FORT Triggering with Single Atoms: Trapped Atoms

TRAPPED ATOMS IN CAVITY QED

Chapter 7 Chapter 7 Trapping a Single Atom Inside a High Finesse Cavity

7.2 IntraCavity FORT

7.2.5 FORT Triggering with Single Atoms: Trapped Atoms

167

evidence that the FORT was xvorking: and t,he next orcier of business was to try to get an i~ldicatior~ of the FORT lifetimi:.

11s on thr: PORT laser AOhI. The waveform of this turll-on edge v,as progarnrnable (wit11 arl arialog function generator, Stanford DS335) and was a coniprornise between t u n i n g the light on too qoickly arid kiclting the at0111 out of the trap versus having tile atoiri drift out of the illode vohur~c: during a slow turn-on period. Furtller1i1ore, tile servo response was iirtimately co~lpied to how t,his waveform behaved, and too

"harii:' a turn on iroilld often causc sigriificarlt ringing.

lower MOT2

PGC ends atoms take 30,ms

1

to fall smm to,cavity

- ,

t = 0 reference this edge asynchronous I for most of (depends on atom arrival) data here

k ^I

near resonant, 2ms intense cp&g pulse from "lattice beams"

CORT trigger window enabled

atom enters cavity, detected by probe

' and triggers detection circuitry

1 1

8 ,

Figwe 7.24: The tinling diagram of Fig. 6.13 is updat,ed here to ir~clude t,he edges rieetled to trigger the FORT Iigllt :and the probe due t,o the presence of a single atom ilisicie the cavity.

:

^{pro& On}

I ,

Ail exiu~iple of FORT triggering hy a single atom and subsequent trapping of that atom is shown iii Fig. 7.26. The arric-a-1 of at1 at0111 is senseci by a reduction

probe turns off, and ... probe on to look again F ~ R T on, (ninimize off time ... FORT turns on for variable time,

for locking servo integratqr hopefully atom is now trapped

FORT off ior zero AC Stark shift

t

while detect atoms (5 ms max)

1 , '

' I I

' 7

1 , '

169

in trarismissiori for tl-I(: probe hear11 (of photon riiimber fi.

-

^0.1). The falling edge of the probe trarisnlission triggers on the FORT field, which then ren~ains o n until being switclled

08

after a fivetl interval. The presence of tile atom at, t,his second o f tinie is likewise detected 1 - 1 ~ 7 niodifictition of the probe

transmission?

demoristrat,ing a trapping tirile of 29 ms for tlie particular event slio\vn in the figure. Note that because the pro'i~abilitics for aton1 trapping given a trigger pt,lts and for detection given a trapped iktom pdjtp are rather small (ptpltgpdllp 0.03)) we operate a,t, ratlier lrigh densities of cold atoms, such that tlre average atom number present in the cavit,y niode at the time of the trigger is

Lv,,,,,,,

^N 0.5 (but wliich then falls off rapidly with t i ) A series of "snapsl-lots" of atom transit signatures around the time of trigger are shown in Fig. 7.25 to aid in det,er~nining

Lv ,,,,,,.

As a consecpence, the attorn that causes tlie trigger is not always the atom that is actually trapped when the FORT is gated on, with such 'phantom' everits estimated to occur in roughly 1 of 4 cases. To aid in rectucing t,his rrnmber, tlie coolirig pulse (timing sllown as an inset to the figt~rc) is delayed to the far. edge of the "transit enivelope" cornpared to the work of Fig. 7.9. The near-resonant cooling had a 1.5 ms duration from t = 33 ms to t = 34.5 rns and the far off-resonant sub-Doppler pulse went another 1.5 n ~ s from t = 34.5 to t ⁼ 36 rus; at which point the trigger window was enabled.

Two rriore examples of FORT triggering and trapping of single atorns are shown

I F i g 7.27(a)?(b). In (a), tlie atorns are again detect,ed as downgoing tramits with a resonant probe. Note that although the probe field is left on for all times in Fig. 7.27(a), there is no a,pparent change in cavity transmission during tlie interval in wliich an atom is purportedly trt~,pped within t,!ie cavity node. Tile absence of atomic sigria,tures during the trapping time; but riot before or aftt:r, is due t o AC- Stark shifts iissociated wit11 the FORT i>nd/or the rnismatclled antinodes between

( I) I11 (b), an off-resonant probe is useti to det,ect upgoing transits in tlie hopes of inlparting less heating to tile atoms during the det,ection period and thns maxirnizing the c h a ~ ~ c e s of the at,orrls actually being capt,ured by t,lic FORT.

Arthermore, the probe here is once again triggered o f for a significant fraction of

trigger time for both

i

,

^{u ,..:J.k,,.w- ur$h}

30 35 1 40

time after downstairs PGC [msl

Figure 7.25: In order to doterrrrine tllo probability that an atoll1 iruitle tlie cavitv will be trappetl by the FORT on any giver1 drop cycle of t,lle MOT, this data was used to det,errninc the averagc at0111 nulnl~er Ntt2',,,,,,, ^-

-

^{0.5 in} the cavity mode at tile time tile trigger xvm enabled. The dasheci line indicat,es the beginning of the "FORT trigger enable" ~vi~idotv of Fig. 7.24.

i

\

50 ^I 60 ^I ! 70

; time after downstairs PGC [ms]

/

turn off FORT to

cool and detect FORT triggered on by a single transit

ramp down FORT to probe again

Figure 7.26: Sirlgle atom t,ransit,s were used to siuiulta~leously trigger on the FORT and trigger

08

the probe bea~n, in order that the atom not be accidexitally heated by the probe while it is trapped. This at0111 was trapped for 29 ms.

the FOItT on time, hut typically neither of these changes (off-resonant det,eci:ion;

probe switclied 0 8 ) seem to maice much of a difference.