A review of silicon microfabricated ion traps for quantum information processing
© Cho et al.; licensee Springer. 2015
Received: 12 December 2014
Accepted: 13 January 2015
Published: 23 April 2015
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.
KeywordsMicroelectromechanial System (MEMS) Microfabrication Ion traps Quantum information processing (QIP) Quantum computing
Quantum information processing (QIP) is a novel information processing method based on quantum mechanics [1-3], and uses two quantum states in a quantum system as a basic unit of information, instead of two voltage levels in conventional information processing based on electronics. This basic unit is called “qubit”, an abbreviation for quantum bit. The information stored in a single qubit exists in a superposition of two quantum states which indicates an arbitrary linear combination of two orthonormal basis. Since a single qubit can occupy either of two states simultaneously, N qubits can represent 2N states of information. Moreover, using a quantum teleportation process , two qubits can provide the same measurement results, regardless of the distance between the qubits. Based on these phenomena in the quantum regime, QIP is expected to achieve noticeable increases in the speed in information processing problems. Therefore, many QIP applications such as quantum communication [5-7], quantum computer [8-12], and quantum simulator [13-15] have been proposed and are being actively researched.
For the physical implementation of the qubit, a quantum system which is sufficiently isolated from their surroundings and can be individually manipulated is required. Individual manipulation means qubits are initializable, controllable and measureable. A single atomic ion confined by a physical platform which is called “ion trap” satisfies the requirements [16-19]. Thus the ion trap has become one of the leading technologies among the various qubit platforms including superconducting circuit [20-22], optical lattice [23,24], nuclear magnetic resonance (NMR) [25,26], and quantum dot [27,28]. The ion trap was initially developed by Wolfgang Paul and Hans Georg Dehmelt who are the co-winners of the Nobel Prize in Physics in 1989. Since Cirac and Zoller have proposed using trapped ions as a physical implementation of qubit , the feasibility of ion qubits has been verified through many experiments [19,29,30]. Recently, in 2012, Serge Haroche and David Wineland received the Nobel Prize in Physics owing to the measurement and manipulation of individual quantum systems, using cavity quantum electrodynamics (QED) and 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 conventional precision machining method and careful manual assembling, with the advances in MEMS, recent ion traps are based on silicon microfabrication technologies. The basic principles of ion traps are presented in Types of ion trap section. Then, Development history of Paul trap section discusses a history of the Paul trap, which is the type of ion traps mainly covered in this paper. In MEMS-based microfabrication section, two MEMS microfabrication methods for ion traps are explained. Finally, the current issues and the future development directions of microfabricated ion traps are presented in Future directions section.
Types of ion trap
An ion trap is a device which can trap charged particles in space by using electric or electromagnetic fields. Trapping a charged particle with static potential alone is impossible because the static potential (φ) obeys one of Maxwell’s equations ∇2 φ = 0 . Wolfgang Paul used an oscillating electric field together with the static electric field , and Hans Georg Dehmelt added a magnetic field to the static electric field to trap a positive ion . The ion traps built by Paul and Dehmelt are called “Paul trap” and “Penning trap”a  respectively. In this paper, we cover only the Paul trap, because the Paul trap is currently widely used for QIP applications.
Development history of Paul 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  and mass spectroscopy [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.
Cirac and Zoller  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 machining and assembling techniques. Each research group has a different electrode structure. Some typical electrode structures include rods [45,47], blades [48,49] and sheets . Figure 2(b) shows a blade type linear trap of the Innsbruck group . Many ion trap research groups are still using a variation of these 4-rod linear traps. In general, when compared to the surface traps (explained in the following sub-section) the 4-rod linear traps have a higher trap depth, which in turn provide a longer ion life time and more stable trapping of ions. However, the linear traps do not offer the design freedom of the surface traps, and currently more research efforts are being expended to the surface traps.
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 operations are held, a memory region that stores ions conserving qubit states, and a region for loading ions.
In addition to the Si-based surface traps mentioned in the above, surface traps with a single metal layer on a non-conductive substrate, fabricated by patterning Au electrodes on quartz or sapphire substrates [54,66-69] have been reported. A surface trap has also been fabricated on printed circuit boards [70-72].
Although many results have been reported on trapping ions with MEMS-fabricated traps, the process details to fabricate the trap chips are very scarce in the literature. Fabricating ion traps requires thick dielectric films to withstand several hundred volts of RF voltages. However, the dielectric layer should be as invisible as possible as seen from the RF null point where ions are trapped, since dielectric charging phenomena can alter the null position and can induce the micromotion of trapped ions. In this section, we introduce two fabrication methods developed by us.
Junction ion trap
As discussed in Development history of Paul trap, the number of ion qubits trapped in an ion trap array inevitably must increase in order to adapt more complex quantum algorithms . For trapping and manipulating large numbers of ions, a multi-zone ion trap composed by a number of ion trap arrays is proposed. In this multi-zone ion trap, the trapping zones are connected by “X” or “Y” junctions, and the information stored in ions can be transferred from one zone to another through the junctions. For shuttling the ions in an axial direction, the location of DC null point is moved along the axial direction by applying time-dependent potentials to the outer DC control electrodes. Ion transports via junctions however require not only applying DC control voltages, but more complex techniques, because pseudopotential barriers created by RF voltages exist near the center of the junctions. Therefore, the geometries near the junctions should be optimized by an iterative algorithm to minimize the magnitude of the pseudopotential barriers.
3-D ion trap fabricated by microfabrication technology
This paper reviewed the operation principles and the development history of ion traps. Ion trap has a huge potential to be used in quantum information processing and computing. By applying MEMS-based microfabrication methods as well as conventional machining techniques, various ion traps for QIP experiments have been built and demonstrated. This paper also showed two variations of MEMS fabrication method for surface ion traps. It is expected that the ion trap technology can contribute to realize novel quantum information processing methods with exponential speed-up that we have never experienced so far. It is also expected and anticipated that MEMS fabrication technologies will be crucially instrumental in realizing complex yet inexpensive ion traps for quantum information processing and computing.
aPenning trap: The Penning Trap was named after F. M. Penning by Hans Georg Dehmelt because Dehmelt stated getting the inspiration of the trap from the vacuum gauge built by F. M. Penning .
Aluminum gallium arsenide
National Institute of Standards and Technology
Buffered hydrogen fluoride
Nuclear magnetic resonance
Buffered oxide etching
National Physical Laboratory
Charge coupled device
Plasma enhanced chemical vapor deposition
Chemical mechanical polishing
Deep reactive ion etching
Electron multiplying charge coupled device
Quantum information processing
Scanning electron micrograph
Low pressure chemical vapor deposition
Sandia National Laboratory
Through silicon via
Taehyun Kim was supported by ICT R&D program of MSIP/IITP. [10043464, Development of quantum repeater technology for the application to communication systems].
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