Bose-Einstein condensation on a permanent-magnet atom chip

Abstract

We have produced a Bose-Einstein condensate on a permanent-magnet atom chip based on periodically magnetized videotape. We observe the expansion and dynamics of the condensate in one of the microscopic waveguides close to the surface. The lifetime for atoms to remain trapped near this dielectric material is significantly longer than above a metal surface of the same thickness. These results illustrate the suitability of microscopic permanent-magnet structures for quantum-coherent preparation and manipulation of cold atoms.PACS numbers: 03.75. Kk, 34.50.Dy, 39.25.+K, 03.75.Be Atom chips are making rapid progress in the quantumcoherent manipulation of microscopic cold atom clouds [1,2,3] with a view to fundamental studies of quantum gases [4], interferometry [5], and quantum information processing [6]. Small-scale magnetic field patterns for atom chips can be made using either microfabricated current-carrying wires or microscopic structures of permanent magnetization. This second idea is attractive because there is no power dissipation in a permanent magnet and because very tight atom traps with oscillation frequencies of ∼ 1 MHz (10 nm ground state size) are possible [7]. Previous studies have used audiotape [8], floppy disks [9, 10], videotape [11], magnetic and magneto-optical films [7,12,13], and hard disks [14]. Until recently, permanently magnetized films were used only for mm-or cm-scale manipulation of atom clouds by reflection, but now cold atom clouds have been loaded into the microtraps [12,15]. For many applications of these magnetic microtraps, the next significant step is to prepare a Bose-Einstein condensate (BEC) on the chip as a source of coherent matter waves for interferometry or as a low-entropy reservoir for quantum information processing.In this paper we describe the production of a BEC on a permanent-magnet atom chip made from videotape, which forms an array of waveguides. We have observed the propagation of the BEC along one of these guides. With the ends of the guide closed, we study center-ofmass and length oscillations of the trapped gas, demonstrating that the videotape chip is a practical way to manipulate cold atoms. We also show that the spin relaxation time of the trapped atoms is significantly longer above the dielectric surface of the videotape than above a metal surface of the same thickness. soidal pattern of magnetization written along the length of the videotape (x direction) with the form M 0 cos(kx)x, as illustrated in Fig. 1. This produces a field (B x , B y ) = B sur e −ky (− cos(kx), sin(kx)),where B sur is the field strength at the surface (y = 0). The waveguides appear, as shown at the top of Fig. 1, when a bias field B bias is added in the x-y plane. Near the center of each guide, the magnetic field has a quadrupole structure with a gradient of field strength given by B ′ = kB bias . An axial bias B z prevents the total field from going to zero at the center. For small-amplitude transverse oscillations this makes a harmonic trap with frequencywhere µ B g F m F is t...

Evaporative cooling of cold atoms at surfaces

“…Since the advent of microchip traps for cold atoms [1][2][3][4][5][6][7][8][9][10][11][12], interest in developing quantum hybrid systems, which exploit the long coherence times of Bose-Einstein condensates with the flexibility of modern micro-and nanoelectronics, continues to grow. There is potential to use such systems as quantum memory devices [13][14][15], precision measurement devices [16][17][18][19] and even rewritable electronic systems [20].…”

Effect of magnetization inhomogeneity on magnetic microtraps for atoms

“…Since the realization of Bose-Einstein condensates (BECs) on atom chips [2,3], pioneering experiments have studied single-mode propagation along waveguides [4], transport and adiabatic splitting of a BEC [5] and recently on-chip atom interferometry [6,7]. Permanent magnets are particularly attractive for atom chips as they can provide complex magnetic potentials [8] while suppressing current noise that causes heating and limits the lifetime of trapped atoms near a surface [9]. To date, permanent magnet atom chips have been developed with a view to study one-dimensional quantum gases [10,11,12], decoherence of BEC near surfaces [9,13], hybrid magnetic and optical trapping configurations [14], and self biased fully permanent magnetic potentials [15].…”

“…Permanent magnets are particularly attractive for atom chips as they can provide complex magnetic potentials [8] while suppressing current noise that causes heating and limits the lifetime of trapped atoms near a surface [9]. To date, permanent magnet atom chips have been developed with a view to study one-dimensional quantum gases [10,11,12], decoherence of BEC near surfaces [9,13], hybrid magnetic and optical trapping configurations [14], and self biased fully permanent magnetic potentials [15]. It has been found, however, that in addition to current noise, atom chips have other limitations, as undesired spatial magnetic field variations associated with the current-carrying wires or magnetic materials act to fragment the trapped atoms.…”

Stability of a trapped-atom clock on a chip

“…Nevertheless, it needs to be investigated, whether better detection and/or imaging at long time-of-flight, may reduce this error. The magnetic field stability may be improved by refined power supplies, the use of multi-wire traps [75], microwave dressing [57] or ultimately the use of atom chips with permanent magnetic material [76][77][78]. If the magnetic field fluctuations can be reduced, the temperature fluctuations may also reduce.…”