Cooling chromium a dipolar quantum gasCr 3+

Tilman Pfau University of Stuttgart

Outline Lecture 1 Cooling atoms to degeneracy The case of Chromium Laser cooling and its limits Magnetic trapping Good and bad collisions Optical dipole traps and evaporation to BEC Demagnetization cooling

Outline Cooling techniques Uncontrolled dipolar interactions: a pain in the neck Controlled dipolar interaction: Demagnetization cooling Quantum ferro fluid

chromium in technics

chromium in the literature

Cr in periodic table of elements

70Yb

Cr propertiesisotopic distribution Bosons (I=0): 52Cr (83.8%), 50Cr (4.3%), 54Cr (2.4%) Fermion (I=3/2): 53Cr (9.5%) Versatile level scheme new cooling mechanisms 6 unpaired electrons electronic configuration: [Ar]3d54s1 S=3

large magnetic moment: = 6 B

Interacting quantum systems in atomic physicscontact interaction van der Waals dipole dipole interaction Coulomb interaction

U coul (r) =short range isotropic long range anisotropic long range isotropic

q1 q2 40 r

MIT

Innsbruck

The phase transition movie

tof = 5 msec Decreasing T

The route to our Cr BEC

Lab tour

BEC Chamber

HOT Atom Source

Preparation of an ultracold Cr sample: Continously loaded Ioffe Pritchard trap (CLIP-trap)J. Stuhler, et al., Phys. Rev. A 64, 031405 (2001) P. O. Schmidt, et al., J. Opt. B 5, S170 (2003)

Compress IP-trap Doppler cooling in the IP-trap at high offset fieldP. O. Schmidt, et al., J. Opt. Soc. Am. B 20, 5 (2003)

2x108 atoms in the ground state phase space density ~10-7 Evaporation

Magneto-optical Trap52

Cregoli ng 42 & t 5. 6 ra nm ppin g7

7

P4

MOT

S3: 6BN= few 106 atoms T=70 K

A. S. Bell, J. Stuhler, S.Locher, S. Hensler, J. Mlynek, T. Pfau, Europhys. Lett. 45, 156 (1999)

co

Magneto-optical Trap52

Cregoli ng 42 & t 5. 6 ra nm ppin g

7

P4

MOT

1E-9

co

1E-10

7

S3: 6BN ~ 106 atoms T=70 K

1E-11

Rb

Li

Na Sr Element

Cr

A. S. Bell, J. Stuhler, S.Locher, S. Hensler, J. Mlynek, T. Pfau, Europhys. Lett. 45, 156 (1999)

Too bad

Gallagher Pritchard PRL 63, 957 (1989)

Continuous Loading Scheme

Continously Loaded Ioffe-Pritchard (CLIP) TrapJ. Stuhler, et al. Phys. Rev. A 64, 031405 (2001), P.O. Schmidt, et al. Journal of Optics B, 5 (2003)

CLIP Trap(Continously Loaded Ioffe-Pritchard Trap) 2D-MOT + 1D molasses weak axial magnetic curvature field atoms are trapped magnetically Advantages: up to 40x more atoms compared to MOT no need for polarizing the atoms no transfer from MOT to IP

Doppler Cooling in the IP TrapTransfer to ground state Doppler cooling

compress MT

P.O. Schmidt, et al. JOSA B, 20 (2003)

Doppler Cooling - Resultsy ~ 100 ms

z~ 10 ms

Tinitial ~ 1 mK Tzfinal = 124 K = TDoppler Tx,yfinal = 300 K Nfinal = Ninitial PSDfinal / PSDinitial ~ 80

Radial Cooling via Reabsorption

number of reabsorbed photons propto Ilaserx ODr

Experiment & Theory

1/rcool propto Ilaser

1/rcool propto ODrtemperature limited by additional heating effects

Great!

CollisionsmJ = +3 mJ = +3

+gjBB

mJ = +2

elastic Collision GOOD

dipolar Relaxation BAD

Dipolar relaxationdipolar relaxation + spin changing collisions

Very good agreement between theory and experiment no BEC in magnetic trap

atom number

time [sec]

Dipole dipole scatteringExactly solvable in Born approximationS. Hensler, J. Werner, A. Griesmaier, P.O. Schmidt, A. Grlitz, T. Pfau, S. Giovanazzi, K. Rzazewski Appl. Phys. B 77, 765 (2003)

elastic scattering

spin relaxation collisions spin changing collisions

Too bad

Trap atoms in energetically lowest statePumping Optical Dipole trap:the atoms to magnetic ground state:20W fibre Laser @ =1064 nm7P 3

=2B B0

7S 3

1st beam: Pmax=9W w0=30m - Umax~130K

mJ=-3 high field seekerPmax=4.5W w0=50m - Umax~22K

mJ=+3 low field seeker

2nd beam: Dipolar relaxation suppressed

Advantages: operation at any offset field all magnetic substates trapable

Problem Sample still mainly polarized in mJ=+3

Great!

Evaporative cooling in a crossed optical trapTotal evaporation time ~30s incl. RF evaporation in MT Evaporation in ODT is very efficient: Maximum PSD gain of 3 orders of magnitude per Order of magnitude loss in number of atoms Max >90. 000 atoms in condensate phase

http://www.colorado.edu/physics/2000/applets/bec.html

The route to our Cr BEC

Condensate fractionideal gasT=1.1K

corr. for finite size and weak interaction*

x=581 Hz y=406 Hz z=138 Hz Tc~700 nK

T=625nK

exp.A. Griesmaier, et al.

PRL 94, 160401 (2005)* S. Giorgini, L. P. Pitaevskii, and S. Stringari, Phys. Rev. A 54, R4633 (1996)

Dipolar relaxationdipolar relaxation + spin changing collisions

Very good agreement between theory and experiment no BEC in magnetic trap

atom number

time [sec]

Demagnetization of chromium is a pain! BUT1915: Einstein - de Haas for a quantum gas?

could it be useful?

Coupling spin and motion

Coherent Einstein - de Haas effect for

B F40F 43S 41D

42P623.704

laser frequency with respect to 5 P 3/2 -level (Thz)

n=39,l>F 39F 42S 40D

453

41D

n=40

41 D5/2

623.700

Dn (MHz) 0 10 electrical field (V/cm) 20

226

41 D3/2

41P

623.696

0

0

5 electrical field (V/cm)

10

electrical field (V/cm)

experimental resultscomparison theory - experiment623.704 453

laser frequency with respect to 5 P 3/2 -level (Thz)

41D

n=40

41 D5/2

623.700

Dn (MHz) [MHz]

laser frequency and electric field varied Stark map excellent agreement theory experiment

226

41 D3/2

623.696

0

10 electrical field (V/cm)

20

0

0

5 electrical field (V/cm)

10

levels split up, according to |mJ| in electric field, states mix excitable from 5P3/2A.Grabowski, R. Heidemann, R. Lw, J. Stuhler and T. Pfau arXiv:quant-ph/0508082

623.7047

laser frequency with respect to 5 P 3/2 -level (THz)

623.704

laser frequency with respect to 5 P 3/2-level (Thz)

623.7046 623.7045 623.7044 623.7043 623.7042 0

623.702

623.700

623.698 0 10 electrical field (V/cm) 20

5 electrical field (V/cm)

10

Reminder: Stark map of Rubidiumlaser frequency with respect to 5 P 3/2-level (THz)140

relative Frequenz (MHz)

n=40,l>F 40F 43S 41D

BEC exp.

120 100 80 60 40 20 0

43S1/2

0

42P n=39,l>F 39F 42S 40D

1 2 3 elektrisches Feld (V/cm)

4

41P

electrical field (V/cm)

Separation of ions from Rydberg atoms

Red shift: ions Blue shift: van der Waals

Rydberg Rydberg interactionblockade condition

C6 ablock6

Max(, )

ryd

ablock 5 m3 nBEC ablock

104

MOT work: Storrs, Paris, Michigan, Freiburg,3 nMOT ablock < 10

ablock 5 m

Collective states super atomsE

G

1 E = { ryd , g , g ,..., g + g , ryd , g ,..., g + ... + g , g , g..., ryd N G = g , g , g ..., g

}

= N 0

Collective coherent time scale

Coherent collective scaling

N 0

collective and coherent !

Rn

0.49 0.06 g,0

1.1 0.1 0

ng,0 0

Change 0

ng,0 = 7.2 1013 cm-3

Fit:

N ryd (t ) = N (1 esat ryd

sat Rt / N ryd

)

Blockade time scale:

R 0 ng

ng,0 = 2.9 1012 cm-3

Rydberg excitation of a BECBEC survives Rydberg excitation BEC

thermal cloud

Rydberg excitation of a BEC

T