/Stanford U., Phys. Dept.

/UC, Irvine /Laurentian U. /Neuchatel U. /SLAC

DeVoe, R.

Fairbank, W., Jr.

Fierlinger, P.

Flatt, B.

Gratta, G.

Green, M.

Hall, K.

Hargrove, C.

Leonard, D.S.

LePort, F.

Moe, M.

Mong, B.

Montero Diez, M.

Neilson, R.

O'Sullivan, K.

Piepke, A.

Pocar, A.

Sinclair, D.

Strickland, V.

U., /Alabama

U., /Carleton

U., /Colorado State

Waldman, S.

Wodin, J.

Leonard, D. S.

Fairbank, Jr., W.

UNT (University of North Texas) Digital Library

Work supported in part by US Department of Energy contract DE-AC02-76SF00515Observation of single collisionally cooled trapped ions in a buffer gasM. Green,1 J. Wodin,1 R. DeVoe,1 P. Fierlinger,1 B. Flatt,1 G. Gratta,1 F. LePort,1 M. Montero Dı´ez,1R. Neilson,1 K. O’Sullivan,1 A. Pocar,1 S. Waldman,1, ∗ D.S. Leonard,2 A. Piepke,2 C. Hargrove,3 D. Sinclair,3V. Strickland,3 W. Fairbank Jr.,4 K. Hall,4 B. Mong,4 M. Moe,5 J. Farine,6 D. Hallman,6 C. Virtue,6 E. Baussan,7Y. Martin,7 D. Schenker,7 J.-L. Vuilleumier,7 J.-M. Vuilleumier,7 P. Weber,7 M. Breidenbach,8 R. Conley,8C. Hall,8, † J. Hodgson,8 D. Mackay,8 A. Odian,8 C.Y. Prescott,8 P.C. Rowson,8 K. Skarpaas,8 and K. Wamba81Physics Department, Stanford University, Stanford CA, USA2Department of Physics and Astronomy, University of Alabama, Tuscaloosa AL, USA3Physics Department, Carleton University, Ottawa ON, Canada4Physics Department, Colorado State University, Fort Collins CO, USA5Physics Department, University of California, Irvine CA, USA6Physics Department, Laurentian University, Sudbury ON, Canada7Institut de Physique, Universite´ de Neuchatel, Neuchatel, Switzerland8Stanford Linear Accelerator Center, Menlo Park CA, USA(Dated: April 1, 2007)Individual Ba ions are trapped in a gas-filled linear ion trap and observed with a high signal-to-noise ratio by resonance fluorescence. Single-ion storage times of ∼5 min (∼1 min) are achievedusing He (Ar) as a buffer gas at pressures in the range 8× 10−5 − 4× 10−3 torr. Trap dynamics inbuffer gases are experimentally studied in the simple case of single ions. In particular, the coolingeffects of light gases such as He and Ar and the destabilizing properties of heavier gases such as Xeare studied. A simple model is offered to explain the observed phenomenology.PACS numbers: 52.20.Hv, 39.25.+k, 29.30.EP, 23.40.-sThe discovery of ion trapping [1, 2] followed by pio-neering experiments on single trapped ions in vacuum[3] has opened the door to a wealth of physics, suchas laser cooling [4, 5], quantum computation [6], im-proved atomic frequency standards [7], mass spectrom-etry [8], and measurements of metastable atomic life-times [9]. Traps are usually operated in ultra-high vac-uum (UHV, . 10−10 torr), and laser cooling techniquesare utilized for precision measurements on a single con-fined ion. Buffer gas filled RF Paul traps, on the otherhand, use collisions to thermalize ions with the buffer gas.These devices have proven useful, in particular, for massspectrometry on ion clouds, as well as cooling stages inheavy isotope accelerators [10, 11]. Buffer gas coolingis effective at cooling many ions at a time, and chargedspecies with many degrees of freedom such as ionizedbiological molecules [12]. The observation of individualbuffer-gas-cooled ions via resonance fluorescence is chal-lenging, primarily due to Doppler broadening of the ion’snatural linewidths. This limits the sensitivity of the tech-nique as a tool to detect trace amounts of ions. Further-more, increasing the number of trapped ions to obtainmore fluorescence makes it difficult to study ion dynam-ics and their interactions with the buffer gas, since ion-ioninteractions produce non-negligible effects.In this work, single Ba ions are trapped and con-tinuously cooled by collisions with a room temperaturebuffer gas (He or Ar) at pressures between 8× 10−5 and4× 10−3 torr in a segmented quadrupole linear RF Paultrap. The 6S1/2 ↔ 6P1/2 and 6P1/2 ↔ 5D3/2 transi-tions are driven with CW diode lasers, producing reso-nance fluorescence at 493 nm and 650 nm [3]. Individualions are stored for ∼50 s to ∼500 s and imaged. Effec-tive “Doppler” ion temperatures are measured from thebroadening of the 6P1/2 ↔ 5D3/2 transition. Also, single-ion unloading rates are measured at different pressuresof He, Ar and He/Xe mixtures, providing insight intothe cooling behavior of light gases and the de-stabilizingproperties of heavy ones. A simple collisional unloadingmodel is found to be able to describe the data.Linear RF Paul traps confine ions radially in aquadrupole RF field applied across electrodes placedsymmetrically about a central axis [13]. The trap usedin this experiment is shown schematically in fig. 1. Athree dimensional potential well is obtained at segment14 (S14) by creating a DC potential minimum along thez-axis of the trap, in addition to the radial RF pseudopo-tential minimum. The well profile is parabolic radially(x-y plane in fig. 1) and longitudinally (z-axis), withdepths Dr and Dz in volts [11, 12].The confinement of an externally injected ion of chargee requires a kinetic energy dissipation mechanism, suchthat the ion is cooled to an energy less than both eDr andeDz while at the trap minimum. One such energy dissi-pation mechanism is laser cooling, which requires UHVconditions in addition to cycling transitions used for cool-ing. Energy dissipation in the system described here, incontrast, is achieved by collisions with a buffer gas. Theion comes into thermal equilibrium with the buffer gas,settling into the well at S14 with energy ∼0.03 eV for agas at 293 K. Over the range of pressures relevant here,Ba ions undergo 1-100 collisions with the buffer gas be-SLAC-PUB-12500physics/0702122Submitted to Phys.Rev.Lett.2......0VS1 S2 S3 S16S14S13 S15(1)yx (2) (3)z-8V40 mm4 mm-1.2 V -1.2 VUDCFIG. 1: Schematic drawing of the buffer-gas filled linear RFPaul trap used in this work. Ions are created in S3 via electronimpact ionization, and collisionally cooled by the buffer gasinto the potential well at S14. UDC is the potential applied toeach segment. The ion is identified via resonance fluorescenceat 493 nm. (1) Spectroscopy lasers, (2) Individual Ba ion, (3)EMCCD camera.fore reaching S14 from the injection region at S3.The details of collisional cooling have been investigatedanalytically [14, 15, 16] and numerically [11, 15, 17, 18]for mI ≫ mB, mI ∼ mB, and mI ≪ mB, where mIand mB are the masses of the ion and buffer gas atoms.Consider an ion with initial kinetic energy EI , interact-ing with a buffer gas at temperature TB < EI/kB, wherekB is Boltzmann’s constant. If mI ≫ mB, each colli-sion perturbs the ion’s momentum by a small amount,bringing the ion into thermal equilibrium with the buffergas after many collisions. Data relevant to this situa-tion are presented in the following section, where Ba ions(mI ≈ 137 amu) are trapped in He (mB ≈ 4 amu) andAr (mB ≈ 40 amu) buffer gases. If mI ∼ mB, the ionis cooled to the temperature of the buffer gas with farfewer collisions, at the expense of a higher probabilityper collision of unloading from the trap. This unloadingis due to RF-heating, whereby a large momentum changeof the ion per collision can cause the ion to absorb energyfrom the RF field. Buffer gases composed of He/Xe mix-tures (mXe ≈ 131 amu) allow for the study of deconfiningeffects in this case, while retaining trapping times longenough to comfortably observe a single ion. In the thirdcase, mI ≪ mB, the ion is not cooled, leading to unload-ing. This case, treated in [19], is not discussed furtherhere.The construction and operational details of the iontrap used here are described elsewhere [20, 21]. The trapis housed in an all-metal UHV tank, coupled to a buffergas purification and injection system. The trap is splitinto fifteen 40 mm segments (S1-S13, S15-S16 in fig. 1),and one 4 mm segment (S14) defining the center of thetrapping region. The total trap length is 604 mm, chosento be long enough to thermalize highly energetic, exter-nally injected ions in future experiments.Each segment consists of four, 6 mm diameter cylin-drical stainless steel electrodes. Radial confinement isachieved by applying a 150 Vpk sinusoidal voltage at1.2 MHz to two diagonal electrodes, spaced 11.3 mmapart center-to-center, while keeping the other two at RFground. Longitudinal confinement is achieved by apply-ing a DC potential, UDC, to each segment as shown in fig.1. This configuration, similar to that used in ion cool-ers at heavy-ion facilities [10], gives a radial trap depthDr = 3.8 V, and a longitudinal trap depth Dz = 6.8 V[11, 21].External cavity diode lasers at 493 nm (frequency dou-bled 986 nm) and 650 nm, locked to the optogalvanicresonance of a Ba discharge lamp [22], create resonancefluorescence from individual trapped ions. The beamsare directed along the longitudinal axis of the trap, asillustrated in fig. 1. The 493 nm and 650 nm laser pow-ers are set to 65 µW and 200 µW, respectively. The493 nm fluorescence from individual Ba ions is imagedonto an electron multiplying CCD[26] (EMCCD) via a64 mm working-distance microscope outside the vacuumchamber. A spherical mirror located inside the cham-ber doubles the acceptance of fluorescence photons. Thefluorescence intensity is obtained by integrating the EM-CCD signals over a region of interest centered aroundthe trapping minimum. The total fluorescence detectionefficiency is estimated to be ∼ 10−2.Detection of the fluorescence from a single ion in theconditions described here is challenging because of theDoppler broadening of the spectral lines and the largervolume of space occupied by the ions. At 293 K, theDoppler broadened lineshapes are a convolution of thenatural ones with the ion’s velocity profile along the laseraxis. The Doppler linewidth isΓD =(ω0c)√8kBT log 2mI(1)where c is the speed of light, ω0 is the frequency of the493 nm or 650 nm transition, and T is the Maxwell-Boltzmann temperature of the ion’s motion. The naturallinewidths of the two transitions are Γ493/2π = 15.2 MHzand Γ650/2π = 5.3 MHz [23]. Doppler broadening re-duces the fluorescence from each of these transitions bya factor ΓD/Γ493,650, or 42 and 92, respectively. Theion’s spatial distribution in the trap is determined bythe gas temperature and the geometry of the potentialwell. In this experiment, the ion is confined to a volume∼ (500 µm)3. Imaging over this large of an area increasesthe amount of scattered laser light accepted by the detec-tion optics. The simultaneous optimization of the laserbeam quality, fluorescence collection optics, trapping seg-ment size, and potential well depth is therefore essentialto observe individual ions.The buffer gas is purified by a hot Zr alloy getter[27], that nominally reduces the contamination from elec-3Time [s]0 200 400 600 800 1000 1200 1400 1600 1800Fluorescence [au]650700750800850900950310×4 ions3 ions2 ions1 ionBackground level0Red detuning [MHz]-400 -200 0 200 400Fluorescence [au]FIG. 2: Intensity of the 493 nm fluorescence from single Baions trapped in S14 as a function of time. The ions are con-tinuously cooled by 8.6× 10−4 torr He at 293 K. Inset is the6S1/2 ↔ 6P1/2 spectrum of a single ion, used to calculate theion’s temperature.tronegative impurities to . 1 ppb. To avoid the buildupof impurities from outgassing, the purified gas flows con-tinuously through the vacuum system and is removedby a dry 520 ℓ/s turbomolecular pump. The pressureis controlled by a flowmeter which is servoed to a pres-sure gauge sampling the vacuum chamber near S14. Baions are created at S3 (far from the potential well) viaelectron impact ionization of neutral Ba, emitted from aBa-coated Ta oven placed just outside of the trap. Colli-sional cooling to the trap minimum at S14 is achieved inmuch less than 1 s.In fig. 2, four ions are initially loaded into the trapat 8.6 × 10−4 torr He, and the 493 nm fluorescence in-tensity in the region of interest is continuously observedover time. Individual ions spontaneously eject from thetrap, reducing the fluorescence intensity by quantizedamounts. Each data point in the time series correspondsto 5 s of integrated fluorescence by the EMCCD, in ar-bitrary units of EMCCD counts. Single ions are clearlyidentified with a signal to noise ratio of 10.6 (defined asthe distance, in number of sigmas, between the 0-ion and1-ion cases). The signals from more than one ion, usefulto establish single ion detection, are not used in the restof these measurements.The Maxwell-Boltzmann temperature of a single ionin He is measured by scanning the red laser frequencyover the Doppler broadened 6P1/2 ↔ 5D3/2 resonance(fig. 2, inset). The 493 nm fluorescence intensity is mon-itored, while the blue frequency is kept fixed. The ion’stemperature is extracted by fitting the measured spec-trum to a Voigt profile, taking into account the measuredpower broadening due to the lasers. This profile assumesa Maxwellian velocity distribution along the laser axis.Pressure [Torr]-410 -310 -210]-1Average unloading rate R [s-310-210-110ArgonHeliumFIG. 3: Average unloading rates of a single Ba ion in variousbuffer gases, as a function of pressure.The effective temperature is found to be constant, withuncertainties, between 5×10−5 and 4×10−3 torr He, andit’s value averaged over this pressure range is 256±10 K.While this temperature is close to the 293 K of the buffergas, the parabolic shape of the trapping potentials andthe imperfect alignment of the lasers with respect to thetrap axis may produce further uncertainties not includedin the error given here.The average single ion unloading rates in He and Arare measured by loading a few ions into the trap. Once asingle ion is left, its residence time is measured. This pro-cedure is repeated multiple times at different pressures ofHe, Ar, and various He/Xe mixtures. Assuming the ionhas a constant probability of unloading per unit time (orcollision), the probability of an ion surviving for time tin the trap isPS(t) = e−Rt (2)where R is the average unloading rate (inverse of theaverage storage time). The average single ion unloadingrates as a function of buffer gas pressure and type are de-termined via an un-binned maximum likelihood method,using ≥ 20 independent residence time measurements foreach pressure. These results are shown in fig. 3 for thecase of pure light buffer gases. The vertical errors in thefigure are uncorrelated statistical fluctuations in the life-times. The horizontal errors are the precision to whichthe absolute gas pressure is known near S14.Two qualitative features of the data are worth noting.First, the average unloading rate increases with increas-ing buffer gas pressure. This is consistent with an in-crease in the frequency of RF-heating collisions, or pos-sibly, impurities present in the buffer gas. Second, theunloading rate is lower for He than for Ar. This is likelydue to kinematic processes such as RF-heating, whichdepend on the buffer gas mass [15, 19].4 [Torr/Torr]He/PXeP0 2 4 6 8 10 12 14 16-310×]-1Average unloading rate R [s-310-210-110FIG. 4: Measured unloading rate of a single ion stored in 3.7×10−4 torr of He/Xe mixtures at different Xe concentrations.The single ion unloading rate in various He/Xe mix-tures is measured as a function of Xe concentration, asshown in fig. 4. In this case, the He acts as a contin-uous cooling mechanism, whereas collisions between theBa ion and Xe atoms have a significant probability ofejecting the ion from the trap.A simple phenomenological model is fit to the data,based on the following argument. The time intervals be-tween Ba+-He and Ba+-Xe collisions are each exponen-tially distributed with rate constants RHe = CHePHe andRXe = CXePXe, respectively, where PHe,Xe are the He andXe buffer gas partial pressures. Collision physics detailsare absorbed into the coefficients CHe,Xe. The probabil-ity that a Ba ion collides once with a Xe atom in a timet isPXeC (t) = 1− e−RXet = 1− e−CXePXet (3)As evident from the magnitude of the ejection rates in fig.4, compared to the Ba+-Xe collision frequency (∼ kHz),a single Ba+-Xe collision is not sufficient, on average,to unload an ion. Instead, unloading appears to requiremultiple consecutive Ba+-Xe collisions on a timescale lessthan the Ba+-He collision time, R−1He . The probability ofn Ba+-Xe collisions during a time t ≤ R−1He is∏nPXeC (t ≤ R−1He ) =[1− exp(−CXePXeCHePHe)]n(4)For PXe ≪ PHe,∏nPXeC (t ≤ R−1He ) ≈(CHePXeCXePHe)n(5)Hence, the rate of this process is proportional to the nthpower of the Xe concentration. Using this model, thedata in fig. 4 is fit to the three parameter function,R(PXePHe)= C0 + C1(PXePHe)n(6)where C0 is the unloading rate due to He alone, C1 rep-resents a combination of the Ba+-Xe and Ba+-He col-lision physics constants, and n is the average numberof collisions required to unload an ion. The fit yieldsn = 4.5 ± 0.6 collisions (and C0 = (1.8 ± 0.3) × 10−3,C1 = 30± 4), with a χ2/dof = 8.6/5.The ability to perform clean measurements on singleions in the presence of different background gases mayprovide important insights with regard to the dynam-ics of these systems. From a practical point of view,traps operated in the regime discussed here may helpto improve the sensitivity of detecting very rare atomicspecies in less-than-ideal situations, where some gas con-tamination is unavoidable. In particular this techniqueis relevant for the development of a very large double-beta (ββ) decay experiment in which individual 136Ba+,produced in the ββ decay of 136Xe, need to be identifiedwith high resolution spectroscopy in the presence of Xecontamination [24, 25].This work was supported, in part, by DoE grant FG03-90ER40569-A019 and by private funding from StanfordUniversity. We also gratefully acknowledge a substantialequipment donation from the IBM corporation, as wellas very informative discussions with Guy Savard.∗ Now at Caltech, Pasadena CA, USA† Now at University of Maryland, College Park MD, USA[1] W. Paul et al., Z. Naturforsch Tiel A 8, 448 (1953).[2] R. Post, Tech. Rep. UCRL 2209, University of CaliforniaRadiation Laboratory (1953).[3] W. Neuhauser et al., Phys. Rev. Lett. 41, 233 (1978).[4] W. M. Itano and D. J. Wineland, Phys. Rev. A 25, 35(1982).[5] D. J. Wineland and W. M. Itano, Phys. Rev. A 20, 1521(1979).[6] M. Nielsen and I. Chuang, Quantum Computation andQuantum Information (Cambridge Univ. Press, 2000).[7] W. Oskay et al., Phys. Rev. Lett. 97, 020801 (2006).[8] D. Douglas et al., Mass Spectr. Rev. 24, 1 (2005).[9] N. Yu et al., Phys. Rev. Lett. 78, 4898 (1997).[10] A. Kellerbauer et al., Nucl. Instr. Meth. A 469, 276(2001).[11] F. Herfurth et al., Nucl. Instr. Meth. A 469, 254 (2001).[12] R. March et al., Quadrupole Ion Trap Mass Spectrometry(Wiley-Interscience, 2005).[13] W. Paul, Rev. Mod. Phys. 62, 531 (1990).[14] H. Dehmelt, Adv. At. Mol. Opt. Phys. 3 (1967).[15] R. DeVoe, to be published.[16] Y. Moriwaki et al., Jpn. J. Appl. Phys. 31, L1640 (1992).[17] T. Kim, Ph.D. thesis, McGill University (1997).[18] S. Waldman, Ph.D. thesis, Stanford University (2005).[19] Y. Moriwaki et al., Jpn. J. Appl. Phys. 37, 344 (1998).5[20] B. Flatt et al., to be published.[21] J. Wodin, Ph.D. thesis, Stanford University (2007).[22] E. Badareu, Br. J. Appl. Phys. 15 (1964).[23] G. Janik et al., J. Opt. Soc. Am. B 2, 1251 (1985).[24] M. Breidenbach et al., proposal to US DoE, Unpublished.[25] M. Danilov et al., Phys. Lett. B 480 (2000).[26] Andor model iXonEM+[27] SAES MonoTorr model PF3C3R1

Observation of Single Collisionally Cooled Trapped Ions in a Buffer Gas

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Observation of Single Collisionally Cooled Trapped Ions in a Buffer Gas

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