The Vacuum System

TRAPPED ATOMS IN CAVITY QED

Chapter 6 Chapter 6 Cold Atoms and High Finesse Microcavities - Experimental

6.2 Delivering Cold Atoms to the Cavity

6.2.3 The Vacuum System

The desigri of the vacuum systerri was the overriding concern in tliis experinierit, because it was felt that the everrtual lirriit to trap lifetirr~e could be dorniriated by col- lisiotls with the backg~ound gas if rile desigri wasrl't done carefilliy. A cluick estin~ate of t l ~ e collisiorlal tirnc scale T,.,,~,: giveri the collisiori cross-section n and mean velocity

with 7' the gas temperature and r r ~ atomic mass. For our sysrerm, wit11 a tller~rmal Cs ( m = 133 arriu) backgroiind vapor at 300 K and a collisional cross-sectiori cr 1 x lo-" in2 [l42j, the collisio~ial lifetiriie is about 5 s at a pressure of 10-"orr.

Hence, much ofiort was irivested in improvi~lg upon tlie typical vacuum levels of lo-"

Figure 6.3: In this sketch of a typical MOT geometry, the arrows representing the direction of the laser beams are meant to be orthogonal to the faces of a cube. A set of anti-Helmholtz coils lies along one of the principal axes to provide a iinear magnetic field gradient,. Zeroing coils (not shown) lie along all threce axes to compensate for stray magnetic fields. This geometry is used for both MOT1 anti MOT2.

Torr used to d a k in these experirncnts in our ggronp.

In any design it is in~portant t,o t,ry to ensure the success of trapping one atorn inside t,he rnode volurr~e on each experimerltal cycle. Due to georrietric constraints, MOT2 can he no closer than aboiit 5 mni from the cavity mode voliime, and the mode has n cross-scctior~ of about 40 p m x 20 pnl. If the initial MOT2 cross-sectiorial x e a

is 1 rr~xn~, an atom has a probibilit,y of about I x 10-%f hitting t,he cavity mode volume and being detected. If we assume

-

^1% of these are actually trapped, then it is necessary to collect about iVhloT, ^N 107 atonw above the cavit,y 111ode volume.

The stea,dy sta,te ni~nltjer of atoms in a MOT is given by j140, 1431

n:,

⁼

- ^4!c r:<:>,,,t

2 pcs

+

Po ~ ~ , l , ~ ~ c < > l l

3fi

(

^pcs

1

^d4,

with tile scattering ra.te I?,,. written as

r e ( 7 ,I ) are the (recoil, thermal, trap collisional) velocities respectively,

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I I I , is the ratio of tlie light iritensity to tlie saturation intensity; d is the beam diaineter, ( p c s ; p o ) asc the Cs and background vapor pressures and ( 6 ; ~ ~ ) are the detuning of tlie laser light from the atomic transition arici the excited sta,t,e half- lii~eu,ititll respi:clively. Clearl); this expression is maximized if pc, >> pm In the UHV c~lvironrrient silrrour~ding tlic cavity. ho~vcver: we require the total pressiue p =

p ~ ,

+

^p, to be dominated by pn (i.e., pn

>

pc,)

.

This rilles oill the possibility of loading the t,rap from a bac:kgrourid vapor in t,hc vicinit,y of the cavit,y. .4n attractive opt,ion was to load T\IOT2 from a cold atomic beam [l44]. The LVIS tecliriique [145j, for example, T I T L ~ optimized for atomic flux, hut was eve~~tually dcerned unnecessary because of the relative simplicity of the "doubleT\~IOT' technique.

Fig. 6.4 shows tlie overall layout arid specific detail of tile vacuum system that was finally built. A double-MOT was adopted because of the flexibility of this strategy and its capability of separating rhe cold atom "source" from the physics 60 be done with those cold atoms [146]. In this system, many atoms (AVhloT,

-

^{5 x lo8) are}

collected in an upper chamber from a dilute vapor si~cli that pc,

>>

pfj in order t o maximize

NT,

as in Eq. (6.2). Tliese atoms are transported (in our case, cfropped) in t,he form of a cold atomic bear11 througli a differential pumping hole to a UHlr chamber below.

The differential pumping hole geometry of lerigt,h L = 2 c111 and dianieter D = 4 rnnl was designed to support the pressure difference between the two clia~nbers. Given a pumpirig speed S

-

²⁰ 11s and a conductance C for a cylindrical tube of

I)"

C ⁼ 12.1 l/sj if D. L [cni],

L +

^{1.33D (6.4)}

the pressure cliffereiicc A P is given by

wtiictl indicates this geometry should easily allow 2 orders of inag~~itude in pressure between the upper arid lower cl.iarnl,css. The hole size of 4 inrri still allows many

W = window

to Cesium reservoir

2.75" conflat

rential pumping hole m diameter, 10 cm length) 25 cm

to ZOO Us ion pump (Varian)

heavy copper block

I t ^I

NOT TO SCALE

chamber base

Figure 6.4: This skctcll (not to scale) provicies all irifornlntio~i re1c:vant to tlie layout, of the vacuurn system; inclndirig an itlcntification of the rr~ajor cornporierits used and ari indii-ation of tlie important distances. Eote tlrat t,he clifYerentia1 p u n ~ p i ~ l g hole was inserted man~.~alIy into the assembleci systern.

atoms to pass fro111 the npper cliamber to the lo~irer, given typical expansion rat,es (te~nperat,nres) tuld initial sizes for MOTI.

The Cs reservoir ~ v a s a very simple design corisisting of a stainless steel cup sur- rounded t)y insulation and attached t,o several peltier ileating elements to allow tem- perature control of the Cs vapor pressure in the upstairs cliamber. The ternperdure was inonitored by a tilermistor arid m7as t,ypicaliv llcld at 5 "C except for a couple of hoim every mornirrg when the temperature .rvould be raised to 20 'C to allow more Cs to enter the npstairs chamber. A valve was included to isolate this reservoir during the balteont period ant1 in the everit of any power out,age.

Standard vacuum t,echniclues were used to prepare an(1 assemble all of the compo nents; but there were several significant challenges worth mentioning. First,, bakeout teniperatures of bhe assen11)led system were ultimately limited t o about 200 "C by the bakeability of the materials associated with the cavity. These included viton for vibration damping, epoxy to hold the assenlbled cavity structure, solder to make elect,rical connect,ions, kapton wires and PZT mat,erials for allowir~g cavity tuning.

Significant surface areas of copper a r ~ d aluminum were also necessary for the cavity rnounting and snpport struct,ure. For this reason: an aggressive cleaning protocol [I471 consisting of ultrasonic cleaning, vapor degeasing; alcohol rinsing, acid etching (for tile copper) and vacuum oven bakeout to 250 ' C before assenibly was nsed to help compensate. A further step of baking all the stainless steel con~ponent,~ in air to 400

" C was not iwed but -,vould probably give even better result,^. After assembling the chani1,ers and all of the conlpoilent,s, a comprehensive tr,vo-\ireek bakeout procedure under vacuunl at, 200 "C was initiatcd. Each significant coupling poirit and window was covered by a band heater and terirperaturc semors (tire ion purnps had their own separate elerncnts) and the entire structure was covered by Inany layers of oil-free alrur~inurn foil.

At tL~e end of the bakeout, period; the system was isolated from the turbo punrping system and t,he ion pumps were tnrnetl on. The steady state pressure after cool-down reached 2 x 10-"orr in the upstairs chanlber as monitored 11y the iori pump current, and 3 x 10-lo Torr downstairs as rnet~~ured by a nil(le ion gauge. After alniost two

73

years, t,he downstairs pressure has reached a. rnini~liurn of 1.5 x 10-lo Torr: but this nunit)er Ructtiates seasonally by up to 50% depending on the relative humidity in the lab. Tlie upstairs pressure fiuctuates depending on the Cs backgroulid vapor pressure, but typically sits at 1 x 1W8 iorr, which verifies the differential pumping geometry tlcsig11.