Bose 2.2 User Manual download pdf (Page 46)

From the exit of the ZS, the atoms travel only a short distance until they reach the

center of the trapping chamber, where they are captured in a magneto-optical trap (MOT).

While standard three-dimensional MOTs [129, Ch.9.4] are widely used in experiments

with alkali atoms [132, 133], a large inelastic two-body loss coeﬃcient for excited state

collisions severly limits the maximum number of chromium atoms that can be accumulated

in such trap [123, 134]. Fortunately, the large magnetic dipole moment of chromium allows

for an alternative loading mechanism: the

ontinuous

oading of a

oﬀe-

ritchard

(CLIP) magnetic trap [135, 136], illustrated in Fig. 3.2(a). The magnetic quadrupole

ﬁeld, produced by the pair of cloverleaf coils, is used for operating a two-dimensional

MOT in radial direction plus a molasses on the

-axis, using the

↔

cooling

transition. With a branching ratio 1

250

000 the atoms decay from the excited state

to the

meta-stable state. There they are decoupled from the MOT cycle, but

remain magnetically trapped. Owing to the small branching ratio, the atoms are cooled

approximately to Doppler temperature (

= 124

) in the MOT cycle, before they

arrive in the meta-stable state. After around six seconds of loading time, the population

in the

state saturates at

N ∼

. We then switch oﬀ the MOT light and excite

the atoms from the

to the

state by a 20

light pulse from the repump-laser at

= 663

. From there the atoms decay back to the

ground-state, as shown in

Fig. 3.2(b).

As a next step, we compress the atomic cloud in the magnetic trap (MT) by maximally

ramping up the current through the cloverleaf coils. The sample heats up to

T ∼

and we then perform Doppler cooling by ﬂashing the axial MOT beams onto the dense

sample [137]. Without loosing atoms, we gain two orders of magnitude in phase-space

density (PSD) [138]. After this process, the PSD is

ρ ∼

−7

which provides good starting

conditions for the RF-induced evaporation cooling

[128, 139–141].

In most experiments using alkali elements, evaporative cooling in a magnetic trap is

used to reach the ultra-low critical temperature for Bose-Einstein condensation [1–3, 128].

However, it turned out that this is not possible in chromium: due to the anisotropic

dipole-dipole interaction, the magnetic quantum number

is not a conserved quantity

in a two-body scattering process, which leads to a heating of the chromium sample by

the so-called dipolar relaxation [126, 142–144]. As the atoms are initially trapped in the

magnetic sublevel of highest Zeeman energy

(

= +3) spin-changing collisions are

accompanied with a release of energy leading to the heating of the sample. By adjusting

daily the magnetic oﬀset ﬁeld during the RF evaporation to the minimum possible positive

The RF-induced evaporation technique is based on the selective removal of the hottest atoms from the

sample which, after a rethermalization time, will possess a lower temperature than before.

Only the so-called ’low-ﬁeld seeking’ states are magnetically trappable as local minima of magnetic

ﬁelds can be produced, while the creation of local maxima is impossible as a consequence of Maxwell’s

equations [145].

1 2 ... 41 42 43 44 45 46 47 48 49 50 51 ... 154 155

Comments to this Manuals

No comments

Bose 2.2 User Manual Page 46

Comments to this Manuals

Related products and manuals for Soundbar speakers Bose 2.2