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The Future of Quantum Information Processing

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The Future of Quantum Information Processing SPECIALSECTION INTRODUCTION The Future of Quantum Information Processing IN A WORLD OVERWHELMED BY INCREASING AMOUNTS OF DATA, FINDING NEW WAYS to store and process information has become a necessity. Conventional silicon- based electronics has experienced rapid and steady growth, thanks to the progres- sive miniaturization of its basic component, the transistor, but that trend cannot continue indefi nitely. Quantum In conventional devices, information is stored and manipulated in binary form: The elementary components of these devices—the so-called bits—have Information two states, each of which encodes the binary 0 or 1. To move beyond the binary system, one can exploit the laws of quantum mechanics. A quantum-mechanical Processing object with two energy levels at its disposal can occupy either of those two levels, but also an arbitrary combination (“superposition”) of the two, much like an CONTENTS electron in a two-slit experiment can go through both slits at once. This results in infi nitely many quantum states that a single quantum bit, or “qubit,” can take; together with another strange property of quantum mechanics—entanglement— Reviews it allows for a much more powerful information platform than is possible with 1164 Scaling the Ion Trap Quantum conventional components. Processor Quantum information processing (QIP) uses qubits as its basic information C. Monroe and J. Kim units. QIP has many facets, from quantum simulation, to cryptography, to quantum 1169 Superconducting Circuits for computation, which is expected to solve problems more complex than those within Quantum Information: An Outlook the capabilities of conventional computers. To be useful for QIP, a qubit needs to be M. H. Devoret and R. J. Schoelkopf both isolated from its environment and tightly controllable, which places stringent 1174 Quantum Spintronics: Engineering requirements on its physical realization. But this is only the fi rst step; to build a and Manipulating Atom-Like Spins quantum computer, for example, we must also have a scalable architecture and in Semiconductors error correction that can be performed in parallel with computation; in addition, D. D. Awschalom et al. efficient quantum algorithms must exist for solving the 1179 Topological Quantum problem at hand—a considerable theoretical challenge. Computation—From Basic Online A number of qubit types have been proposed and exper- Concepts to First Experiments sciencemag.org imentally realized that satisfy at least some of these crite- A. Stern and N. H. Lindner Podcast interview ria, and tremendous progress has been made over the past with author decade in improving the fi gures of merit, such as the coher- See also Science Podcast D. D. Awschalom (http://scim.ag/pod_ ence time. In this special section, four Reviews look into 6124a). the future of QIP in some of its most promising physical realizations. On p. 1164, Monroe and Kim discuss the chal- lenges of scaling trapped ion architectures to hundreds and thousands of qubits and beyond. Devoret and Schoelkopf (p. 1169) speculate on the future of super- conducting circuits, whereas Awschalom et al. (p. 1174) focus on the many prom- ising qubit fl avors based on spins in semiconductors. Finally, Stern and Lindner (p. 1179) lay out the prospects for quantum computation using the entirely differ- ent approach of topologically protected states. The future of QIP appears bright in spite of the many remaining challenges. As a bonus, overcoming these challenges will probably also advance basic research. – JELENA STAJIC PHOTO: CURT SUPLEE AND EMILY EDWARDS, JOINT QUANTUM INSTITUTE AND UNIVERSITY OF MARYLAND; CHIP FABRICATION: SANDIA NATIONAL LABORATORIES WITH SUPPORT FROM THE INTELLIGENCE ADVANCED RESEARCH PROJECTS ACTIVITY FROM THE INTELLIGENCE ADVANCED WITH SUPPORT LABORATORIES SANDIA NATIONAL CHIP FABRICATION: JOINT QUANTUM INSTITUTE AND UNIVERSITY OF MARYLAND; EDWARDS, SUPLEE AND EMILY PHOTO: CURT www.sciencemag.org SCIENCE VOL 339 8 MARCH 2013 1163 Published by AAAS Quantum Information Processing magnetic traps, suspending atoms in free space REVIEW so that they do not uncontrollably interact with background atoms, molecules, or surfaces. There are several compelling proposals for quantum Scaling the Ion Trap computer architectures based on trapped neutral atoms and optical lattices, although the weak Quantum Processor interaction between neutral atoms leads to diffi- culties in controlling their entanglement, and re- C. Monroe1* and J. Kim2,3 search in this area is still exploratory (3). Here, we focus on the trapping of electrically charged atoms, Trapped atomic ions are standards for quantum information processing, serving as quantum memories, or ions, for which high-fidelity quantum opera- hosts of quantum gates in quantum computers and simulators, and nodes of quantum communication tions and measurements are now commonplace. networks. Quantum bits based on trapped ions enjoy a rare combination of attributes: They have exquisite Thetypicaliontrapgeometryforquantum coherence properties, they can be prepared and measured with nearly 100% efficiency, and they are information purposes is the linear radio frequency readily entangled with each other through the Coulomb interaction or remote photonic interconnects. (rf ) Paul trap, in which nearby electrodes hold The outstanding challenge is the scaling of trapped ions to hundreds or thousands of qubits and static and dynamic electrical potentials that lead beyond, at which scale quantum processors can outperform their classical counterparts in certain to an effective harmonic confinement of the ions, applications. We review the latest progress and prospects in that effort, with the promise of advanced like a bowl full of mutually repelling marbles (2). architectures and new technologies, such as microfabricated ion traps and integrated photonics. When ions are laser-cooled to very low temper- atures in such a trap, the ions form a linear crystal uantum physics can be distilled to two that the encoded information of 2400 ∼ 10120 val- of qubits, with the Coulomb repulsion balanc- disjointed and counterintuitive rules. First, ues is more than the number of fundamental par- ing the external confinement force (Fig. 1A). Qan isolated quantum system is represented ticles in the universe; such a computation could Ions are typically loaded into traps by creating a by a “wave function,” or a mathematical never be performed without the parallel process- neutral atomic flux of the desired particle and entity that evolves according to a wave equation ing enabled by quantum mechanics. In a sense, ionizing them once in the trapping volume. Ions on March 8, 2013 and is shaped with external controls. Second, when entanglement between qubits acts as an invisible can remain confined for months, with lifetimes a quantum system interacts with a measurement wiring that can potentially be exploited to solve often limited by the level of vacuum. Elastic col- apparatus or its surrounding environment, the certain problems that are intractable otherwise (1). lisions with residual background gas occur roughly wave function probabilistically and irreversibly The requirements for large-scale quantum com- once per hour per ion at typical ultrahigh-vacuum “collapses” into a particular state. The incompati- puter hardware are daunting, given the exponen- pressures (∼10−11 torr) and do not necessarily eject bility of these two quantum rules is seen most tial sensitivity of such large superpositions to errors the ion, although inelastic collisions can change clearly in a quantum superposition state, in which, and leaks to the environment. However, there exist the species of the trapped ion. Cryogenic chambers for instance, an isolated particle’swavefunction error-correction codes that allow arbitrarily com- can virtually eliminate these collision events by is delocalized between two or more positions. The plex quantum superposition states to be generated further reducing the background pressure. www.sciencemag.org second rule ensures that such states are never di- and stabilized (1), giving us hope that useful fault- Appropriate atomic ion species should have rectly seen in the macroscopic world. However, tolerant quantum computers will eventually be a strong closed optical transition that allows for when a system is left isolated without interacting realized despite the steep technical requirements laser-cooling of the motion, qubit state initializa- with its environment, the (microscopic) superpo- far beyond current experimental capability. tion, and efficient qubit readout. This rules out sition persists and can be exploited to store mas- In the search for quantum information pro- almost anything other than simple atomic ions sive amounts of information in parallel. cessing hardware, one needs qubits that are ex- with a lone outer electron, such as the alkaline- A quantum information processor encodes tremely well isolated from the environment yet earths (Be+,Mg+,Ca+,Sr+,andBa+) and partic- Downloaded from information in an array of quantum bits or qubits, can be precisely controlled with external fields ular transition metals (Zn+,Hg+,Cd+,andYb+). which can hold superpositions of classical bit to affect interferences through the operations of Qubits are represented by two stable electronic values 0 and 1. When N qubits are prepared in quantum logic gates. Moreover, we must ulti- levels within each ion, sometimes characterized their most general state, we have a quantum super- mately couple the qubits to the outside
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