Cho et al. Micro and Nano Systems Letters (2015) 3:2 DOI 10.1186/s40486-015-0013-3 REVIEW Open Access A review of silicon microfabricated ion traps for quantum information processing Dong-Il “Dan” Cho1*, Seokjun Hong1, Minjae Lee1 and Taehyun Kim2 Abstract Quantum information processing (QIP) has become a hot research topic as evidenced by S. Haroche and D. J. Wineland receiving the Nobel Prize in Physics in 2012. Various MEMS-based microfabrication methods will be a key enabling technology in implementing novel and scalable ion traps for QIP. This paper provides a brief introduction of ion trap devices, and reviews ion traps made using conventional precision machining as well as MEMS-based microfabrication. Then, microfabrication methods for ion traps are explained in detail. Finally, current research issues in microfabricated ion traps are presented. The QIP renders significant new challenges for MEMS, as various QIP technologies are being developed for secure encrypted communication and complex computing applications. Keywords: Microelectromechanial System (MEMS); Microfabrication; Ion traps; Quantum information processing (QIP); Quantum computing Introduction For the physical implementation of the qubit, a quantum Quantum information processing (QIP) is a novel infor- system which is sufficiently isolated from their surround- mation processing method based on quantum mechanics ings and can be individually manipulated is required. [1-3], and uses two quantum states in a quantum system Individual manipulation means qubits are initializable, as a basic unit of information, instead of two voltage controllable and measureable. A single atomic ion levels in conventional information processing based on confined by a physical platform which is called “ion electronics. This basic unit is called “qubit”, an abbrevi- trap” satisfies the requirements [16-19]. Thus the ion ation for quantum bit. The information stored in a single trap has become one of the leading technologies among qubit exists in a superposition of two quantum states the various qubit platforms including superconducting which indicates an arbitrary linear combination of two circuit [20-22], optical lattice [23,24], nuclear magnetic orthonormal basis. Since a single qubit can occupy ei- resonance (NMR) [25,26], and quantum dot [27,28]. ther of two states simultaneously, N qubits can represent The ion trap was initially developed by Wolfgang Paul 2N states of information. Moreover, using a quantum and Hans Georg Dehmelt who are the co-winners of teleportation process [4], two qubits can provide the the Nobel Prize in Physics in 1989. Since Cirac and same measurement results, regardless of the distance Zoller have proposed using trapped ions as a physical between the qubits. Based on these phenomena in the implementation of qubit [16], the feasibility of ion quantum regime, QIP is expected to achieve noticeable qubits has been verified through many experiments increases in the speed in information processing problems. [19,29,30]. Recently, in 2012, Serge Haroche and David Therefore, many QIP applications such as quantum com- Wineland received the Nobel Prize in Physics owing to the munication [5-7], quantum computer [8-12], and quantum measurement and manipulation of individual quantum simulator [13-15] have been proposed and are being systems, using cavity quantum electrodynamics (QED) and actively researched. ion traps, respectively. There has been several review articles on the subject of quantum information processing [18,31-34]. Although the earlier Paul traps were constructed by * Correspondence: [email protected] 1ISRC/ASRI, Department of Electrical and Computer Engineering, Seoul conventional precision machining method and careful National University, Seoul 151-744, Republic of Korea manual assembling, with the advances in MEMS, recent Full list of author information is available at the end of the article © 2015 Cho et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. Cho et al. Micro and Nano Systems Letters (2015) 3:2 Page 2 of 12 ion traps are based on silicon microfabrication technologies. structure and electric fields in the radial plane of a linear The basic principles of ion traps are presented in Types of trap. The hyperbolic electrode of the ring trap is replaced ion trap section. Then, Development history of Paul trap by four rods. Sectors of the circular rods form a hyperbolic section discusses a history of the Paul trap, which is the type shape, and an RF voltage is applied to two opposite rods of ion traps mainly covered in this paper. In MEMS-based with the other rods grounded. The pseudopotential gener- microfabrication section, two MEMS microfabrication ated by the RF voltage confine the ions in the radial direc- methods for ion traps are explained. Finally, the current tion, and the radial position of the ions is at the center of issues and the future development directions of microfabri- the four rods. Because the RF null is expanded along the cated ion traps are presented in Future directions section. axial direction of the rods, the pseudopotential from the RF voltage cannot confine ions in the axial direction. To Types of ion trap An ion trap is a device which can trap charged particles in space by using electric or electromagnetic fields. (a) ground Trapping a charged particle with static potential alone is impossible because the static potential (φ) obeys one of Maxwell’s equations ∇2φ = 0 [35]. Wolfgang Paul used an oscillating electric field together with the static electric field [36], and Hans Georg Dehmelt added a magnetic field to the static electric field to trap a positive ion [37]. The ion traps built by Paul and Dehmelt are ion called “Paul trap” and “Penning trap”a [38] respectively. RF RF In this paper, we cover only the Paul trap, because the Paul trap is currently widely used for QIP applications. Figure 1(a) shows the structural schematic and trapping principles of the Paul trap, which is composed of a ring- shaped hyperbolic electrode and two endcap electrodes located at the top and bottom of the ring electrode. This type of ion trap is called the “ring trap” compared with the ground next generation traps which are called the “linear trap” and the “surface trap”. The details of the linear and surface (b) traps are discussed in Development history of Paul trap. Figure 1(b) shows the ring trap made by Wolfgang Paul [39]. In the ring trap, a charged particle is confined in both radial and axial directions by the RF voltage applied to the hyperbolic electrode, and the endcap electrodes function as an RF ground [40]. Typically, the magnitude of the RF voltage is up to hundreds of volts and the frequency of the RF voltage is from tens to hundreds of MHz. Development history of Paul trap Ring trap In the early stages of ion trap researches, the ring type Paul trap was used for experiments concerned in fundamental physics such as frequency standards [41] and mass spectros- copy [42,43]. Ring traps can be easily constructed because of its simple structure, but has a drawback in trapping large numbers of ions because a potential minimum exists at a specific point and difficult to be expanded to a 3-D space. Linear trap Figure 1 Schematic and picture of “ring trap”. (a) Schematic of a To store more ions, a new ion trap called the “linear ring trap. The red and blue regions indicate RF and DC electrodes trap” was built by Prestage et al. [44]. The first linear respectively. The curved arrows denote the direction of electric field trap held approximately 20 times the number of ions as when RF voltage is positive. (b) A ring trap made by Wolfgang Paul [39]. Ion is trapped inside the ring structure. that of a ring trap. Figure 2(a) shows the electrode Cho et al. Micro and Nano Systems Letters (2015) 3:2 Page 3 of 12 a higher trap depth, which in turn provide a longer ion (a) life time and more stable trapping of ions. However, the linear traps do not offer the design freedom of the sur- DC RF face traps, and currently more research efforts are being expended to the surface traps. Surface trap ion To implement more complicated quantum operations, more ions that can be manipulated in a common motional mode (which refers to the collective oscillation of the whole ion string) should be trapped. Therefore the idea of integrating multiple ion trap arrays in a single ion trap chip was proposed [12,51]. The ion trap chip integrated with multiple ion trap arrays is divided into different regions, as an operation region in which the quantum RF DC (b) Figure 2 Schematic and picture of “linear trap”. (a) Schematic of a 4-rod linear trap. The red and blue circles indicate RF and DC electrodes respectively. The curved arrows denote the direction of electric field when RF voltage is positive. (b) A blade type linear trap of Innsbruck group [48]. (© R. Blatt, University of Innsbruck). confine trapped ions in the axial direction, two endcap electrodes are located at both ends of rod electrodes, and DC voltages are applied to the endcap electrodes to confine the ions axially [45,46]. Cirac and Zoller [16] proposed using trapped ions as a physical implementation of quantum computation. Since then, many research groups have been using linear traps in their QIP experiments. Most of the groups have developed their own linear traps using precision machin- ing and assembling techniques. Each research group has a different electrode structure. Some typical electrode structures include rods [45,47], blades [48,49] and sheets [50].