We demonstrate a macroscopic magnetic guide for cold atoms with suppressed
longitudinal field curvature which is highly desired for atom interferometry.
The guide is based on macroscopic copper tape coils in a copropagating currents
geometry, where the atoms are located between the coils few cm away from each
surface. The symmetric geometry provides a much lower magnetic field curvature
per fixed length that promises longer coherence time for atom interferometers.
A double-tape design of each coil allows a smooth translation of guided atoms
without addition of an external bias field. The guide is also immune from the
current and thermal noise by virtue of the turns averaging and a large working
distance, respectively. We present the experimental results of guide
application to atom interferometry

Prentiss, Mara

Tonyushkin, Alexey

Journal of Applied Physics

English

arXiv

Alexey Tonyushkin

Mara Prentiss

Crossref

Straight macroscopic magnetic guide for cold atom interferometer

We demonstrate a macroscopic magnetic guide for cold atom interferometry, where the magnetic guiding field is generated by a symmetrical array of racetrack coils of copper tape. This system represents a conceptual advance over previous guided atom interferometers based on nonsymmetrical geometries because the symmetry provides a much lower magnetic field curvature per fixed length than equivalent nonsymmetrical geometries, permitting a decrease in system length without increasing the decoherence rate associated with field curvature. We realized a magnetic guide a few cm away from each coil, where smooth translation of the guided atoms is achieved by changing the currents in second array of the multiple-conductor tape.PhysicsAccepted Manuscrip

Tonyushkin, Alexey A

Harvard University - DASH

 Straight macroscopic magnetic guide for cold atom interferometer  (Article begins on next page)The Harvard community has made this article openly available.Please share how this access benefits you. Your story matters.Citation Tonyushkin, Alexey, and Mara Prentiss. 2010. Straightmacroscopic magnetic guide for cold atom interferometer. Journalof Applied Physics 108(9): 094904.Published Version doi://10.1063/1.3506685Accessed February 19, 2015 9:23:43 AM ESTCitable Link http://nrs.harvard.edu/urn-3:HUL.InstRepos:10591650Terms of Use This article was downloaded from Harvard University's DASHrepository, and is made available under the terms and conditionsapplicable to Open Access Policy Articles, as set forth athttp://nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of-use#OAPMacroscopic magnetic guide for cold atomsAlexey Tonyushkin and Mara PrentissDepartment of Physics, Harvard University, Cambridge, Massachusetts 02138, USAWe demonstrate a macroscopic magnetic guide for cold atoms with suppressed longitudinal ﬁeld curvaturewhich is highly desired for atom interferometry. The guide is based on macroscopic copper tape coils in acopropagating currents geometry, where the atoms are located between the coils few cm away from each surface.The symmetric geometry provides a much lower magnetic ﬁeld curvature per ﬁxed length that promises longercoherence time for atom interferometers. A double-tape design of each coil allows a smooth translation ofguided atoms without addition of an external bias ﬁeld. The guide is also immune from the current and thermalnoise by virtue of the turns averaging and a large working distance, respectively. We present the experimentalresults of guide application to atom interferometry.PACS numbers: 3.75.Dg, 37.25.+kPresently, the most common type of magnetic guides forneutral atoms are micro-fabricated structures on dielectricsubstrates [1]. Due to requirements on the potential smooth-ness, such a guide needs to be fabricated with submicron pre-cision [2], which is still considered a challenge. Recently apartial solution to roughness suppression was demonstratedvia rapid current modulation [3]. Roughness can also be re-duced by addition of many wires to compensate for ﬁnitesize [4]. The other disadvantage of the micro-chip is itsclose surface operation, which causes spin ﬂips and thus atomloss [5]. The short distance imparts the requirement for a chipto be placed inside a vacuum system and often integration ofoptical elements onto the chip itself thus limiting tuning ca-pabilities of such systems [6]. One alternative to the micro-chip guide is an Ioffe-type four-rods guide frequently used asa conveyor to transport atoms along its guiding direction [7].The major problem with such guides, however, is the require-ment of a high current of a few hundred amps [8], which re-quires water cooling, and the inability to smoothly translateatoms in the transverse direction. Another alternative is theuse of an electromagnet-based guide with ferromagnetic ma-terials [9]. Because of ﬁeld enhancement in ferro-foils theoperational distance can be large; however, the hysteretic na-ture of ferromagnetic materials does not allow precise controlover the magnetic ﬁeld.The goal of this effort is to create a magnetic guide withthe reduced potential roughness and small ﬁeld curvature ca-pable of smooth translation of the atoms in the transverse di-rection. A ﬂat potential along the guiding direction is requiredto achieve a long interrogation time for atom interferometer-based sensors [10]. To date, none of the demonstrated ﬁnite-size guides for atoms provide both the ﬂat longitudinal poten-tial and high gradient in transverse direction. In this work wepresent the design of such macroscopic guiding structure thatutilizes symmetric geometry. An alternative geometry withlow intrinsic ﬁeld curvature although with low ﬁeld gradi-ent and no radial access is provided by a solenoidal geome-try [11]. Our guide is based on a parallel copropagating cur-rents in a geometry similar to microscopic two-wire guidesused as an atom conveyor [12]. In addition such a guide al-lowsprecisecancellationofmagneticﬁeldandprovidesinitialtrapping potential for a surface magneto-optical trap (MOT).It is easily fabricated and does not require water cooling togenerate large ﬁeld gradients.The guiding structure developed here is based on coils withmacroscopic copper tape (Bridgeport Magnetics) wrappedaround aluminum structures as shown in Fig. 1. Each coilFIG. 1: (Color online) Photography of macroscopic symmetric mag-netic guide structure, the inset picture shows the double tape conduc-tor.has a 150 mm  15 mm aluminum core and consists of 29turns of double conductor tape (6.35 mm wide and 0.25 mmthick) with a 0.025-mm-thick layer of Kapton insulator on oneside. The spacing between two coils h can be varied to accom-modate vacuum cell of different sizes. We enclosed each coilin glass-epoxy dielectric retainers to insure that the coils arestraight and parallel to each other. In the experiment, we placethe guiding structure outside of an ultrahigh vacuum glass cellwith square (4  4 cm) cross-section. The guide is mountedon two translational and two rotational stages that allow initialalignment of the guide axis with respect to the optical beams.In the stationary guide regime we use only one set of conduc-tors from each coil, which are connected in series. Open airprovides enough heat dissipation for the applied current of upto 50 A in the pulsed current regime. In the preliminary designwe used similar coils in a counterpropagating current geom-etry to generate a minimum of the magnetic ﬁeld above thecoils plane. That macro guide, however, suffered signiﬁcantlyfrom the potential variation associated with the respective cur-arXiv:0908.3504v2  [physics.atom-ph]  30 Oct 20092rent noise in two coils, which resulted in the short coherencetime of the atom interferometer.The magnetic ﬁeld properties of the symmetric guide areshown in Fig. 2. In our setup the quadrupole guiding potential(c)(d)-30 -20 -10 0 10 20 30Z,  mm-20-1001020X,mm -50050 X, mm-50050Y, mm-10010Z (a)xx16 G (b)Y,   mm Z, mmB, G B, GIatomsFIG. 2: (Color online) Symmetric guide conﬁguration (a), and mag-netic ﬁeld of the guide for current of 50 A: (b) contour plot of themagnetic ﬁeld vs position (x,z) (the separation between the lines is16G, the tape conductors from two coils and direction of the currentare shown for the reference); (c) magnetic ﬁeld vs x; (d) magneticﬁeld vs y, which gives the curvature of the ﬁeld near the axis.is created along the symmetry axis between coils 2 cm awayfrom the outer turn of each coil. A peak current I = 50 A inthe guiding regime provides a ﬁeld gradient of 80 G/cm anda guide depth of  2:7 mK for the jF = 1;mF =  1 >ground state of 87Rb. The magnetic ﬁeld and radial gradientof the ﬁeld close to the symmetry axis scale with coils sep-aration as / 1=h2. Microscopic separation of the coils canprovide ﬁeld gradients of the order of a few kG/cm, whichare highly desirable for ultracold atom experiments in low di-mensions [13]. Another advantage of our guide is favorableﬁeld line orientation with respect to polarization of the opti-cal molasses beams in the mirror MOT regime that results inthe increased capture volume by a factor of ﬁve compared tothe electromagnets used in counterpropagating current geom-etry. Figure 2(d) shows the ﬁeld variation near the symmetryaxis along the guiding direction, the estimated curvature of theﬁeld @2B=@y2  8:710 8 G/cm2, which is many orders ofmagnitude less than curvatures of the similar size guides in acounterpropagating current geometries.The typical experimental procedure is similar to the onedescribed for a ferro-foils guide in Ref. [14]. We ﬁrst usethe guiding coils in a low current mode (I1 = 6   8 A,rB  12   16 G/cm) to capture and cool atoms that are op-tically transported from a source chamber along z-axis intoa mirror MOT that is generated by four 45-oriented laserbeams bounced from a mirror in the inner surface of the glasschamber. Atoms collected by the mirror MOT and Doppler-cooled for 500 ms, after which the optical molasses beams aregradually turned off and the atoms are optically pumped intoa low-ﬁeld-seeking hyperﬁne magnetic sublevel, at the sametime the current in the coils is ramped up by the current regu-lated power supply to the peak value of the guide. We exper-imentally found that there is no need for the longitudinal biasﬁeld since there is always a residual offset ﬁeld present. Weestimate the transverse trapping frequency !? = 2  40 Hzwith the atoms temperature T  30K. The loading efﬁ-ciency is maximized by monitoring the absorption of a weaknear-resonant probe beam propagating along the guiding di-rection with the typical guide absorption of up to 90% thatcorresponds to the optical depth of more than 20 along theguide.The main goal of our work is to develop a guide that al-lows us to enclose a large area in the atom-interferometer-based rotational sensor by translating atoms over the largedistances [14]. In contrast to the early demonstration of thetranslated guide, our guide allows fast translation over cm dis-tances. Figure 3 shows translation of the guided atoms oversuch macroscopic distance along z-axis. Unlike micro-chips-20-1001020-20-1001020-20-1001020-20-1001020-20-1001020-10 -5 0 5 10 15 20Z,mm-20-1001020X,mm zxxztime(b) (a)I1=50A,I2=0AI1=40A,I2=10AI1=30A,I2=20AI1=20A,I2=30AI1=10A,I2=40AI1=0A,I2=50AI1 I2FIG. 3: (Color online) Guide translation: (a) simulations of the ﬁeldcontours show ﬁeld minimum translation over 7 mm for I1 = 50  0A;I2 = 0 50A (the tape conductors are shown for reference); (b)experimental data of absorption images of guided atoms across theresonant probe beam; the guided atoms are smoothly translated over 2 mm distance (from top to bottom) limited by the probe diameter.used for atom transport above the surface, symmetry insuresthat the atoms do not experience bumps in the x-axis direc-tion over the course of travel. To get the absorption images3of guided atoms we direct a weak probe beam on-resonanceto the D2 F = 1 ! F0 = 2 transition through the atomsalong the guiding direction. The image of the probe cross-section and the absorbed atoms are taken by a fast CCD dig-ital camera. Figure 3(b) shows a series of images for differ-ent values of a current in two sets of coils: I1 = 50 A andI2 = 0   10 A (from top to bottom); the respective guidetranslation is  2 mm (as limited by the cross-section of theprobe). The maximum translation is limited by the width ofthe tape conductors; in our case it can reach over 1 cm accord-ing to the current switching algorithm as shown in the simu-lation in Fig. 3(a). The other important application for such aguide would be heat-free coherent transport of guided atomstoasurfaceforatom-surfaceinteractionstudies[15]. Totrans-port atoms over several cm distance a multiple-conductor tapecan be used.To demonstrate an application of the guided atoms in themacro-guide we carried out an atom interferometer exper-iment in a time domain similar to the free-space interfer-ometer [16] and an interferometer with ferro-foils-guidedatoms [14]. The interferometer scheme and preliminary re-sults are presented in Fig. 4. The interferometer signal is de-xyPDgAI pulses 87Rb atoms at F=12 Q k ≈ (a)xyPDgAIpulses 87Rb atoms at F= F= F 12 Q k ≈ (a)xyPDgAI pulses 87Rb atoms at F=12 Q k ≈ (a)024681 01 21 405101520253002468 10 12 14-30-25-20-15-10-505     T,  m  s Data Parabola fit T, ms Phase, radAmplitude, arb. u. (b)05 0 100 150 200-25-20-15-10-5051015202530   T,  m  sφ=QT0gα(T-T0)(c)Amplitude, arb. u.Phase, radFIG. 4: (Color online) (a) Atom interferometer (AI) experimentalscheme (here, PD is a photodiode, Q is a grating vector); (b) exper-imental results with three pulse scheme in the symmetric magneticguide: contrast of the interferometer vs. interrogation time; the insetshows the phase component, which is proportional to the gravita-tional acceleration component along the guiding direction; (c) exper-imental results with four-pulse scheme; the formula gives the linearﬁt, where T0, , g are the time separation between ﬁrst and secondpulses, the guide tilt to horizon, and gravitational acceleration, re-spectively.tected by a heterodyne technique as a function of the interro-gation time T. Figure 4(b) shows a single run data of a con-trast decay (an amplitude) and a phase shift for the case whenthree off-resonant optical standing wave pulses were used asdiffraction gratings and Fig. 4(c) shows the results of a four-pulse scheme [17]. In the ﬁrst case the coherence is preservedfor about 9 ms and in the second case for more than 200 mswith the actual wave-packet separation of d = 0:5m. Asmall component of gravity along the guiding direction con-tributes to the quadratic phase shift [the inset of Fig. 4(b)] andthe linear phase for the four-pulse scheme [Fig. 4(c)]. As areference, we also present the typical results of three pulse in-terferometry experiments with a regular conﬁguration of themacroscopic electromagnet guide as shown by blue circlesin Fig. 4(b); the symmetric guide provides larger and longerdecay of the interferometer contrast. The coherence time ishighly affected by the misalignment of the optical beams withrespect to the guiding axis. The data presented here was ob-tained using only mechanical stages to align the guiding axiswith respect to the optical beams. We hope to increase the co-herence time by improving the design of the mounting struc-ture and by implementing high resolution optical beam steer-ing for the alignment.In summary, we developed a straight and smooth macro-scopic magnetic guiding structure for cold atoms that is apromising tool for atom optics and atom interferometry exper-iments. The guide can also be used for coherent transport forsurface probing and nonlinear optics experiments with coldatoms.We thank T. Kodger for technical assistance. This work wassupported by the DARPA from DOD, ONR and U.S. Depart-ment of the Army, Agreement Number W911NF-04-1-0032,and by the Draper Laboratory. Electronic address: alexey@physics.harvard.edu[1] N. H. Dekker et al., Phys. Rev. Lett. 84, 1124 (2000); W. Hanselet al., Nature (London) 413, 498 (2001); R. Folman et al., Adv.At. Mol. Opt. 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Macroscopic magnetic guide for cold atoms

Macroscopic magnetic guide for cold atoms

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