The 2010 Benjamin Franklin Medal in Physics presented to J. Ignacio Cirac, David J. Wineland and Peter Zoller

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Abstract

A quantum computer is a device that makes explicit use of quantum mechanical phenomena, such as superposition and entanglement, to perform operations on data. General ideas of quantum computing were discussed in information theory since the 1970s and a possibility of constructing quantum computers from atoms or photons was suggested by Richard Feynman in 1981. A quantum computer would allow one to solve problems that a typical traditional computer cannot. This is because quantum effects make it possible to simultaneously process huge amounts of data, as opposed to processing them sequentially by a classical computer. In 1994, Ignacio Cirac and Peter Zoller proposed an experiment to construct a quantum computing device using cold trapped ions. This concrete description of a realistic quantum logic gate, the fundamental device in the computer that performs basis logic functions, allowed David Wineland and coworkers to build such a gate within a year. It was the first realization of quantum computer logic at the level of individual atoms. This work has demonstrated that quantum computing—with its potential of vast increases in computing power—may be possible.

Introduction

A quantum computer is a machine which differs very essentially from a classical one. Classical computers operate on bits: each bit is a device that can be in one of two possible states: 0 or 1. Quantum computers operate on qubits which in contrast to classical bits can be simultaneously in both states. This is possible due to the laws of quantum mechanics which allow to construct wave functions describing states of quantum systems as arbitrary linear combinations of other wave functions. An example of a qubit can be an atom with spin 1/2: such spin can be up or down corresponding to a classical bit, but also in states being mixtures of the two. A quantum computer is a device using sequences of qubits (registers). A register with n qubits: 10001010… can exist in 2n different “classical” arrangements, but also simultaneously in all of them. A classical computer can operate on only one arrangement at a time. A quantum computer can operate on all arrangements at the same time. For large n, this means that much more data can be processed in one step. However, there is a problem with getting out an answer from a processed state since a quantum measurement collapses the wave function to a particular arrangement. Quantum algorithms show that by applying proper types of measurements useful answers can be extracted.

The concepts of quantum information science including quantum computation and quantum cryptography appeared in the 1970s. The initial formal developments were in information theory, such as the 1976 paper by Roman Ingarden [1]. In 1981, Richard Feynman [2] gave a talk entitled “Simulating Physics with Computers” where he argued that quantum systems can be most effectively simulated on quantum computers. Although Feynman's simulator is a different kind of device than a general purpose quantum computer, this talk suggested that a computer based on quantum logic might be more powerful than one based on classical logic. These ideas were expanded on in 1985 by David Deutsch [3], who described the first universal quantum computer and showed that certain computations could be performed more efficiently using such systems. In 1993, Seth Lloyd [4] considered some possible practical implementations of quantum logic. A significant breakthrough was due to Peter Shor [5] who showed in 1994 that a quantum computer could factor large numbers much more efficiently than any classical computer. Since factorization is the most common method used in cryptography, this work generated a lot of interest and changed the status of the field from a curiosity at the best to a critically important research priority.

However, even as late as in 1994, few people believed that a quantum computing device, or even the elementary quantum logic gates needed for a quantum computer, could ever be built. This situation has changed due to the work of Ignacio Cirac and Peter Zoller [6]. Stimulated by Artur Ekert's talk at the International Conference for Atomic Physics in 1994, Cirac and Zoller made a critical link between the formal quantum-logic developments and the cold ion experiments of David Wineland. They realized that by manipulating ions stored in the linear traps with laser pulses one can perform the elementary operations needed to construct a quantum computer. The Cirac and Zoller analysis suggested that an experimental system very similar to one already existing in David Wineland's group could be utilized for this purpose. This group had already worked on quantum entanglement of ions to create exotic states relevant for improvement of atomic clocks. Indeed, nearly immediately Wineland's group demonstrated [7] the first quantum-logic gate. This truly pioneering experiment not only realized the first quantum information processing, it was also a paradigm-changing step for the whole of physics, as it brought our ability to control quantum phenomena to an entirely new level.

The work of Cirac, Zoller, and Wineland in the area of quantum computing did not end with these landmark achievements. These three researchers have remained leaders in the field of quantum information. For example, Cirac and Zoller collaborated to generate many of the key theoretical ideas [8] for using neutral (rather than ionized) atoms for quantum information processing, as well as for a related topic of using atomic gases to simulate the full quantum mechanical dynamics of the complex many-body systems found in condensed matter [9]. Wineland and his group in the meantime devised new ideas for building ever-larger registers of trapped ions [10]. They have also elucidated [11] many of the central features needed to achieve scaling of the size of quantum devices. These developments are important steps on the road to bringing the scale of quantum computations to the level of competitiveness with classical computers. This group has also used the quantum logic gate techniques to engineer an improved type of atomic clock [12] in one of the most striking examples where quantum information ideas are feeding back into the fields from which they sprung.

While the general ideas for quantum information science have been laid by Ingarden, Feynman, Lloyd, Deutsch, Shor, Ekert, and others, it was the seminal paper by Cirac and Zoller, followed by the implementation by Wineland and co-workers that caused the field of quantum computation and quantum information science to erupt. The number of papers in this field increased from just a handful to several thousands. Many countries have launched special programs on quantum information science. While we are still far from a quantum computer which could rival current classical ones, small-scale devices were used to perform operations like factorization of small numbers or approximate solutions of Schrödinger's equation for very simple atomic systems. Devices based on quantum logic gates are also used in quantum cryptography which is on the verge of commercialization. An important impact of Cirac, Zoller, and Wineland's work is a realization of how much progress in science, not only in physics, can be achieved if we can control and manipulate the specific aspects of quantum mechanics that allow the accomplishment of tasks that are impossible classically.

Note

For the research described here Wineland received Nobel Prize in 2012 whereas Cirac and Zoller received Wolf Prize in 2013.

Section snippets

J. Ignacio Cirac

Juan Ignacio Cirac was born in 1965 and grew up in Spain. He attended the Universidad Complutense de Madrid for his undergraduate and graduate degrees, earning them in 1988 and 1991, respectively. Between 1991 and 1994, during several research visits to JILA in Boulder, Colorado, Cirac began working with Peter Zoller on their ideas for quantum computing. After appointments at Universidad de Castilla-La Mancha and Leopold Franzens Universität Innsbruck, in 2001 Cirac began research at the Max

The Benjamin Franklin Medal in Physics medal legacy

Previous laureates in physics who, like J. Ignacio Cirac, Peter Zoller, and David Wineland, focused on issues related to the quantum nature of matter include:

  • 1952 Wolfgan Pauli (Franklin Medal):

    For work in the understanding of atomic physics and formulation of exclusion principle.

  • 1959 Charles H. Townes (Ballantine Medal):

    For development of the MASER.

  • 1978 William Klemperer (Wetherill Medal):

    For molecular beam techniques to study molecular structure.

  • 1982 Charles V. Shank (Longstreth Medal):

    For

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