc.):==>Universal Conductance Fluctuation (UCF)--- phase coherence transport
and analog of speckle for electron wavefunction
•mesoscopic: phase coherence L~ sample size•cold atoms/BEC excellent lab to study mesoscopic effects!
R.Webb’84
Strong Disorder:
VD(all 6ms TOF)
0Hz
B~720G (interacting)
8ms TOFVD/μ1
0.6
0.3
0.1
0.03
0
0ms (in-situ)
“tight binding”limit(random Josephson)[also in Rb exp]