Bose-Fermi Degeneracy in a
Micro-Magnetic Trap
Seth A. M. Aubin
University of Toronto / Thywissen Group
February 25, 2006
CIAR Ultra-cold Matter Workshop, Banff.
Work supported by NSERC, CFI, OIT, PRO and Research Corporation.

Outline
 Motivation
 Micro-magnetic traps and apparatus
 Boson and Fermion degeneracy
 Surprises in Rb-K scattering
 Future experiments

Why ultra-cold bosons and fermions?
Objectives:
 Condensed matter physics.
 Boson-fermion mixtures.
 Atom interferometry.
Why on a chip?
Advantages:
 Short experimental cycle.
 Single UHV chamber.
 Complex multi-trap geometries.

Micro-Magnetic Trap
Technology:
 Electroplated gold wires on a silicon substrate.
 Manufactured by J. Est ève (Aspect/Orsay).
Z-trap current
Trap Potential: Z-wire trap I z defects
Evaporated Ag and Au (B. Cieslak and S. Myrskog)
RF for evaporation

Light-Induced Atom Desorption (LIAD)
Conflicting pressure requirements:
• Large Alkali partial pressure
 large MOT.
• UHV vacuum
 long magnetic trap lifetime.
Solution: Use LIAD to control pressure dynamically !
 405nm LEDs (power=600 mW) in a pyrex cell.

High Efficiency Evaporation of 87 Rb
10 -13 10 -6 1 10 5
PSD thermal atoms
MOT magnetic trapping evap.
cooling
BEC
Evaporation Efficiency
 d ln(PSD) d ln(N)

3 .
95

0 .
1

RF@1.740 MHz:
N = 7.3x10
5 , T>T c
87 Rb BEC
RF@1.725 MHz:
N = 6.4x10
5 , T~T c
RF@1.660 MHz:
N=1.4x10
5 , T<T c
Surprise! Reach T c with only a 30x loss in number.
(trap loaded with 2x10 7 atoms)
 Experimental cycle = 5 - 15 seconds

of fermionic 40 K with bosonic 87 Rb
10
4
10
2
10
0
10
-2
10 -4
10 -6
10 -8
10 5 10 6 10 7
Atom Number
Cooling Efficiency

 ln(PSD)
 ln(N)

8

Fit:
E
F
Non-Gaussian Distribution
1 st signature of Fermi Degeneracy
0 200
Radial distance (
 m)
400
N = 4

10 4
T
F
= 960 nK
T/T
F
= 0.14(2) z = 1.4

10 3
Residuals:
0 200
Radial distance (
 m)
400


2
|
Fermi

0 .
9


2
|
Gaussian

2 .
2
Non-Thermal
Distribution

Pauli Pressure --
2 nd signature of Fermi Degeneracy
E
F
Fermi
Boltzmann
Gaussian Fit kT
Rb
/E
F

Naïve Scattering Theory
Collision Rates
Rb-Rb
 
RbRb n
Rb

RbRb v
RbRb
Rb-K
 
RbK n
Rb

RbK v
RbK a
RbRb
8
 a
2
RbRb

5 .
238 nm


RbK
RbRb

2 .
7 a
RbK
4
 a
2
RbK
 
10 .
8 nm
Sympathetic cooling should work really well !!!
Sympathetic cooling 1 st try:
 “Should just work !” -- Anonymous
 Add 40 K to 87 Rb BEC  No sympathetic cooling observed !

Experiment:
Sympathetic cooling only works for
evaporation
Evaporation 3 times slower than for BEC

Cross-Section Measurement
Thermalization of 40 K with 87 Rb

What’s happening?

… come see the poster
Pauli Blocking of light scattering:
 Fermi sea reduces number of states an excited atom can recoil into.
 Atomic lifetime increases, linewidth decreases.
B. DeMarco and D. Jin, Phys. Rev. A 58 , R4267 (1998).
Species-specific trapping potentials ?
 Bosons and fermions in different trapping potentials.
 Isothermal “cooling” of fermions with bosons.
 Boson-mediated interaction of fermions in an optical lattice.
… or use a “magic” wavelength for Rb and K.
C. Precilla and R. Onofrio, Phys. Rev. Lett.90 , 030404 (2003).

 87 Rb BEC with up to 2

10 5 atoms.
 cycle time as short as 5 s.
 40 K Fermi degeneracy: T/T
F with 4

10 4 atoms.
~0.1
 Sympathetic cooling to 0.1T
F
 cycle time of 30 s.
in 6 s.
 Observation of severe reduction of Rb-K scattering cross-section at high T.
 Bose-Fermi degeneracy in a chip trap.
First time on a chip !
arXiv: cond-mat/0512518
E
F

Colors :
Staff/Faculty
Postdoc
Grad Student
Undergraduate
S. Aubin B. Cieslak M. H. T. Extavour L. J. LeBlanc
D. McKay S. Myrskog A. Stummer J. H. Thywissen