carsten klempt et al- direct observation of vacuum fluctuations in spinor bose-einstein condensates

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  • 8/3/2019 Carsten Klempt et al- Direct observation of vacuum fluctuations in spinor Bose-Einstein condensates

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    Direct observation

    of vacuum fluctuations

    in spinor

    Bose-Einstein

    condensates

    Carsten Klempt, Oliver Topic, Manuel Scherer,Thorsten Henninger, Wolfgang Ertmer, and Jan ArltInstitute of Quantum Optics

    Gebremedhn Gebreyesus, Philipp Hyllus, and Luis SantosInstitute of Theoretical Physics

    of

    in

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    Vacuum fluctuations in spinor BECs

    Introduction

    Nonlinear crystal

    Optical spontaneous parametric down-conversion

    pair creation parametric

    amplification

    amplification ofvacuum

    fluctuations

    entangled pairs (EPR)

    squeezing

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    Vacuum fluctuations in spinor BECs

    Spinor BECs

    Confined spinor BECs

    Vacuum fluctuations

    Introduction

    Contents

    Experiments

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    Vacuum fluctuations in spinor BECs

    Spinor BECs

    Confined spinor BECs

    Vacuum fluctuations

    Introduction

    Contents

    Experiments

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    Vacuum fluctuations in spinor BECs

    Atoms with non-zero spin

    Spinor gases:

    Alkali-atoms

    with non-zero spin

    Here:

    87Rb F=2 mF=0

    E= p m + q m2

    p ~ B

    q ~ B2

    < 0

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    Vacuum fluctuations in spinor BECs

    Energy scales

    Competing energy scales define the ground state

    mF=0

    mF=1

    Repulsive interaction

    quadratic Zeeman energy interaction energy

    mF=0

    mF=1

    Attractive interaction

    quadratic Zeeman effect

    interactions

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    Vacuum fluctuations in spinor BECs

    Ground states of spinor BECs

    H. Schmaljohann et al., PRL 92, 040402 (2004).

    Ground states ofF =2 Spinor Bose-Einstein Condensates

    Ciobanu et al., PRL 61, 033607 (2000).

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    Vacuum fluctuations in spinor BECs

    Spinor BEC in F=2, mF=0

    ( ) ( ) ( ) ( )

    ( )( )( )( )

    ( )

    +

    =+=

    +

    +

    r

    r

    r

    r

    r

    rrrr

    r

    r

    r

    r

    r

    rrrrrrr

    2

    1

    0

    1

    2

    00

    0

    0

    0

    0

    ( ) ( ) ( ) ( )rrUrVM

    rrrrhr

    0

    2

    000

    22

    02

    ++

    =

    Condensate Excitations

    Scalar Gross-Pitaevskii equation for m=0

    Field operator for the spinor BEC in F=2 mF=0

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    Vacuum fluctuations in spinor BECs

    Hamiltonian for spin excitations

    ( )[ ]( )[ ]

    ++

    +

    =

    +

    +=

    +

    1111

    1

    3

    r

    qrH

    rdHeff

    m

    meffm

    r

    r

    ( ) ( )rnUreffrr

    011

    Hamiltonian (linear regime)

    ( ) ( ) ( ) ( ) +++

    rnUUrVMrHeff

    rrhr01100

    22

    2

    Veff

    Veff

    Spin changing collisions

    This Hamiltonian describes the time evolution of the mF=1 components

    homogeneous case

    mF=0

    Identical to optical parametric amplification !

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    Vacuum fluctuations in spinor BECs

    Spin Bogoliubov modes

    2 possibilities [Lamacraft, PRL 98, 160404 (2007)]

    1) Real Eigenvalues: system is stable

    no population in mF=1

    2) Complex Eigenvalues: system is unstable

    Eigenmodes of the Hamiltonian

    are Spin Bogoliubov modes in mF=1with wavevector k

    which are exponentially amplified: e2 Im(E) t

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    Vacuum fluctuations in spinor BECs

    Stability diagram (homogeneous)

    q>0

    stable

    qcr

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    Vacuum fluctuations in spinor BECs

    Measurements on homogeneous system

    87Rb F=1

    Creation of spatial structure (broken symmetry)Sadler et al., Nature443, 312 (2006)

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    Vacuum fluctuations in spinor BECs

    Spinor BECs

    Confined spinor BECs

    Vacuum fluctuations

    Introduction

    Contents

    Experiments

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    Vacuum fluctuations in spinor BECs

    Confined case

    What changes when weconsider the trapping potential ?

    quadratic Zeeman effect

    interactions

    confinement

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    Vacuum fluctuations in spinor BECs

    Hamiltonian for spin excitations

    ( )[ ]( )[ ]

    ++

    +

    =

    +

    +=

    +

    1111

    1

    3

    r

    qrH

    rdHeff

    m

    meffm

    r

    r

    ( ) ( )rnUreffrr

    011

    Hamiltonian (linear regime)

    ( ) ( ) ( ) ( ) +++

    rnUUrVMrHeff

    rrhr01100

    22

    2

    Veff

    Veff

    This Hamiltonian describes the time evolution of the mF=1 components

    confined case

    mF=0

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    Vacuum fluctuations in spinor BECs

    Spin Bogoliubov modes

    2 possibilities

    1) Real Eigenvalues: System is stable

    no population in mF=1

    2) Complex Eigenvalues: System is unstable

    Eigenfunctions of the Hamiltonian

    are Spin Bogoliubov modes in mF=1composed of the eigenstates of the eff. Potential

    which are exponentially amplified

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    Vacuum fluctuations in spinor BECs

    Stability diagram (homogeneous)

    q>0

    stable

    qcr

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    Vacuum fluctuations in spinor BECs

    Spinor BECs

    Confined spinor BECs

    Vacuum fluctuations

    Introduction

    Contents

    Experiments

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    Vacuum fluctuations in spinor BECs

    Experimental setup

    Dispenser

    MOT cell(10-9 mBar) science cell

    (< 10-11 mBar)

    differentialpumping stage

    LIAD: an efficient, switchable atom sourceC. K., T. van Zoest, T. Henninger, O. Topic, E. Rasel, W. Ertmer und J. Arlt, Phys. Rev. A 73, 13410 (2006).

    4109 atomsat 45 K

    87Rbmolasses

    40Kmolasses

    5107 atomsat 260 K

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    Vacuum fluctuations in spinor BECs

    Optical dipole trap

    Optical dipole trap ( =1064 nm, 2 W)whorizontal = 25 m, wvertical = 60 m

    T = 1,0 K

    center of coils:(100 Hz / 15 Hz)

    6 106 87Rb-atoms

    3 106 40K-atoms

    4 106 87Rb-atoms

    2 106 40K-atoms

    T = 1,5 K

    dipole trap(370 Hz / 170 Hz)

    transfer

    evaporation

    3 105 87Rb-atoms

    3 105 40K-atoms

    T = 270 nK

    B = 8 mG @ 550 G

    B = 4 mG @ 1 G

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    Vacuum fluctuations in spinor BECs

    F=2

    mF=-1

    mF=0

    mF=2

    mF=1

    mF=-2

    78 80 82 84 86

    -8

    -6

    -4

    -2

    0

    2

    4

    6

    8

    Energyofc

    oupledsystem[

    MHz]

    Radiofrequency [MHz]

    Radio frequency

    sweep in 1ms

    78 Mhz 82 MHz

    B-Field=120G

    mF

    = -1

    mF=0

    mF=2

    mF

    =1

    mF= -2

    mF=1

    mF=0

    mF= -2

    mF

    = -1

    mF=2

    Spin preparation

    87Rb

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    Vacuum fluctuations in spinor BECs

    Hold time with

    magn. field

    Stern-Gerlach and

    detection

    mF=0 preparationat 120 G

    297.5mG 395mG 512mG 570.5mG

    x=46Hz

    y=132Hz

    z=176Hz

    NBEC

    =4x104

    thold=21ms

    Measuring sequence

    Purification:strong gradient

    0

    -1

    +1

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    Vacuum fluctuations in spinor BECs

    Spin dynamics resonances

    exp

    theory

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    Vacuum fluctuations in spinor BECs

    Ground state and excited states

    1st Resonance 2nd Resonance

    mF=1

    mF=0

    mF=-1

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    Vacuum fluctuations in spinor BECs

    Spinor BECs

    Confined spinor BECs

    Vacuum fluctuations

    Introduction

    Contents

    Experiments

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    Vacuum fluctuations in spinor BECs

    Triggering of spinor dynamics

    2 possibilities

    1) System is stable

    no population in mF=1

    2) System is unstable

    Eigenfunctions of the Hamiltonian

    are Spin Bogoliubov modes in mF=1composed of the eigenstates of the eff. Potential

    which are exponentially amplified

    What triggers

    the spinor dynamics ?

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    Vacuum fluctuations in spinor BECs

    Seed

    2 possibilities

    1) Spurious radiofrequencies or magnetic field noise

    produce atoms in mF=1 in the spatial mode of the

    initial BEC classical seed

    2) No atoms present in mF=1 vacuum fluctuations

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    Vacuum fluctuations in spinor BECs

    Experimental verification

    exp

    theory

    Produce a classical seeddeliberately:

    Radio-frequency pulsewith extreme small amplitudecouples the BEC in mF=0symmetricallyto the states mF=1

    (after purification)

    V fl t ti i i BEC

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    Vacuum fluctuations in spinor BECs

    Classical vs quantum seed

    exp

    theory

    classical

    seed

    vacuum

    fluctuations

    V fl t ti i i BEC

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    Vacuum fluctuations in spinor BECs

    Amplification of classical and quantum seed

    classical fluctuations vacuum fluctuations

    mF=1

    mF=0

    mF=-1

    Vacuum fluctuations in spinor BECs

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    Vacuum fluctuations in spinor BECs

    Overlap of classical seed

    How relevant is the classical seed ?

    Vacuum fluctuations in spinor BECs

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    Vacuum fluctuations in spinor BECs

    Outlook

    Analyze spatial structurequantitatively

    Quantify fluctuations

    Demonstrate squeezing(measure quadratures)

    Extract EPR pairs ? Investigate effects ofdipolar interaction

    Vacuum fluctuations in spinor BECs

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    Vacuum fluctuations in spinor BECs

    People

    Carsten Klempt, Oliver Topic, ManuelScherer, Thorsten Henninger, WolfgangErtmer, and Jan ArltInstitute of Quantum Optics

    Gebremedhn Gebreyesus, Philipp Hyllus,and Luis Santos

    Institute of Theoretical Physics