Table Of ContentEvidence of sympathetic cooling of Na+ ions by a Na MOT in a hybrid trap
I. Sivarajah,1 D. S. Goodman,1 J. E. Wells,1 F. A. Narducci,2 and W. W. Smith1
1Department of Physics, University of Connecticut, Storrs, Connecticut 06269
2Naval Air Systems Command, EO Sensors Division,
Bldg 2187, Suite 3190 Patuxent River, Maryland 20670, USA
(Dated: October23, 2012)
A hybrid ion-neutral trap provides an ideal system to study collisional dynamics between ions
andneutrals. Thissystemprovidesageneralcoolingmethodthatcanbeappliedtospeciesthatdo
2 not have optically accessible transitions, and can also potentially cool internal degrees of freedom.
1 The long range polarization potentials (V ∝ −α/r4 ) between ions and neutrals result in large
0 scatteringcrosssectionsatcoldtemperatures,makingthehybridtrapafavorablesystemforefficient
2
sympathetic cooling of ions by collisions with neutral atoms. We present experimental evidence of
t sympathetic cooling of trapped Na+ ions, which are closed shell and therefore do not have a laser
c
inducedatomictransitionfromthegroundstate,byequalmasscoldNaatomsinamagneto-optical
O
trap (MOT).
2
2
I. INTRODUCTION ited by the ratio between the ionic (m ) and the atomic
I
] (mA) masses. Only when mI/mA > 1 can the cooling
h overcome the atom-ion rf heating [32]. However, unlike
p Hybridtrapsareidealexperimentalsystemsforstudy-
a buffer gas, which is dilute and extends throughout the
- ing collisional dynamics between neutral atoms and
m trapping region, a MOT is very dense, localized cloud
atomic or molecular ions. Trapped cold ions are poten-
of neutral atoms. As a result, certain approximations
o tial candidates in studies related to precision measure-
made in Ref. [23] that lead to m /m > 1 do not ap-
t ments [1, 2], quantum computing [3, 4] and ultracold I A
a ply [27]. Therefore, ion collisions with the MOT result
. quantum chemistry [5–12]. In most of these applica-
s in little to no atom-ion rf heating until the ions secu-
c tions the trapped ions are requiredto be cooled down to
lar motion amplitude is smaller than the radius of the
i lowtemperaturestoextendtheirstoragetimesandspec-
s MOT. This allows ions to be sympathetically cooled by
y troscopic resolution. Various cooling mechanisms such
equalmassneutralatoms[10,27,28]. Sympathetic cool-
h as laser cooling [13, 14], resistive cooling [15–17], sym-
ing is advantageous because it can be applied to atoms
p pathetic cooling by other co-trapped cold ions [18–20]
[ or buffer gas cooling [21–23] have been regularly imple- andmoleculesthatdo nothaveopticallyaccessibletran-
sitions [13, 14, 25] and has been theorized to be able to
2 mented.
cool the internal degrees of freedom of molecular ions
v As originally proposed by W.W. Smith, our hybrid [25, 33]. Since Na+ is a closed shell ion, the conclusions
6
trap consists of a magneto-optical trap (MOT) concen- of this study should be applicable to other atomic and
0
1 tric with and encompassed by a linear Paul trap (LPT) molecular ions that do not have a laser induced transi-
2 [24, 25]. The hybrid trap apparatus has recently been tion.
. used in several experimental measurements of charge-
9
exchange rate constants [7–9] and creating a cold molec-
0
ular ion source [26]. Using localized cold or ultracold
2
1 atomicgasestocoolionsinahybridtraphasbeeninves-
: tigated using cold atoms from a MOT (up to 200 Rb+
v
ions) [27] or a Bose-Einstein condensate (BEC) (for a
i Ion-neutral interactions at low energy are dominated
X single ion) [10–12]. This paper describes experimental by the long range polarization potential V ∝ −α/r4,
r evidence of effective cooling of an initially large popula- whereαisthedipolepolarizabilityoftheneutralspecies.
a tion (∼103−104) of Na+ ions by elastic scattering and
The collision cross sections between ions and atoms are
resonantcharge-exchangecollisionswithcoldequal-mass
considerably larger than the cross sections between two
Na MOT atoms in a hybrid trap.
neutralsatcoldtemperatures[25,34,35]. Wepreviously
Sympatheticcoolingisachievedwhenthetranslational investigatedthefeasibilityofsympatheticcoolingofNa+
kineticenergyofonegasislowereddue toelastic,inelas- andCa+ ions by aNa MOT via simion simulations[28].
ticandcharge-exchangecollisionswithanothercoldergas In this paper, we present experimental evidence of sym-
[20, 28]. When collisionally cooling one ionic species of pathetic cooling of an Na+ ion cloud by an Na MOT.
m with another pre-cooled ionic species of mass m , it This paper is organized as follows: In Sec. II, we dis-
I C
wasshownthattheminimummassratioism /m >0.5 cuss our experiment, which includes a description of the
I C
[29]. Recentexperimentalstudieshaveshownthatisnot apparatus as well as trap loss mechanisms. Trap loss
the lower bound for cooling [30, 31]. In the case of sym- mechanisms are important to understand the results of
pathetic cooling by a neutral buffer gas, as first demon- our experiments, which are presented in Sec. III. We
strated by Major and Dehmelt [23], the cooling is lim- conclude in Sec. IV.
2
MOT cloud, which in a hybrid trap is impeded by the
LPTapparatus. Atomsarereflectedbackintotherecap-
ture region by the trap electrodes, systematically lower-
ing the effective temperature measured. Therefore, the
measurement was considered a lower limit of the actual
MOTtemperature,whichislikelyanorderofmagnitude
higher [38, 42].
The Na+ ions necessary for this experiment are cap-
turedbyutilizing anLPT.OurLPTconsistsoffourseg-
mented metal rods assembled as shown in Fig. 1 and
discussed in further detail in Ref. [28]. The four center
segments, termed rf segments, are used for radial con-
finement of charged particles, while the eight end seg-
ments (four on each end) are used for axialconfinement.
The rf segments are supplied with a voltage of the form
±V cosΩt,withthevoltageonthepairofrodsalongthe
rf
◦
x axis 180 out of phase from the voltage on the pair
1
along the x axis. This configuration effectively creates
2
a rotating quadrupole saddle potential at the center of
the LPT [13,43–46]. Unless otherwise specified, the am-
FIG. 1. (Color online). Diagram of the hybrid trap appa-
plitude was set to V = 36 V and the driving frequency
ratus. A Na MOT (orange) is formed concentric with an rf
was set to Ω/2π=729 kHz. Confinement along the axis
ion cloud (grey) inside a segmented linear Paul trap (LPT)
is provided by a DC potential V = 35 V on the end
with six 589 nm MOT beams (yellow) and a pair of anti- end
Helmholtz coils (exterior to the chamber). A 405 nm beam segments with the center segments at dc ground.
(blue)alignedco-linearlywithoneoftheMOTbeamsisused The total time dependent electrical potential near the
for REMPI. Fluorescence measurements of the MOT can be center of the LPT due to these applied fields is aproxi-
made with a photo-multiplier tube (PMT) or a CMOS cam- mated by (for x2+x2 ≪r2)
1 2 0
era. An electrically biased mesh is placed between the LPT
andthechanneltronelectron multiplier(CEM) whichisused x2−x2
for ion detection. Inset: Axial view of Paul trap with Carte- Φ(xi,t)≈Vrf cos(Ωt) 1r2 2
sian coordinate system. 0
ηV x2+x2
+ end x2− 1 2 (1)
z2 (cid:18) 3 2 (cid:19)
0
II. EXPERIMENT
where x is the magnitude of the position vector with
i
the coordinates given in Fig. 1 (inset), the distance be-
A. Apparatus
tweentwodiagonalelectrodesis2r =19mm,the length
0
of the rf segment is 2z = 48 mm and η=0.14 is a unit-
0
ThehybridtrapconsistsofaNaMOTconcentricwith less efficiency factor dependent on the geometry of this
an ion cloud confined within a linear Paul trap (LPT). particular trap.
A standard Na type II MOT [28, 36–38] is formed as The motion of a single ion within this oscillating elec-
shown in Fig. 1. In addition to the anti-Helmholtz coils tric field is describedby the Mathieuequation[23]. This
requiredfortheMOT,magneticshimcoilsarealsoplaced motion can be divided into a slow secular motion and a
outside the vacuum chamber, enabling the MOT to be rapid micromotion at the rf driving frequency Ω [47].
translated for better overlap with the ion cloud. A Na The sequence of loading, trapping and detecting the
source (Alvatec or SAES) inside the vacuum chamber ions is depicted in Fig. 2. Ions were loaded into the
provides the ≈ 1000 K background Na gas from which LPT via a resonance enhanced multiphoton ionization
the MOT is produced. The vacuum chamber was main- (REMPI) method [48]. A laser diode at 405 nm (RG-
tained at a constant pressure on the order of ∼ 10−9 BLase) drives excited Na atoms into the continuum,
Torr by continuous pumping with an ion pump. Using thereby producing Na+ ions. The 405 nm laser beam
standardfluorescencemeasurementstakenwithaphoto- is co-linear with one of the MOT beams (Fig. 1). The
multiplier tube (PMT) and/or a Complementary Metal initialnumberofionsloadedwithintheLPTcanbecon-
Oxide Semiconductor (CMOS) camera [39, 40] a peak trolled by adjusting either the 405 nm laser intensity or
MOT density of ∼ 1010 cm−3 was inferred for the type the exposure duration t .
Load
II MOT. A destructive ion detection method was employed us-
The release and recapture [41] technique was imple- ing a Channeltron electron multiplier (CEM) positioned
mented to measure the MOT temperature, which was adjacent to the LPT along its trap axis as shown in
found to be 0.2(1) mK. The model typically used with Fig. 1. An electrically biased mesh placed between the
this measurement assumes ballistic expansion of the LPT and the CEM isolated the CEM from the trapped
3
(I)5
V) 4 (a)
al ( 3
n
Sig 2
on 1 (b)
I
0
0 20 40 60 80 100 120
tLoad(s)
(II)
V) 8
gnal ( 6 N Sae2+c Rualadri al Na+ & Na2+ 2nd Harmonic
Si 4 Frequency Radial Secular Frequency
n
o 2
I
0
(gIII) 0 20 40 60 80 100 120 140
n
aini 100 Frequency (KHz)
m
e 80
R
FIG. 2. (Color online). (a) The TTL pulse used with the s 60
n
4Nse0ag5+mneimonntsslacvsleoiarsedtshtioetdoRetEthoMelCPoIEadMm(elitoshalooddwin.egr(ebtdi)maVteetndhdeenotofitmtehdeeobffyoeuxtrLtoreaandcd)- nitial Io 2400 (b) (a)
tion to generate a dipolar field between the ends of the LPT of i 0
to direct the ions towards the CEM. (c) A typical ion signal % 0 20 40 60 80 100
from the CEM and preamplifier.
tTrap(s)
FIG. 3. (Color online) (I) (a) Na+ and the photodissociated
2
ions thereby ensuring that the trapping fields were not Na+ producedbytheMOT.(b)Thenullionsignalaftermass
disturbed by the high operating voltages of the CEM. selectiveresonantquenchingappliedattheNa+radialsecular
2
frequency. (II) A radial ac quenching field frequency scan of
ThetrappedionsareextractedbyloweringV closest
end Na+ andNa+ ionsproducedbytheMOTduetoAI.Wefind
totheCEMfromtheinitial35Vto-7Vandtheopposite 2
resonancesat theNa+ first harmonicat 36±1kHz,theNa+
V from 35 V to 0.5 V producing a dipolar electric 2 2
End second harmonic at 71±1 kHz, and the first harmonic of
field between the two sets of end segments accelerating Na+ at 75±1 kHz (III) An illustration of the ac side-effect
and directing the ions into the CEM [27, 49]. The CEM heatingwithout theMOT. (a)Relativetrap loss ofNa+ ions
signal is fed through a preamplifier which outputs the without the ac quenching field applied to the trap segments
integratedion signal. The amplitude (peak value) of the of theLPT and (b) with theac field set at the radial secular
ion signal from the preamplifier is directly proportional frequency of Na+2 ions at an amplitude Vsec = 1.8 V. (The
to the number of ions in the trap at the beginning of error bars are smaller than thedata points).
t . Additionally, the MOT can be turned on or off
Extract
via an electronic shutter on one of the 589 nm retro-
B. Trap loss mechanisms
reflected beams during an adjustable percentage of the
t and/or loading time t .
Trap Load
TheLPTcanonlytrapionswhoseenergyisbelowthe
The number of trapped ions was determined by cali-
trap depth.The radial and the axial trap depths [51] are
brating the CEM to the observed photoionization from
defined as
the MOT. The photoionization rate was measured using
the methods described in Ref. [50]. This calibration is eq V eηV r2
D = 1 rf − end 0 (2)
accurate to an order of magnitude. Radial 4 2z2
0
Operating the LPT in the presence of the magnetic
and
field produced by the anti-Helmholtz coils used to gen-
erate the MOT was experimentally established to have D =ηeV (3)
Axial end
little impact on the trapping and detection of ions. Sim-
ilarly the LPT rf fields did not affect the number of cold The LPT stability parameter is defined as q1 = m4IerV02rΩf2
MOT atoms in any measurable way. [28].
4
FormostoftheresultspresentedinSec.III,whenq ≈ thermalizing with the ion sample under study through
1
0.32,D ≈2eV.Similarly,duringthe trappingtime coulomb interactions, ultimately resulting in increased
Radial
D was always ≈ 5 eV. When an ion’s mean secular ion-ion rf heating.
Axial
energy rises above the trap depth due to heating within
To quench unwanted ions from the LPT, mass selec-
theLPTitwillevaporatefromtheLPTresultingintrap
tive resonant excitation was implemented [59, 60]. Fig-
loss. ure. 3(I)(a) shows the Na+ and the photodissociated
2
Heating mechanisms, which can result in ions being Na+ produced during t from the MOT. An exter-
Trap
lost from the trap, have to be overcome by the sympa- nal ac field was introduced on the rf trap segments in
thetic cooling to achieve low ion temperatures. Trapped a quadrupole configuration set at the radial secular fre-
ions decay exponentially from the LPT, and ions which quency of the trapped Na+ ions. The amplitude of the
2
have been sympathetically cooled have been demon- external ac quenching field (V ) is set much lower than
sec
strated experimentally to have longer lifetimes due to that of the rf driving field amplitude V (V ≪ 5% of
rf sec
elastic scattering andnon-radiativecharge-exchangecol- V ) to prevent any disturbance of the trapping poten-
rf
lisions [34] with cold neutral atoms [23, 27, 52]. tial. The frequency of the external ac quenching field
Ion collisions cause energy to be exchanged between was scanned while monitoring the ion signal with the
their micromotion and their secular motion, a process CEM as shown in Fig. 3(II). Ions are ejected from the
calledatom-ion(ifthe collisionisbetweentheion(s)and trapwhen their secularfrequency resonateswith the ap-
atoms) or ion-ion (between co-trapped ions) rf heating plied external ac field because the ions’ energy is reso-
[22, 45, 53]. Another ion trap heating mechanisms is nantly driven above the trap depth. Our experimental
excess micromotion heating [11, 47]. It is caused by im- secularfrequency measurementsshowedgoodagreement
perfections in the construction or the alignment of the with simion simulations. For example, the values for
trap electrodes, the relative phase of the electric fields, Na+ ω and ω found via simulations were ≈ 36
Axial Radial
and stray fields present within the trapping region due kHz and ≈76 kHz; experimentally we found them to be
to other electric devices as well as charge build up on 35±1 kHz and 75±1 kHz respectively.
insulating materials in the chamber. In addition to test-
Althoughthistechniquesufficientlyquenchedextrane-
ing trap lifetime extension via sympathetic cooling, we
ous ions, it presented a new heating mechanism referred
experimentally found that increasing individual heating
to as ac side-effect heating [28]. When mass selective
mechanisms reduced trap lifetimes in the LPT. quenching is implemented resonantly at the Na+ radial
2
Sinceion-ionandatom-ionrfheatingareaninevitable secular frequency, the motion of the trapped Na+ ions is
by-productofiontrappinginanLPTandaredependent perturbed. As shownin Fig. 3 (III), when the MOT was
on the value of the q1 stability parameter [54], we had turned off during tTrap, the presence of the ac quenching
to experimentally determine the most favorable Vrf for field on the trap segments leads to additional trap loss.
our LPT. In the absence of any cooling, we scanned V
rf The ions created from AI within the MOT prevents
andfoundthelargestionsignalandlongesttraplifetime
us from studying a small number of ions. Even when
corresponded to q = 0.5. Similar results were found in
1 quenching is on, the CEMmeasuresa backgroundsignal
Ref. [55]. This is likely due to the balance of competing
from the AI ions. In order to keepa high signalto back-
factors such as single ion stability, atom-ion rf heating,
ground ratio, the minimum initial number of Na+ ions
ion-ion rf heating, and trap depth. We found that with
loaded in the trap ranged from 103−104.
a q < 0.5 reduced heating (as was seen in simulations
performed in Ref. [28]), but q < 0.2 would significantly The amplitude of the secular field, Vsec, must be cho-
reduce the trap lifetime, likely due to the reduced trap sen to balance sufficiently quenching the Na+2 ions and
depth. Therefore, in trap lifetime measurements, which minimizing ac side effect heating. At the value of q1 for
comprise the majority of the results in Sec. III, use q1 ≈ Na+ thatweused(q1 ≈0.32)wefoundaminimumvalue
0.32. of Vsec ≈ 1 V was required to sufficiently quench the
Na+ background signal. We found that when V was
In the presence of 589 nm light, Na MOT atoms 2 sec
increased from this minimum value by as little as 20%,
(uniquelyamongthealkalimetalatomsbutcommonlyin
the ac side effect heating would begin to overwhelm the
alkalineearthatoms)areadditionallysubjecttophotoas-
sociative ionization (AI) reactions which produce Na+ sympathetic cooling from the MOT.
2
molecularions[56,57]. Na+ionsareproducedfromthese Both q and V are inversely dependent on the mass
1 sec
Na+ ions via two mechanisms: resonant photodissocia- of the species trapped. Since Na+ is twice the mass of
2 2
tion caused by the 589 nm photons and collisional pho- Na+, it experiences half the q and half the trap depth
1
todissociation caused by collisions with excited Na(3p) experienced by Na+. Therefore, as q of Na+ is lowered,
1
atoms [58]. As a result the Na MOT is itself a source of the minimum necessary V is also lowered. as a result
sec
Na+ and Na+ ions. Since the time of flight to the CEM the ac side-effect heating is also lowered. This gives fur-
2
isnotresolvedforthetwoionicspecies,anychargedpar- ther motivation for using q < 0.5. Experimentally, we
1
ticles (atomic or molecular) introduced during t in- found that if q < 0.2 the Na+ trap depth and stabil-
Trap 1 2
creases the ion signal. These MOT born ions due to AI ity parameter are so low that the minimum necessary
caninterferewiththesympatheticcoolingexperimentby V ≈0 V, i.e Na+ is not trapped.
sec 2
5
100 100
(a)
g (a)
n
ni g 80
mai nin
e 10 ai
R m 60
s e
n R (b)
o (b) s
al I on 40
niti 1 of I
f I % 20
o
%
0
0.00 0.01 0.02 0.03 0.04 0.05 0.06
0 20 40 60 80
tTrap (s) | DRadial| (eV)
FIG. 5. (Color online). Fraction of ions remaining in the
hFaIGve.a4n. (eCxpoloonreonntilainled)e.cNaya+shdowecianygcduiffrveersenocneainsetmraip-lologssscbaele- trap, at an initial DRadial ≈ 0.6 eV, as a function of a sud-
tweenionsthatweresympatheticallycooled bytheMOT(a) den change in trap depth (∆DRadial) before extraction after
and ions that were not exposed to the MOT (b). (The er- a fixed tTrap = 5 s. Curve (a) is with MOT cooling. Curve
(b)iswithoutMOTcooling. (Theerrorbarsaresmallerthan
ror bars are smaller than the data points where they are not
thedata points).
visible).
energy distribution of the ion cloud changes. Therefore,
III. RESULTS
a hotter ion cloud should yield a larger fraction of ions
lost after a sudden drop of the LPT’s trap depth [61].
Since ground state Na+ does not have an optically After t = 5 s with a D of 0.6 eV, the radial
Trap Radial
accessible transition, direct fluorescence based Doppler trap depth was lowered suddenly by ∆D for 10 ms
Radial
temperature measurements were not possible. Instead, duration (t ) by reducing V immediately prior to
Drop rf
four different indirect measurements were taken to extraction. After suddenly lowering V the ions were
rf
demonstrate sympathetic cooling of Na+ ions by the Na detected using the CEM. As ∆D is increased, the
Radial
MOT. ionsthatarenotcooledbegintoevaporatefromthetrap
Figure 4 showsa semi-logplot of a typicalion lifetime atamuchsmaller∆D thanwhenthe ionsaresym-
Radial
measurement with and without sympathetic cooling by pathetically cooled (Fig. 5).
the MOT.The ionswereinitializedfromthebackground This experiment (Fig. 5) was conducted at an initial
Na gas resulting in an initial time averaged mean ion Na+ q = 0.18 value, for which the AI-produced Na+
1 2
cloudkineticenergyof≈1 eV (accordingtosimionsim- andNa+ ions arenotcapturedinthe LPTandtherefore
ulations). TheexternalquenchingofNa+ionswasimple- secularquenchingwasnotnecessary. Atthislowq value,
2 1
mented during this process and any residualbackground trap lifetime measurements were not feasible due to the
signal was subtracted from the final experimental values low D . However, this test produced similar results
Radial
toeliminateanycontributionsfromionsproducedduring at various q , t and t values.
1 Drop Trap
tTrap. As shown in Fig. 4, Na+ ions that were cooled by Thethirdmethodemployedtoshowsympatheticcool-
the Na MOT stayed in the trap longer. Similar results ing was by changing the percent of trapping time an ion
were obtained for a smaller number of ions in Ref. [27] sample was exposed to the MOT during a fixed t =
Trap
with Rb+ ions and a Rb MOT. 8 s. The ion signal increases linearly as a function of
The shape of the decay curves shown in Fig. 4 are increasing MOT exposure time, i.e., increased exposure
typicalwhethertheionsareloadedviaREMPI,fromthe time leads to a larger fraction of ions cooled below the
MOTorthe backgroundgas. Thereforethe temperature trap depth (Fig. 6).
of the source of the neutrals from which the ions are TheoverlapoftheMOTwiththeioncloud,whichwas
producedhas little effect onthe traplifetime or the final demonstratedto have a significanteffect onsympathetic
temperature, as predicted in Ref. [28]. coolinginRefs[10,28],wastestedandprovidedafourth
Thesecondtestusedtodemonstratesympatheticcool- and final test to demonstrate cooling. Moving the MOT
ing measuresthe traplossas afunction ofchangingtrap with respect to the ion cloud was accomplished by using
depth. When the ion cloud is cooled by the MOT the a magnetic shimcoil. The trapped ionsignalis obtained
6
ence in trap lifetime, trap loss due to changing trap
depth, variable MOT exposure time and MOT overlap
3.5
were tested experimentally and the results support sym-
pathetic cooling of Na+ ions by an equal mass Na MOT
3.0
as hypothesized. Because of our previously published
2.5 (a)
V)
al (2.0 5.0
n
Sig1.5 4.5
n
o 4.0
I1.0
3.5
)
0.5 (V3.0 (a)
(b) al
0.0 n2.5
g
0 20 40 60 80 100 Si2.0 Change in MOT Position
% MOT Exposure n (b) As Viewd From CEM
o1.5
I
1.0
FIG. 6. (Color online). Ion signal (proportional to the num-
ber of ions) plotted against the fraction of a fixed tTrap = 8 0.5
(c)
s during which the ions were exposed to the MOT. In curve
0.0
(a) the ions signal is seen to increase linearly as MOT expo-
sure approaches 100% of the fixed trapping time. In curve -4 -3 -2 -1 0 1
(b) the background reading shows effective quenching of AI
MOT position (mm)
MOT produced ions. (The error bars are smaller than the
data points where they are not visible).
FIG. 7. (Color online). (a) Ion signal (proportional to the
number of ions) is plotted against the translated position of
after a fixed t = 7 s. As portrayed in Fig. 7(a), the theMOT, which is a caused by an applied shim coil current.
Trap
ion signal reached a maximum as the MOT was trans- Position = 0 mm corresponds to the point of maximum ion
lated across the x −x plane. Although the ion cloud signal. A Gaussian fit of curve (a) gives a full width at half
1 2 maximum of3.2±0.2 mm. (b)Effect oftheshim coil on the
cannot be optically imaged, the relative MOT position
trapped ions when the MOT is not present. (c) Background
at which the maximum ion signal occurs is likely where
MOT born AI ion signal showing that translating the MOT
the MOT is concentric with the ion cloud. A Gaussian
across the LPT axis does not affect the quenching process.
fit of curve (a) Fig. 7 yields a full width at half maxi-
Inset: Shows the MOT moving (+) to (-) as (from right to
mum of 3.2±0.2 mm. This measurement can be inter- left) as viewed from the CEM. (The error bars are smaller
preted as an upperbound of the size of the ion cloud in than thedata points where they are not visible).
the MOT translationdirection. The shim coil itself does
not dramatically affect the ion signal [see Fig. 7 (b)] or
theresonantquenchingofextraneousions[seeFig.7(c)]. work in simulating this system [28], this result was sur-
prising to us (but not inconsistent) that the hybrid trap
is able to effectively coola relatively largenumber of co-
IV. CONCLUSION trapped ions.
WedemonstratedsympatheticcoolingofNa+ionsbya
cold Na MOT in a hybrid trap. Since the Na MOT also V. ACKNOWLEDGMENTS
produces Na+ and Na+ ions via photoassociative ion-
2
izationandsubsequentphotodissociation,measureswere We would like to acknowledge support from the NSF
taken to quench this external ion production in order to under Grant No. PHY0855570. One of us (F.A.N.)
demonstrateaclearcoolingeffect,whichweobservedde- would like to thank the University of Connecticut group
spite this experimental obstacle. for their hospitality during numerous visits. We also
Evidence of sympathetic cooling by cold MOT atoms thank Jian Lin and Oleg Makarov for their preliminary
was investigated using four different methods. Differ- work on the ring-dye laser and the Na MOT.
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