TRAPPED ATOMS IN CAVITY QED
Chapter 6 Chapter 6 Cold Atoms and High Finesse Microcavities - Experimental
6.2 Delivering Cold Atoms to the Cavity
6.2.4 The Upstairs MOTl
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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.
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Exarnirlation of the expression for in Eq. 6.2 shows that t,he easiest way to t,rap inariy atoixs is to use large bear11 diarriet,ers d arid maint,ain enough power in the beams such rlrat they are well above the saluration intensity of I,
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3 m\5Jjcm2 for t,kie Cs D2 6S1;?, F = 4 -+ 6P3;?, F' = 5 line at 852 rim. The windot\! size for the upstairs char~iber limited the beam size t,o 1.5 cm in diaiieter so that about 6 mil' of porver was necessary for each of the three (retroreflected bearas) to saturat,e the transition. In addition, an optical fiber ?rt7as used t,o eliminate the day-to-day llassle of having to realign the MOT beams. This is typically necessary due to the Lit,trow rnounting of the gratirlgs on our hornemade external cavity diode lasers [42], which will cause the beam to move t~orizontally if the grating is adjusted for wavelength tuning. Tho drawback of the fiber is hhe typical 50% retlluction in porvcr due to coupling losses. F~lrtherrnore~ there is oft,en a, need for great. flexibility in frequency and intensity adjustment of the light in order to optimize the MOT t,ernperat,ures, u.hich implies that several acousto-optic modulators with finite (single pass-
80%,double-pass
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60%) efficiencies are necessary.111 the end. the decision was made t,o go to an injection-locking scheme [I481 to eiiruir~ate some of these losses frorn tlie bealri path. For example, very little power
(approximately 10 ,uW or less) of rnasttx laser light is necessary to injection lock a slave laser, so that nluch of the frequency n~anipulation can be done to tlie nlast,er laser without compromising tlle ultiniate output power of the slave. The assumption is tlmt tlie injection lock is fast enough that the slave h e r \will follow t,lle intentional frequency ctrarlges of the master. One further improverllent t,o the set-up would be to remove the grating frorr~ the sltm laser and ii~jectioii-lock the freortinrlirlg diode, because tliere is irltririsic competitior~ het\\rew the injection-lockin~g light, and tlie feeiiback frorr~ t,he external grating. The pating was used ir~itially or~ly to provide a si~nple uray to tune the slave laser wavelength close enough to t,he niaster suck1 that the irijectio~i lock .ivonld " g r a l ~ ~ " wllile at tlie same tirne avoiding t,ht: need for temperature :tilt1 currer~t t,iming of the diode waveleng?h.
The master laser was locked using a simple Pound-Drever-Hall rf saturated ab- sorptiori locking teclmique [52j, and a beat-note between the master and slave was
repumping master trapping slave trapping
diode iaser diode i e ~ e r
anamorphic - 85 MHz traPPing: ~ 4 . 3 . 7 MHz
prism pair PO cooling: 0 4 . ~ . 41 MHz
to cesium spaCtromotsr (monitor ooppiar profile) +lo? MHz
to Ceslum saturated
absorption spectrometer trapping:
(iock to 3 - U 3 4 crossover ,205 MHz
resonance at wJ4 - 203 MHZ) to Cesium saturated PG cooling:
absorption spectiometer r 170 MHz
(iock to 4414-5 crossover reeenance at - 126 MHz)
Figure 6.6: The basic layout for tile t i m e upstairs diode lasers are shown, along .cvit,h the most relevant iriforrnation about the laser frequencies. Xote that, for simplicity, rrlost routine beam shaping optics (such as l(+r~iies) mid polarizatiorl determining optics (such as polarizers and wavepIat,es) have been omitted to keep the overall layout as clear as possible.
rnonitored daily on a homemade fast photodiode. Another added bonus of iisirig an injection-locked slave for t,rappiilg is that it does not carry ariy residual frequency sidebands from its locking tecl~uiqi~e which could tend t,o shift a portion of the light closer to resoriance arid cause heating of the atoms. In addit,ion, a repumping iaser locked to the CtSllz, E = 3 -+ F' = 4 line is necessary to repopulate those atoms that end up decaying to the F = 3 state due to off-resona~rt (uninteiltional) driving of tlie 6Sif2! F = 4 -+ 6FSl2, F' = 1 tra~~sitiori (see Table 6.1). Fig. 6.6 gives tlie optical table layout for t,Eie upstairs ?JOT laser scheme.
A few wrcls are in order about tire diode lasers used in tile MOT work. They are all i~orneruade of tile exterrlal cavity g r l z t i r g - b i l e t,ype 1421, using Lit,trow corlfiguration mith weak feedback of about 13% from t l ~ e grat,ing (the diodes, SDL 5421-G1 200 nriV singlomode, are not AR coatecf). Tile grating stabilized linewidti1 is ahoi~t. 200 kHz. Tlie low ~ioise ternperatnro and current corltrollers are ever-evolving, but all gerlcrations can be traced back readily to the original Lihbrecht-Hall work 1149;. The lasers; which are locket1 elect,ronically to a Cs linej ernpioy rf moiiuiation for
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t l ~ e Pound-Drever-Hall tecll~liqile on their injection current and a dual-stage feedback loop to both the current and temperature controllers. As bot,li the spectroscopy technique for tlie ger~eration of the error signal arid the design of tile PI servo loop isself haw been discussed extensivelyt orlly a few details will be mentioned here. An excellellt iliscussion can be found in [150] for diode laser work. In practice, it was fourid that the best lock mas obtained with a handwitlth of about 10 Hz to the grat,illg and 200 k137, t,o the laser injection current; and that feedback down to DC t,o the laser ci~rrent helped tire long-term stability. Finally> actual ir~tegration was found t,o be rrruch preferred to low-pass filtering. Note that the details of the saturated absorption spectroscopy a r ~ d electror~ic feodback loops are omitted fron~ the figures (sucli as Fig. 6.6) which atterript to give overvievvs of the optical table. Tbese locks rtTere critical because muc11 of the data involved amaging and cour~ting sessior~s which could last up to 112 hour long. Furthermore, t l ~ e end of t,he lab with the diode lasers tended to havt: major temperature Ailctuations until the door at that enti of the lab was perrr~anently closed.
Table 6.3 shio~vs all of t,he relevant physical pararr~eters for both of the MOTs.
The upstairs MOTI roufir~ely collected about 2 x 10' atorns as estimated by the cloud ~rolurnc of about 1.5 m ~ n " and the atomic cicnsity phloTl
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1 x 10'' atoms/cm3 ir~ferred from the irltensit,y of scattered light (fluorescence) frorrl the atom clo~ld.Note that this rrurirber is consistent with Eq. 6.2 above axid t,he dat,a in Table 6.3 for MOTI. %'it21 this success in hand, the next task was to aclueve as cold a temperature as possible to maxirnize tile transfer efficiencgr to the lower h;10T2.
The Doppler fi~rrit in a %IOT is tluc to tlie near-resoilant (6
- -r)
frequericy of the beams necessary for efEcierit capture of t,hc atom?. That is, Doppler coolirig relies upon efficient scattering of light by the atoms, and they scatter rr~ost efficiently when that light is close to resonance. Tlris limit is rouglily kuTD-
fir/2: which forCesiurrl correspontis to coolirig frorri rooin temperature to about, 120 pK. Ho~vever;
with snch a c:old sainple already prepared, it is possible to cool below this limit a s will be expIained below. Even tllougii advanced atomic coolillg tecl~niques were very d l known 11511 by this point in the atonlic physics cornmunit,yj no serious effort at
Table 6.3: Cor~tparison of the partmeters for the upstairs and downstairs MOTS.
Coil radius
Number of turns Coil separation Coil current
Coil turn-off time Magnetic field gradient Trapping beam diameter Trapping beam power Trapping beam detuning Repurnping power
MOT size
achieving sub-Doppler temperat,ures had ever been neeessay in our labs