Efficient fluorescence collection and ion imagingwith the “tack” ion trap

Gang Shu,1,2,* Chen-Kuan Chou,1 Nathan Kurz,1,3 Matthew R. Dietrich,1,4 and Boris B. Blinov1

1Department of Physics, University of Washington, Seattle, Washington 98195, USA2Current address: School of Chemistry and Biochemistry, Georgia Institute of Technology,

Atlanta, Georgia 30332-0400, USA3Current address: Nion Company, Kirkland, Washington 98033, USA

4Current address: Argonne National Laboratory, Lemont, Illinois 60439, USA*Corresponding author: shugang@gatech.edu

Received August 26, 2011; accepted September 22, 2011;posted October 3, 2011 (Doc. ID 153525); published November 11, 2011

Trapped, laser-cooled atoms and ions produce intense fluorescence of the order 107 ∼ 108 photons per second.Detection of this fluorescence enables efficient measurement of the quantum state of qubits based on trappedatoms. It is desirable to collect a large fraction of the photons to make the detection faster and more reliable.Additionally, efficient fluorescence collection can improve the speed and fidelity of remote ion entanglementand quantum gates. Refractive and reflective optics, and optical cavities have all been used to collect the trappedion fluorescence with up to about 10% efficiency. Here we show a novel ion trap design that incorporates ametallic spherical mirror as the integral part of the trap itself, being its RF electrode. The mirror geometry enablesup to 35% solid angle collection of trapped ion fluorescence. The movable central pin electrode of this trap allowsprecise placement of the ion at the focus of the reflector. We characterize the performance of the mirror, and mea-sure 25% collection efficiency, likely limited by the imperfections of the mirror surface. We also study the proper-ties of the images of single ions formed by the spherical mirror and apply aberration correction with an asphericalelement placed outside the vacuum system. Owing to the simplicity of its design, this trap structure can be adaptedfor microfabrication and integration into more complex trap architectures. © 2011 Optical Society of America

OCIS codes: 270.5585, 130.3990.

Trapped ion qubits are among the most promising candidatesfor a practical quantum computer [1–3]. The qubits are rea-lized by the long-lived atomic levels, such as the ground-statehyperfine levels [4] or the metastable excited states [5]. Laserand RF control techniques enable single- and multiqubit op-erations with high fidelity [6]. One of the crucial componentsof a quantum computation, the qubit state readout, is accom-plished by the selective electron shelving technique whereone of the qubit states produces strong fluorescence, whilethe other remains dark [7,8]. Efficient collection of singlephotons emitted by trapped ions excited by short laser pulsesis essential for generation of remote ion entanglement, quan-tum gates between distant ions, and quantum state teleporta-tion protocols [9–15]. Both these tasks require high efficiencyof ion fluorescence collection, and an efficient single-modefiber coupling is a must for the latter. In this paper, we addressthis problem from the perspective of integrating optics into atrap structure.

A cold, trapped ion interacting with a resonant laser radia-tion represents an almost ideal point source of light. Thus, thetask of collecting the ion’s fluorescence becomes the problemof a large solid angle imaging of a point source. Additionalrestrictions imposed by the trap structure and the vacuum sys-tem tend to limit the usable solid angle to ∼0:5 sr, or a fewpercent of the total solid angle. Traditional high numericalaperture (NA) microscopy setups are not applicable becauseof the long working distances involved in the trapped ion ima-ging. The multielement refractive optics quickly become toobulky (and expensive) as the NA increases [16,17]. The use of

near-field optics, such as [18], is generally not applicable totrapped ions due to charging of the dielectric surface of theoptics [19]; a notable exception is the diffractive micro-lens used to produce diffraction-limited ion images in [20].Concave mirrors [21] have been proposed as a way to poten-tially increase the collection solid angle to almost 90%; such adeep parabolic mirror can be used in conjunction with the“stylus” ion trap [22]. Another alternative is the use of opticalcavities surrounding the trapped ion [23,24], where a verylarge fraction of the ion’s spontaneous emission would go intothe cavity mode due to the Purcell effect. All of these methodspresent many challenges, and are currently being activelypursued.

Following our success with utilizing a spherical mirror tocollect approximately 10% of fluorescence from an ion iso-lated in a standard linear RF trap [25,26], we designed, built,and tested a novel ion trap—the “tack” trap (Fig. 1). The trapuses a concave conductive optical surface as one trap elec-trode and a sharp needle passing through the axis of the mir-ror as the other, thus resembling the ubiquitous office tool[Fig. 1(a)]. An additional ring electrode is placed above themirror surface to increase the trap depth. The rotational sym-metry of the optical surface forms a trapping potential patterncoaligned with the optical axis [Fig. 1(b)]. This highly inte-grated approach has little optical blocking; therefore, it canhave a very large accessible solid angle solely restricted bythe shape of optics. The needle electrode forms a very largelocal field gradient, resulting in a relatively deep trapping po-tential localized in a small volume [Fig. 1(c) and 1(d)]. The

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ion’s position can then be precisely controlled anywhere alongthe optical axis by moving the needle electrode.

This trap design can be readily applied to any conductiveoptical surface shapes with rotational symmetry, includingspherical, parabolic, and elliptical mirrors. We tested the con-cept with a commercially available, low-cost spherical mirror.As shown in Fig. 2, the trap is built around a 6mm diameter,4mm radius of curvature Al-coated concave spherical mirror(Anchor Optics part number AX2740). A Ø0:75mm hole isdrilled through the vertex of the mirror to accommodate a0:5mm diameter tungsten rod that tapers to a point. The rodis attached to a linear vacuum actuator feedthrough driven bya precision micrometer. The trap assembly is supported by thesuperstructure made from Kimball Physics Inc. e-beam partsand a custom-made aluminum holder [Fig. 2(b)]. Four micro-

motion compensation electrodes are attached to the ceramicrods [Fig. 2(c)]. The top plate may be biased, also for micro-motion compensation. The RF is applied to the metallic sur-face of the mirror, while the tungsten rod is RF-ground.

According to our simulations, the pseudopotential of thetack trap, being symmetric around the optical axis, is strongerin the axial direction, and weaker along the radial. For a ty-pical RF drive voltage of 270V at 23MHz, the trapping depth ison the order of 0:02 eV. This trapping depth is comparablewith that of a surface trap [27,28], which one might expectfrom the topological similarity of these two trap types. How-ever, the trap’s sharp needle and the curved metallic reflectorsurface increase its pseudopotential depth compared to thatof a planar trap with similar dimensions. Our simulations alsoindicate that as the needle is translated along the optical axis,the trapping depth remains approximately the same for a widerange of needle positions. This enables us to first load ionswith the needle extended further out, at the location more ac-cessible to the atomic beam, and then move the ions to theoptimal position for imaging and light collection.

The tack trap is placed in a Kimball Physics 4 1=2 in: “sphe-rical octagon” vacuum chamber and mounted with its axisparallel to the optical table. The trap is operated at the pres-sure of about 3 × 10−11 Torr in a standard ultrahigh vacuumsetup with a 20 l=s ion pump and a titanium sublimation pump.We use 138Ba� to test our trap. The Doppler cooling of thetrapped ions is achieved by combining a frequency-doubled986nm external-cavity diode laser (ECDL) and 650 nm ECDLin a single-mode optical fiber [29], and focusing the combinedbeam almost perpendicular to the optical axis of the trap(which is also one of the trap’s principal axes) with a smalltilting angle of 4 degrees to be able to cool the axial motionof the ions. The atomic barium beam is generated by an ovenlocated to the side of the trap, with the barium beam pointingto the optical axis. Barium atoms are ionized using a resonanttwo-photon transition using a 791nmECDL laser and a 337nmnitrogen gas laser [30], both aligned perpendicular to the Ba

Fig. 1. (Color online) Concept of the tack trap and (a) pseudopotential simulation: (b) near the trapping center,(c) along the trap axis of symmetry,and (d) along the radial direction. The numbers are given in electron-volts for typical trapping RF parameters: RF frequency of 23MHz and RFvoltage of 270V.

Fig. 2. (Color online) The construction of the tack trap. (a) Fourspherical mirrors with holes drilled are placed on a U.S. penny forscale. (b) The overview of the trap structure showing the aluminumframe and structural other elements, as well as the needle and needleguide. (c) The side view showing the electric connection of the trap,with conductors shown in gray color and insulators in light blue.

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atomic beam. A two-stage long working distance microscopeis set up along the optical axis and a 50=50 beam splitter isused to both image the ions using an Andor iXon electron-multiplied charge coupled device camera and to countphotons using a Hamamatsu photomultiplier tube (PMT).The sharp needle tip, illuminated by the focused laser light,is used as a reference point to align and focus the opticalsystem.

The ions are trapped at a distance of 0:541mm from theneedle tip, consistent with our simulation prediction of0:55mm. At the background pressure of 3 × 10−11 Torr, for asingle 138Ba� ion, we observed >30 min dark lifetime and>48h lifetime with laser cooling. We also loaded and laser-cooled various two-dimensional ion crystals, as shown inFig. 3. The two-dimensional nature of these crystals is easilyunderstood based on the strength and the symmetry of thetrapping potential. This trap is in a way complimentary tothe more typical linear RF ion trap, where the axial directionis the weakest, and elongated ion crystals are formed. The“pancake” crystals formed in the tack trap were stably trappedfor many hours. While micromotion cannot be nulled for theions located off center in these crystals, this trap design maybe potentially used as a platform for quantum simulations withtrapped ions [31].

By adjusting the needle electrode position, ions can bemoved over a fairly large distance (>1:5mm), covering theentire range from the mirror focus to its curvature center.We measured the position of the trapped ion relative to thereading on the micrometer translating the needle, and founda nearly linear dependence, which only starts to deviate fromthe straight line at the largest needle extensions. The needlecan be reliably moved over the entire range without ion loss.To further characterize the trap, we measured the trap’s

secular frequency by sending in a 10V amplitude oscillatingvoltage to one of the compensation electrodes and scanningits frequency from 100 kHz to 4MHz. The dips in ion fluores-cence correspond to the radial and the axial frequencies,which were found to be about 200 kHz and 420kHz, respec-tively. A closer look at the radial frequency reveals small non-degeneracy in the radial direction, which is probably due toslight misalignment between the needle and the main op-tical axis.

Our mirror electrode covers approximately 38.6% of the so-lid angle around its focal point, which is defined to be exactlyhalfway between the vertex of the mirror and its curvaturecenter. But for such a small focal ratio system, the huge sphe-rical aberration makes the definition of the focal point ratheruseless. Additionally, due to the small size and high curvatureof the mirror, the light reflected from it cannot be compen-sated with a Schmidt-like corrector that we used in our earlierdesigns [26], since the rays reflected from different points inthe mirror will have intersected by the time they reach theoutside of the vacuum chamber. Therefore, we designed a dif-ferent type of aspheric lens, such that it collimates the lightfrom the ion while compensating its spherical aberration.The setup is shown schematically in Fig. 4(a), and the ex-pected diffraction-limited image point spread function inFig. 4(b). For best compensation results, instead of being atthe focus of the mirror, the ion is moved 0:25mm further awayfrom the mirror. We numerically calculate the shape of thecorrector using an algorithm similar to that described in [26]and arrive at a high-order nonlinear curve.

To test the effectiveness of this compensation scheme, wemachine the aspheric lens prototypes out of clear acrylic. Theobvious improvement of the image quality can be seen inFig. 5. Here, in Fig. 5(a), an uncompensated image of an ioncrystal is shown, with the bright dots in the center formed bythe paraxial rays from the mirror, while much of the light islost into the large, diffuse rings. In Fig. 5(b), a single ion imagewith the compensation scheme is shown. Although the aber-ration is greatly reduced, it is obviously not fully compensatedwith our current device. This is largely because both the sphe-rical mirror and the aspherical lens are far from perfect.Notably, a direct measurement of the mirror surface with a

Fig. 3. (Color online) Images of Ba� ions in the tack trap. (a) A singleion, (b) a symmetric 7-ion crystal, (c) an asymmetric 10-ion crystal,and (d) a larger (24-ion) crystal. All 10 ions are visible in (c), while(b) and (d) have some dark ions representing different isotopes ofbarium. (a), (b), and (c) are directly formed by a single micro objec-tive; (d) is formed by being reflected off the mirror and collimated byadditional commercial lenses.

Fig. 4. (Color online) The aberration compensation scheme. (a) Theschematics of the optical setup showing the mirror, the vacuum view-port, the aspheric element, and the multielement low-NA micro objec-tive. (b) The ideal point spread function calculated from simulations(0:97 μm) reaches diffraction limit (1:15 μm).

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measuring scope (OGP SmartScope) shows that its radius ofcurvature is at least ∼10% larger than its nominal value; thesurface is not a true sphere either. The precise control andmeasurement of the plastic homemade corrector surface iseven more difficult. Taking these factors into account, we be-lieve that our compensation scheme performs exceptionallywell, and we clearly demonstrate the proof-of-principle.

Geometrically, this spherical mirror covers about 35% ofthe solid angle from the ion placed 0:25mm away from thefocus. To calibrate the mirror’s efficiency for light collection,we use a single photon generated by repeated shelving anddeshelving of the single ion, using the scheme described in[26]. In the test we counted 9957� 611 photons per 1,000,000excitations of the ion, corresponding to 1% overall photon col-lection and detection efficiency. After factoring out the quan-tum efficiency of the PMT, and the transmissive and reflectivelosses of all optical elements, we arrive at approximately 24%of the entire 4π solid angle to be subtended by the mirror,which is equivalent to about 3:0 sr, or an NA of 0.85. Thereare several possible reasons for the reduced efficiency, in-cluding the nonuniform spatial distribution of the ion dipoleradiation pattern, and the degradation of the unprotectedaluminum surface of the mirror during the baking of thevacuum system [32].

Our trap design can be readily applied to other shapes ofreflectors, such as parabolic or elliptical mirrors. Parabolicmirrors can in theory reach solid angle close to 100% [21]and produce diffraction-limited, collimated light from anion located in the focus of the mirror. A proposal exists tocombine such a mirror with the stylus trap designed by theNIST group [22]. We believe that a simpler design similar

to that described here may be more practical. It does notrequire microfabrication and, with a mirror surface beingthe RF electrode, the tack trap can sustain higher RF voltagesthan the stylus trap, thus improving the trapping depth.

Elliptical mirrors are also interesting, since they can pro-duce, in one of their foci, a diffraction-limited image of a pointsource located in the other focus. Thus, ions placed in onefocus point can be directly imaged into a single-mode opticalfiber located at the other focus point without additional op-tics. For free space optics, elliptical mirrors can be used as anNA converter by reducing a large NA at one focus into a smal-ler NA at the other; the successive imaging can then be donewith a low-NA microscopy setup. Additionally, elliptical mir-rors appear to be suitable for microfabrication and integrationinto state-of-the-art chip-scale ion traps, as discussed below.

An important criterion of an ion trap design, especially inquantum information applications, is its feasibility for scaling—both down in size and up in number. The simplicity of the

Fig. 5. (Color online) Demonstration of the effectiveness of the op-tical compensation scheme. (a) An ion crystal image without correc-tion, where the large rings can be clearly seen due to sphericalaberration, where ∼90% of the light from the ions is spread. (b) A sin-gle ion image formed by the mirror-corrector system, and (c) the pro-file of the image. The full width at half-maximum of the peak is lessthan 100 μm, which is a significant improvement, but still far fromdiffraction limit (1:15 μm for this setup).

Fig. 6. (Color online) A proposal for microfabricaton of mirror traps.(a) First, a substrate with appropriate conductive layers is prepared.(b) A series of step guides are etched with standard microelectrome-chanical systems (MEMS) techniques, such as DRIE. A thin slot par-allel to the section (not shown) can be etched to connect all the stepsto allow plating fluid access to the entire surface. (c) An ellipsoid beadfalls into the guide, finishing a self-assembly. (d) Current is runthrough all the conductive layers to plate metal into the gap betweenthe bead and the substrate; the bead is then removed with chemicaldissolution. (e) The substrate can then be polished down to the appro-priate height. (f) The mirror surface can then be segmented and elec-trical connections are made. Note that different segments areinsulated from each other. Two pseudopotential simulations showhow the height of the trapping zone can be controlled by routingRF and ground electrodes to different parts of the mirror, whereRF is represented by yellow and ground by green. (g) When theRF is applied on the upper two segments of the mirror, the trappingzone is lower, while (h) if the RF is applied to the top segment only,the trapping zone is raised higher, close to the focus of the ellipticalmirror. The simulations predict deeper trapping potentials than in aplanar trap of a similar scale.

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tack trap permits large-scale fabrication and integration withthe microfabricated planar traps and trap arrays. One chal-lenge here is to fabricate the highly curved surface controlla-bly. There are efforts currently under way to integrate curvedmirrors into planar ion traps using more traditional methods[33,34], but it is challenging to produce a large focal ratio op-tical surface while controlling its exact shape. We propose aself-assembly molding procedure, with which full control ofthe shape and quality of optical surface is possible [35,36].

Micrometer-scale spherical or ellipsoid beads with opticalquality surface are available from either commercial sources,or by in situ lab fabrication (e.g., by melting glass fiber into amicrosphere with laser blasting [37]). With single point di-amond turning technology, the mold or even the surface itself,can be directly machined with ultrahigh precision [38]. Theconcave reflector surface can then be fabricated by electro-plating high-reflectivity metal, such as aluminum or gold, withthe bead serving as a mold. The possible fabrication steps areoutlined in Fig. 6. After the substrate is prepared with a seriesof conducting and insulating layers, a guiding structure isetched to ensure alignment of the electroplated mirror withthe other parts of the trap. Then the substrate is patternedwith a thin conductive layer for electroplating. When thebeads and the substrate are put together (typically insidesome liquid), the beads will fall into the guides due to gravity,thus finishing the self-assembly. During the electroplating, themetal fills the gap between the bead and substrate [Fig. 6(d)].The beads can then be removed by chemical dissolution thatwould keep the metal surface intact. The finished metal sur-faces will have the exact shape and smoothness of the molds.The fabrication of the needle electrode can be adapted to dif-ferent trap scales. For example, for a trap with the character-istic structure size of 100 μm or above, the needle can befabricated separately by the deep-reactive ion etching (DRIE)process [39] and precisely aligned and bound to the mirrorsubstrate. For smaller trap sizes, the needle can be eliminatedaltogether by dividing the mirror surface into different seg-ments acting as the RF and the ground electrodes, as shownin Figs. 6(g) and 6(h).

In summary, we present a novel ion trap design for efficientfluorescence collection from trapped ions. We achieve aneffective NA of 0.85, which, to our knowledge, is by far thelargest solid angle for the ion traps. We discuss and test thepossibility of aberrations compensation, and propose practi-cal approaches for scaling down and integrating this “tack”trap with a micofabricated planar trapping system using theavailable microelectromechanical systems technology. Thistrap design can be used in many trapped ion quantum compu-tation and information, as well as for quantum simulations.

ACKNOWLEDGMENTSThis research was supported by the National Science Founda-tion (NSF) grants 0758025 and 0904004, the United StatesArmy Research Office (USARO), and the University ofWashington Royalty Research Fund.

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