FORT Lifetime Measurement

TRAPPED ATOMS IN CAVITY QED

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

7.2 IntraCavity FORT

7.2.6 FORT Lifetime Measurement

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.

detect downgoers

latom in cavity li

1 triggers FORT on I' FORT turned off,

/

atom detected

13.5 ms trapping time FORT transmission through the cavity

40 45 50 55

time after downstairs PGC [ms]

detect upgoers ms trapping ti+e

I

W

,probe turned off

(b) / whtle FORT on

<

30 40 50 60

time after downstairs PGC [ms]

Figure 7.27: In (a)! a tcchnicjue similar to Fig. 7.26 .ivas uscd to detect and trap a single: atom; but t,lle probe heam ^~;?s rrot switcheti off during tlie FORT detc:ction period. In tht: lirnit of a large FORT, there should be ~nirlilrlal heating from this. In (h), upgoing atom transits were used to trigger the FORT on arid off, in the lropes that the upgoing atoms rnigllt he a little colder before being loadetl into the trap. In both cases here, the attorn \vas trapped for about 14 ms.

everts.

.

enably FORT trigger window

time after downstairs PGC [ms]

Figure 7.28: The probability that ari atorn remained trapped in the FORT a f u n d o n of time after the end of t,he downst,airs PGC cycle. The beginning of the

"FORT t,rigger enable" pulse of Fig. 7.24 is shown for reference. For this data, the FORT was triggered on by the presence of a sirlgle atom; arlrl the probe field triggered

of/ This allo~vs the detemiination of a trap lifetime close t o 28 ms. Background courts have been subtracted from this data as will be explained in Fig. 7.29.

Note that at eac11 of the time delays in Fig. 7.28; a subtraction of "backgroilrtd events (atomic transits dt:layeci by the int,racavit,y cooling beams) has been made froni the set of total detected events, wit,h a record of these background events show11 in Fig. 7.29. 'his background was det,errnirled by way of nleasurements following the same protocol as in Fig. 7.26; but wit,hoilt the FORT beam. For tirr~es t < 49 rns, the backgrounii dominates the signal by roughly 50 fold, precluding an accurate me~~iirement of trapped events. Hoxvever, becaitse it has a rapid decay time 3 ms;

for tirr~es greater tila11 about 52 ms it makes a nt:gligible cor~trihi~t,ion.

This trap lifetime was collfirrnecl in an irldependerit experiinerlt where the FORT is turned on and off at predetermineci times without transit,-triggering or probe trig- gering, yielding T ; = ~(27 i ~6) rns as shown in Fig. 7.30. As mentioned in the ~

disciissiori of Fig. 7.26, our ability to load the trap wit11 reasonable efiiciericy via asyn- c l ~ o ~ ~ o u s turn-on is due to operatio11 with large

&,,,,,,.

It is interesting to note orie big differer~ce between the data of Figs. 7.29 ar~cl 7.30, namely that the background

3 I. background counts I

I o net events with

1 ""triggered FORT

~- n

cooiing and

* 8 FORT

. .-...

30 40 50 60 70 80 90 1 00

time after downstairs PGC Ems]

Figure 7.29: At times t < 49 nLs for tlie dat,a of Fig. 7.28, there are far too many residnal atoms falling through t,tie mode left over from the original MOT dropi which does not allomr ari accurate determination of t,he FORT lifet,irne at short times. How- ever, this background falls extremely rapidly to very close to zero couvrt,s by 70 ^{1 s .} Subtracting these backgronnci counts for times greater than al~oist t = 49 ms allows a i1c:terrnination of the FORT tirrie coilstant.

cotnits showed a very similar fast dct:ay, but with the decay occurring at different points in tinre offset At

-

3 nrs along the time =is. In the case wliere the probe was not triggered off (Fig. 7.30): the background atorrrs decayed earlier (at

-

^{49 ms) .}

This was presumably due to t,lre fact that the atoms interacted wit,li the resonant probe (left on in tlie absence of a FORT) and its concomitant riieclianica,l potent,ial of Fig.7.11 (c)> with the resnlt t,lrat they left t,he mode volume rnore quickly. Note that the probe field xvtts not triggert:d during tlre backgroisrid rneasurernerit of Fig. 7.30 so that this hackground was tletermiriccl in a consist,er~t fashion to the way the trapped atoms were measisred xvitl-i tlie FORT on.

Using the data of Fig. 7.30> ari indication of how tlie bac:kground beiraverl for much earlier times is presented in Fig. 7.31 to substantiate tlre cli~iiri rriade above tl-vat tlie backgroimti was too overwlielmir~g to r n ; h reasona~ble rnet~~~rrenierits of tlie trapping 1irobahilit.y at earlier times. The extremely large number of background events at tlicse times are presinnahly diie to residnal atorris from the dropped MOT "envelope"

of atoms (see the tirne history of atom transits for a single drop of the downstairs

I ebackground counts

/

*net events with untriggered FORT

'7 0.3 -TI

L ⁱ j! ,

here without

fit trap lifetime (Ile) 27 ms FORT

- , j I

1

30 40 50 60 70 80 90

time after downstairs PGC [ms]

Figure 7.30: The FORT lifetitne of Fig. 7.28 was indeperideritly verified in a separate experirr~ent in which 6he FORT was not triggered by tlie presence of a single atom, but was gated on a t the fixed time indicated. The resonant probe beam was also left on for t,his experiment. Here, a frt reveals a lifetime of about 27 ms. The background counts associated witti this measurement are also shown.

MOT, albeit with an extrerni:ly large atom number, iri Fig. 7.3(c)). Finally, care was taken to check t i ~ o xiumber of everits seer1 u41e1r tire atom source was blocked to rule out any other spurious signal source. This measurenicnt always produced zero counts independciit of time.