TRAPPED ATOMS IN CAVITY QED
Chapter 6 Chapter 6 Cold Atoms and High Finesse Microcavities - Experimental
6.2 Delivering Cold Atoms to the Cavity
6.2.6 The Downstairs MOT2
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temperature of 3 /aK after the PGC protocol. This was niore than adequate to allow the 'tonis to fall the 25 cril distance through the differential puniping hole to the position where they would be capt,ured in the lower hcIOT2.
P G C duration Iir n q
Table 6.4: Coniparison of the polarization gradient cooling (PGC) paranleterrs used for tlie two MOTS.
Orlee this labor-intensive protocol had been followed once, a short-cut was found which allowed tlie PGC to he tweaked easily every day. At the very end of the PGC, instead of dropping the atom cloud, it ms exposed to a brsst of rt:sonaiit light. Tlie cloud would get "blown away" because of the resoriant nature of tlie light, and the exact way it would expand as seen on a video moriitor could be correlated with its temperature. A signature of good PGC was a very slow "puRing" of the cloud even under intense; resoriant excitation.
I I I I
0 0.5 1 .O 1.5
position [cm]
Figure 6.8: Tile temperature of the at0111 cloud is rneasured by deterlnining its ex- pailsion rate. Tile change in the width of the cloud's fli~oresccnce irnage fro111 (a) at 24 11% nft,er the end of the PGC cooling cycle to (b) at 36 111s aft,er PGC gives an upstairs MOT1 t,eruperatixe of approximately 3 pK, allowing tho cold ball to pass easily t21rough t-he differer~t,ial pumpirig hole into the loxver clmmber.
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lower vacuum charnber. A point 1 cm helo\v the enti of tile bob tlleii defined the correct position of the cavity, assurrlirig the atorrs fell st,raight dovr~l only uncler the ilith~encc of gravity fro111 rhe PG cooled clouci of hlOTi. Great care was haken to avoid introducing coirtarniriant.s onto the cleaned vacuum parts arid the assembled cavity throughout this procedure.
Fig. 6.9 shows the table layont for the lot:king, freqiiency; and inte~isity control of the downstairs lZIOTz hearris. The downstairs lasers use a somcwhat different locking teclt~lique irnplementat,ioil to avoid putting frequency sidebands on the trappirig laser.
Here, the re-puruping laser is locked with an FL,f Pound-Drever-Hall [52] punrp-probe sclieme as usual, but now it is also used as the weak probe in a second saturated absorption sclierile irivolving the trapping laser as the strong pump. In this case, tht:
rrrodulatiori of the re-pumper is trarsferreti throu,gii the excited state arid can be used to derive an error signal for t,he trapping laser. This is knowxi as a modulat,ion-transfer t,ypc spectroscopy [152]. The trapping light is then double-passed through another AOM for fiecpency and int,e~lsity control, as mentioned above, before being coupled into a fiber. The fiber again serves to both improve its spatial ntode structure (so that roughly 11 rriW of trappirig light is available) arid to aid in minimizing MOT2 rrrisalignnlerit.
The actual aligrlnie~rt of the beams is rriuch t,rickicr irr this case. First; the 852 nrn lighit ca.nnot he seen by t,ht: naked eye. In addit,ion; there is very little inside the vannnn chamber which clips the beanis to dlow then1 to scatter and be seen by an infrared viewer. Finally, there is rio backgronrrd atomic vapor aroiind as in LEOTI:
whose fluorescence car1 be used to align each set of bearrs to the (%her. In the end, the easiest tliing to do is irse t,he georiletry of the vacuunt cltaxnber itself and align the bcarrts to be ge(>metrically centered on t,he cylindricit1 chanther structure. This procedurt: teririeti to work %irell as quantified by tthc rozighly 10% ~ransfer efficiency of at,oms from tile upstairs MOTl to the dowristairs MOT2> which is corsisterlt with other rti~rrrbers reported for tllis type of doublehIOT loading unaided by any mag- ttetic gtlidirig. This nnmber w i ~ s fonrrtl to be n~ost sensitive to the pararnet,ers of the .upstairs MOTi, wilereas up to variations in h.IOT2 pararnekers (such as beam
Figure 6.9: A scllematic similar to Fig. 6.6 is shown riow for the dowr~tairs t,ralt- ping laser set-up. In this case, an ir~jcction locking scheme was not used: but ext,ra complication for these lasers arises fvonl the fact that they are also used to derive the
"lattice" or "cooling" bearns wllicll are discussed in Sectiolr 6.2.7.
irit:cnsity; det,m~ing ancl coil current) typically did not significantly effect the capturing capabilities.
Figure 6.10: An image of the downstairs MOT2 resting approximately 5 mm above the cavit,y (glowing frorn scattered light) shows the approximately 10% transfer efficiency of the 10' atorns from the upstairs MOT1.
The rnagnetic field coils were again placed on a three-axis translator for this part, of the experiment,. Independent of the previous discussion about correct placement of the field zero crossing, there is another reason for not attaching the coils rigidly t,o the cliarnber. The downstairs coils are geometrically constrained to be Fartller away froln the hIOT center, wliicl~ means more current (approxirxiately 11 A) was necessary to ai:liieve the required trapping field gradient. This currer~t also rieetis to be turried off cluickly, tending to cause some mechaaical motion of the coils. The motiori is enougl~ t:o drive vibrat,ions of the type 304 s t , a i n l ~ s steel (and hence, somewhat magnetic) illia~rl>er! which in t,urn can couple in t o cause vibrations of the cavity aioiirlt,. Physically clecoupling the coils from the chainher as we did here tiefinitely mitigates this problem, hut still tlrere is n residual signature of this turn-off trarrsierit or1 ex7t:ry sirigte laser lock on the table due to rniechanical coupling t11ro11gh the optical table itself. One very int,eresting side not,e here is that the correct corripcr~satioir coil setting changed quite dramatically when the Ti:Sapphire pump l a e r was switciied from a 20 W Ar+ laser (Coherent Innova 100; 20W) to a 5W diode pumped solid
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state laser (Coherent Verdi) after about 1 year. This was due to bhe large magnetic field generateri by tlie confining magnet around the plasma tube of the iori laser which does not exist in the solid state pump laser.
PGC was iiiiplerner~t,ed and optimized in rr~uch the same way as described for the upstairs hIOTlj and Table 6.4 sumxiarizes the final parameters. Vcrificat,ion that thc atoms were indeed falling vertically towards the cavity gap was accomplished by fluorescence imaging the fallirig cloud. The sequence of p i c t i ~ e s in Fig. 6.11 depicts the fallir~g of the atom cloud towards the cavity as a funct,ion time after the 12 111s PGC cooling cycle has ended. Note that, the end of the rlownstairs PGC cycle, denoted as t = 0 for Fig. 6.11, is assumed to be t = 0 for all future discussior~ of atom transits unless otherwise noted.