Prototyping of Atomic Conveyer for Quantum Devices Keisuke Nagato a) , Takeshi Ooi a) , Tetsuo Kishimoto c) , Hidekazu Hachisu b) , Hidetoshi Katori b, c) , and Masayuki Nakao * ,a)
a) Department of Engineering Synthesis ; b) Department of Applied Physics, Graduate School of Eng., The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan. c) Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST) *) Corresponding author (e-mail :
[email protected]) Keywords : atomic conveyer, quantum devices, laser-cooled atoms, micro-electrodes, functional requirement, fabrication process
Abstract We are researching on electric manipulation of laser-cooled atoms, aiming for quantum devices with these atoms. ‘Atomic conveyer’ with micro-electrodes, in which the ultracold atoms are conveyed, has been developed. Because the atomic conveyer requires complicated structures and functions, several microfabrication technologies must be applied. This paper reports a design of fabrication and its prototyping.
1. Introduction Since laser-cooling of atoms [1] had been invented, the possibility of quantum computing [2, 3] with ultracold atoms were proposed. Quantum computer may become a breakthrough to the limit of ‘Moore’s Law’, because N quantum-bits have 2N combinations of information and allows fast parallel calculations. As nano-micro fabrication technology has been developed, manipulation of atoms in micro-electrodes became one of the research interests. Especially the chip, on which atoms are manipulated, is called ‘atom chip’, and it is researched in the field of ultracold atomic physics. Since ions strongly couple to electromagnetic fields, manipulation of ions can be easily performed and has been thoroughly investigated to demonstrate quantum computing with ions [4, 5]. On the other hand, this strong coupling causes a problem, which the strong interactions between neighboring ions or ions and electrodes shorten ions’ coherence time. Further this problem limits the scalability of this system. Similarly, atoms with spins are easily manipulated with magnetic fields [6, 7], while it strongly couples to magnetic fluctuation due to thermal noise from current-carrying wires. To avoid these problems, manipulation of spinless neutral atoms using electric fields (Stark effect) has been proposed [8]. Stark effect is a quadratic interaction between atoms and electric field (U = −1/2 α |E|2, U : Stark potential, α : polarizability of atoms, E : Electric field). Therefore manipulating neutral atoms with electric fields become extremely insensitive against external field noise. Recently we succeeded in demonstrating electric trapping of strontium (Sr) atoms on a ‘Stark atom chip’ with micro-electrodes [9, 10]. In this paper, we describe a prototype of ‘atomic conveyer’ utilizing our recent technique on electric trapping. We determined functional requirements for electric conveying of atoms, designed the fabrication process and actually prototyped the atomic conveyer. The quantum CCD by D. Leibfried et al. [11] leads this atomic conveyer.
2. Scheme of atomic conveyer Atoms are electrically trapped by switching the voltage of the electric pairs (Figure 1(a)) in two phases (±V ↔ Ground) at a constant frequency [9, 10]. For an atomic conveyer, a structure with an array of such four-electrode assembly is considered (Figure 1(b)). The electrode spacing is 50 µm, and the arraying pitch is 25 µm. A trap can be formed by combining four set of adjacent electrodes along the array. By switching the trapping voltages in a controlled sequence along the array axis, the trapping area is transferred and as a consequence the trapped atoms are conveyed. (a) (b)
Figure 1 : (a) Atom trapping,
and
(b) Atom conveying.
3. Experimental procedure and functional requirements 3.1. Experimental procedure Here experimental procedure for demonstration of electric conveying is described. First, atoms are laser- cooled and trapped below the center (electrodes) of the chip (Laser-cooling and trapping : [12]). Figure 2 (a) shows the schematic. Six lasers (three orthogonal axes) are shone to the atoms, two of the six lasers are irradiated from 45 degrees angle, two of them are from the reflect off of the mirror surface, and two of them are irradiated inward and outward of the page. Next, counter-propagating lasers are focused through the center of the chip while a standing-wave (optical lattice) is formed. By raising the frequency of the laser from below, the optical lattice moves upwards. Due to Stark effect, atoms are confined in the optical lattice potential minimum, or the antinode of the standing-wave. By this moving lattice, atoms are transferred to the center of the electrodes (Figure 2 (b)) (Laser-transferring : [13]). Then finally the electric conveying experiment of the transferred cold atoms is carried out. This conveying experiment is carried out in an ultrahigh vacuum such as 10-10 Torr. (a)
(b)
Figure 2 : Transfer of cold atoms to the electrodes. (a) Laser-cooling and trapping of atoms. (b) Laser transferring of atoms by moving optical lattice. 3.2. Functional Requirements Functional requirements (hereinafter partly called FR) for the previous section 3.1 are listed in the following. 3.2.1. FR1 : High optical reflectivity ( φ 17 mm) Atoms are cooled by the lasers reflected from the mirror surface of the chip. Because the diameter of the laser irradiated from angle of 45 degrees is 12 mm, high optical reflectivity is required in the area of φ 17 mm. Reflectivity must be over 95 % at wavelengths of 460 nm and 689 nm. 3.2.2. FR2 : Optical transmission (φ 50 µm) To form an optical lattice for laser-transferring, two lasers are counter-propagated. The diameter of the beam waist is 32 µm and the lasers must transmit through the center (φ 50 µm) of the electrode assembly. The laser wavelength is 800 nm and the optical transmission must be over 90 %. 3.2.3. FR3 : Fabrication precision of electrodes High fabrication precision of electrodes is required for atom conveying. The basic structure is shown in Figure 1 (b), and the precision must be less than several % of the size of the electrodes, i.e. 1 µm or less. This value is calculated by numeric simulations. If this precision of electrodes is not satisfied, symmetrical electric field is not produced and trap size becomes small. 3.2.4. FR4 : Thinness of metallic coat Although no electric current is applied, the atoms near the electrodes can be heated up by thermal current of the metallic surface. This heating can be reduced by using thinner electrodes [14, 15]. Under current conditions (electrode spacing ~ 50 µm), influence of the thermal current by thin film (less than 1 µm) is negligible.
4. Design of fabrication process We here design fabrication process following the requirement in section 3. Because of FR1 (High optical reflectivity, φ 17 mm) and FR4 (Thinness of metallic coat), the chip is made from polished glass substrate (insulator substrate) on which silver sputter thin film (or vapor deposition film) is coated. Considering FR2 (High optical transmission), the center of the electrodes must be clear for laser (λ = 800 nm) so we puncture this area. For FR3 (Fabrication precision of electrodes), we use FIB (focused ion beam), because submicron-scale precision is required although the fabrication volume is large. First, we produce a three-layer structure using optical contact and polishing. Each layer has a thickness of 50 µm and the middle layer has a φ 1 mm hole punctured at the center (Figure 3 (a)). Second, slits whose width is 50 µm and length is 500 µm are fabricated at the center of both the top and bottom layers (while middle layer has a hole at the center) by FIB (Figure 3 (b)). Third, silver thin films are sputtered on the top and the bottom surfaces of the three-layer structure and the inner faces of the slits by four steps (Figure 3 (c)). Fourth, by FIB etching at angles, 3D electrode structures are fabricated (Figure 3 (d)). (a)
(b)
(c)
(d)
Figure 3 (a) ~ (d): Fabrication process of atomic conveyer. (a) Produce a three-layer structure. (b) Fabricate slits by FIB etching. (c) Sputter thin film by four steps. (d) Fabricate electrodes by FIB etching.
5. Fabrication of atomic conveyer 5.1. Fabrication of electrodes Fabricated electrode array is shown in Figure 4. The image is a prototype of a single layer. The thicknesses of silver thin films are 250 nm on upside and backside surfaces, 100 nm on inner faces. Sixteen rows are arrayed. The pitch of electrodes is 25 µm and the width of etched isolating line is 5 µm. The fabrication precision is in 1 µm. The glass is quartz, and the three layer glass contact and polishing is carried out by Japan Cell Co., Ltd. On the actual electrodes, MgF2 thin film are coated about 100 nm by vapor deposition after fabrication of electrodes and laser ablation (described later) to protect from electric discharging or oxidization. The MgF2 thin film is thin enough to avoid distortion of the electric fields for atom conveying.
Figure 4 : Prototyped electrode array. (SIM image, SIM: Scanning Ion Microscopy)
5.2. Fabrication of wiring Sixty-six electrodes on the chip are wired (each thirty-two wiring from front and backside of the three-layer structure and two wiring to ground the mirror surfaces). To wire out from the outer rim of the φ 25 mm chip surface, the pad pitch becomes 1 mm at the rim. Any strong forces or thermal actions must not be applied, since the 150 µm thick electrode chip is fragile. Therefore we come up with a novel method of wiring. Configuration of experimental setup is shown in Figure 5. An AR (Anti-Reflection) -coated glass substrate and the electrode chip are mounted on a vacuum chamber port. Wire-contacts made from a stainless steel sheet are sandwiched in and fixed at the outer side of the chip by two contact-mounts (glass rings). Thin wires are connected to the ends of the wire-contacts and lead to the outside of the chamber. These radial sixty-six wire-contacts are fabricated from an etched pattern whose outer ends are first joined and inner ends are bent to touch the electrode chip (see Figure 5). After sandwiched by two glass rings, the outer ends are cut and all wire-contacts are isolated. Figure 6 shows the fabricated wiring. Because of the slow fabrication speed of FIB, femtosecond laser is used to draw the insulating pattern of the wiring between the padding and the electrode structures fabricated by FIB (area of about 1 mm). ‘Torr Seal’ (epoxy adhesive by VARIAN) is used for bonding assembly, while it has little out-gassing. Further, holes are also patterned at the ends of the contacts for easier wiring operations.
Figure 5 : Experimental setup.
Figure 6 : Fabricated wiring of the chip.
6. Conclusion We designed fabrication process satisfying functional requirements for electric conveying, and actually fabricated the atomic conveyer.
Acknowledgements We would like to thank J. Fujiki for his help in experiments and simulations, and also, T. Hamaguchi and K. Tsuchiya for their advices. References [1] W. D. Philips, “Laser cooling and trapping of neutral atoms”, Rev. Mod. Phys. 70, 721-741 (1998). [2] D. Leibfried et al., “Quantum dynamics of single trapped ions”, Rev. Mod. Phys. 75, 281-324 (2003). [3] J. I. Cirac and P. Zoller, “Quantum computations with cold ions”, Phys. Rev. Lett. 74, 4091-4094 (1995). [4] D. Kielpinski et al., “Architecture for a large scale ion-trap quantum computer”, Nature 417, 709-711 (2002). [5] D. Leibfried et al., “Experimental demonstration of a robust, high-fidelity geometric two ion-qubit phase gate”, Nature 422, 412-415 (2003). [6] J. Reichel et al., “Applications of integrated magnetic microtraps”, Appl. Phys. 72, 81-89 (2001). [7] M. Bartenstein et al., “Atoms and wires: Toward atom chip”, IEEE J. Quantum Electron 36, 1364-1377 (2000). [8] H. Katori and T. Akatsuka, “Electric manipulation of spinless neutral atoms on a surface”, Jpn. J. Appl. Phys. 43, 358-360 (2001). [9] H. Katori et al., “Engineering Stark potential for precision measurements: Optical lattice clock and electrodynamic surface trap” Atomic Physics 19 (Proceedings of the 19th International Conference on Atomic Physics, Rio de Janeiro, Brazil, 2004) pp.112-122 (2005). [10] K. Nagato et al., “Fabrication of atom trap chip for quantum information processing”, Proceedings of the 19th American Society for Precision Engineering Annual Meeting (Orlando, Florida, 2004) pp.121-124 (2004). [11] F. Schmidt-Kaler et al., “Realization of the Cirac-Zoller controlled-NOT quantum gate”, Nature 422, 408-411 (2003). [12] H. J. Metcalf and P. Straten, “Laser cooling and trapping”, Springer Verlag (1999). [13] M. S. Kuhr et al., “Deterministic delivery of a single atom”, Science 293, 278-280 (2001). [14] Y. Lin et al., “Impact of the Casimir-Polder potential and Johnson noise on Bose-Einstein Condensate stability near surfaces”, Phys. Rev. Lett. 92, 050404-1 – 050404-4 (2004). [15] D. M. Harber et al., “Thermally induced losses in ultra-cold atoms magnetically trapped near room-temperature surfaces”, J. Low Temp. Phys. 133, 229-239 (2003).
