B. Naylor (PhD), Johnny Huckans O. Gorceix, B. Laburthe-Tolra, E. - - PowerPoint PPT Presentation

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B. Naylor (PhD), Johnny Huckans O. Gorceix, B. Laburthe-Tolra, E. - - PowerPoint PPT Presentation

Oct 09, 2022 26 likes •256 views

Cooling of a Bose Einstein Condensate by Spin distillation B. Naylor (PhD), Johnny Huckans O. Gorceix, B. Laburthe-Tolra, E. Marchal, L. Vernac, P. Pedri (Theory), M. Robert de Saint-Vincent Have left: A.de Paz, A. Chotia, A. Sharma, B.

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SLIDE 1

Have left: A.de Paz, A. Chotia, A. Sharma, B. Pasquiou , G. Bismut, M. Efremov, Q. Beaufils,

  • J. C. Keller, T. Zanon, R. Barbé, A. Pouderous, R. Chicireanu

Collaborators: Anne Crubellier, Mariusz Gajda, L. Santos (Theory, Hannover), Perola Milman, Rejish Nath

  • B. Naylor (PhD), Johnny Huckans
  • O. Gorceix, B. Laburthe-Tolra, E. Maréchal, L. Vernac,
  • P. Pedri (Theory), M. Robert de Saint-Vincent

Cooling of a Bose Einstein Condensate by Spin distillation

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SLIDE 2

Cooling Mechanisms Evaporation:

Very Efficient up to

Demagnetization cooling:

Lose most energetic atoms, Then system rethermalises

Why do we need new cooling mechanisms? Colder gases? Very recent result, beat most limitations Olf et al. arXiv:1505.06196 (2015)

Transfer of kinetic energy into magnetic energy

Fattori et al. Nature Physics 2,765-768(2006)

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SLIDE 3

Spin ½ interacting Fermions or Bosons Super-exchange interaction Esslinger: short range anti-correlations

  • I. Bloch, T. Porto, W. Ketterle, R.G. Hulet…

Quantum magnetism

Cold atoms in a lattice Magnetic correlations appear when

th B c B

f k T T k N S 6 . 3 6 . 3 /

3

=

  • ≈

Entropy of a saturated cloud:

For a fully saturated gas, the entropy is given by the thermal fraction Removing entropy removing thermal atoms

Observe magnetic correlation

A condensed atom carries no entropy

A gas at a given T has an upper limit to the number of thermal atoms

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SLIDE 4

Stern-Gerlach separation: (magnetic field gradient)

S

m = 1

S

m = 1

S

m = −

  • 2
  • 1

1 2 3

  • 3

Optical dipole traps equally trap all Zeeman state of a same atom

( )

2

( )

S S B

E m m g B B µ α = +

Linear Zeeman effect

Chromium

Large electronic Spin: S=3

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SLIDE 5

Feature introduced by dipolar interactions: Free Magnetization

  • 3 -2 -1 0 1 2 3

dd

V Γ ≈

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SLIDE 6

Spontaneous magnetization due to BEC

BEC only in mS=-3 (lowest energy state) Cloud spontaneously polarizes ! Thermal population in Zeeman excited states T>Tc T<Tc a bi-modal spin distribution

  • 3 -2 -1 0 1 2 3
  • 3 -2 -1 0 1 2 3

Pasquiou et al. PRL 108, 045307 (2012) B=0.9mG

slide-7
SLIDE 7

Spin Cooling: Principle of the Experiment Step 1:

  • 2
  • 1
  • 3

Linear Zeeman

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SLIDE 8

Spin Cooling: Principle of the Experiment Step 1: Step 2:

  • 2
  • 1
  • 3
  • 2
  • 1
  • 3

Linear Zeeman

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SLIDE 9

Spin Cooling: Principle of the Experiment Step 1: Step 2: Step 3:

  • 2
  • 1
  • 3
  • 2
  • 1
  • 3

Linear Zeeman With RF pulse or Magnetic field gradient

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SLIDE 10

A competition between two mechanisms

BEC Thermal

ms=-3 ms=-3, -2, -1, …

(i) Thermal cloud depolarizes (ii) BEC melts to re- saturate ms=-3 thermal gas (and cools it) (iii) Kill spin-excited states

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SLIDE 11

A competition between two mechanisms

BEC Thermal

ms=-3 ms=-3, -2, -1, …

(i) Thermal cloud depolarizes (ii) BEC melts to re- saturate ms=-3 thermal gas (and cools it) (iii) Kill spin-excited states

BEC melts (a little)

? Who Wins ?

Losses in thermal cloud due to depolarization

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SLIDE 12

0.8 0.6 0.4 0.2 1.2 1.0 0.8 0.6 0.4 0.2 0.0

final condensate fraction

A competition between two mechanisms

BEC Thermal

ms=-3 ms=-3, -2, -1, …

(i) Thermal cloud depolarizes (ii) BEC melts to re- saturate ms=-3 thermal gas (and cools it) (iii) Kill spin-excited states

BEC melts (a little)

? Who Wins ?

BEC fraction

Losses in thermal cloud due to depolarization

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SLIDE 13

1.2 1.0 0.8 0.6 0.4 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3

c f

T T

  • c

i

T T

  • A competition between two mechanisms

At high T/Tc, BEC melts (too few atoms in the BEC to cool the thermal gas back to saturation) At low T/Tc, spin filtering of excited thermal atoms efficiently cools the gas Theoretical model: rate equation based on the thermodynamics of Bosons with free magnetization. Interactions are included within Bogoliubov approximation B=1,5mG

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SLIDE 14

Summary of the experimental results as a function of B

Final condensate fraction

(large field, no effect)

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SLIDE 15

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.2 0.4 0.6 0.8

c f

T T

  • c

i

T T

  • Theoretical limits for cooling

There does not seem to be any limit other than practical In principle, cooling is efficient as long as depolarization is efficient Process can be repeated At each spiling, a factor 2 in entropy is gained

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SLIDE 16

Extension to ultra-low temperatures for non-dipolar gases

In our scheme, limitation around 25 nK, limited by (difficult to control below 100 µG)

B g T k

B J B

µ ≈

Proposal: use Na or Rb at zero magnetization. Spin dynamics occurs at constant magnetization

  • 1

1

G kHz B g

B J

/ 8 . 2 = µ

2

/ 70 G Hz q ≈

F=1, mF=-1, 0, 1

We estimate that temperatures in the pK regime may be reached Nota: the spin degrees of freedom may also be used to measure temperature Olf et al. arXiv:1505.06196 (2015)

(0 0) (-1 1)

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SLIDE 17

Shock Cooling

Initial conditions: Thermal Gas at 1µK in every spin state ms = -3 -2 -1 0 1 2 3

Collaboration with M.Gajda et M.Brewczyk

Preliminary Results

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SLIDE 18

Shock Cooling

Initial conditions: Thermal Gas at 1µK in every spin state ms = -3 -2 -1 0 1 2 3 Then Evaporation while monitoring Spin distribution and momentum distribution What happens? Magnetic Order? Bose order?

Collaboration with M.Gajda et M.Brewczyk

Preliminary Results

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SLIDE 19

Shock Cooling

Initial conditions: Thermal Gas at 1µK in every spin state ms = -3 -2 -1 0 1 2 3 Then Evaporation while monitoring Spin distribution and momentum distribution BEC forms only in ms=-3 and ms=-2 gas is saturated but never forms a BEC !

Optical Depth

  • 3

position

  • 2

Saturated Thermal Gas BEC

Collaboration with M.Gajda et M.Brewczyk

Preliminary Results

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SLIDE 20

Shock Cooling

1.0 0.8 0.6 0.4 0.2 0.0 800 600 400 200 condensate fraction in ms=-3 BEC_theo

  • Preliminary results suggest good agreement
  • Simulations give a BEC in ms=-3 and a saturated

thermal gas in ms=-2 with no BEC

Interpretation?

Favorable fast dynamics (-2 -2) (-3 -1)

Time (ms) Fraction condensée dans ms=-3

Collaboration with M.Gajda et M.Brewczyk

Preliminary Results

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SLIDE 21

Conclusion

1.2 1.0 0.8 0.6 0.4 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3

c f

T T

  • c

i

T T

  • New cooling mechanism

to reach very low entropies (in bulk): Use spin to store and remove entropy Should be applicable to non-dipolar species , with pK regime possible

arXiv:1505.05098

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SLIDE 22
  • A. de Paz (PhD), A. Sharma, A. Chotia, B. Naylor (PhD)
  • E. Maréchal, L. Vernac,O. Gorceix, B. Laburthe
  • P. Pedri (Theory), L. Santos (Theory, Hannover)

Thank you

Aurélie De Paz Amodsen Chotia Arijit Sharma

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SLIDE 23

Thanks ! – Come and Visit