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Concepts in Magnetic Resonance

The relationship between ensemble quantum computing logical gates and NMR pulse sequence engineering exemplified by the SWAP operation

Thomas Schulte-Herbr�ggen, Ole W. Sørensen

Published Jan 1, 2000• 14 citations
Publisher

Abstract

This paper describes how to derive nuclear magnetic resonance (NMR) pulse sequences suitable as ensemble quantum computing logical gates by known systematic tools for design of pulse sequences in NMR spectroscopy. This is in contrast to the current trend of developing NMR pulse sequences for ensemble quantum computing independent of established NMR spectroscopic technology, although many of the pertinent spin manipulations are already known in NMR spectroscopy. The SWAP operation is discussed in detail and one version is only (2J)−1long, which compares favorably with 3(2J)−1 in a recent implementation (Linden N, Barjat H, Kupce E, Freeman R. Chem Phys Lett 1999; 307:198–204). Alternatively, the number of π pulses can be reduced in that sequence. © 2000 John Wiley & Sons, Inc. Concepts Magn Reson 12: 389–395, 2000

DOI
10.1002/1099-0534(2000)12:6<389::aid-cmr2>3.0.co;2-t

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This paper describes how to derive nuclear magnetic resonance (NMR) pulse sequences suitable as ensemble quantum computing logical gates by known systematic tools for design of pulse sequences in NMR spectroscopy. This is in contrast to the current trend of developing NMR pulse sequences for ensemble quantum computing independent of established NMR spectroscopic technology, although many of the pertinent spin manipulations are already known in NMR spectroscopy. The SWAP operation is discussed in detail and one version is only (2J)−1long, which compares favorably with 3(2J)−1 in a recent implementation (Linden N, Barjat H, Kupce E, Freeman R. Chem Phys Lett 1999; 307:198–204). Alternatively, the number of π pulses can be reduced in that sequence. © 2000 John Wiley & Sons, Inc. Concepts Magn Reson 12: 389–395, 2000
This paper describes how to derive nuclear magnetic resonance (NMR) pulse sequences suitable as ensemble quantum computing logical gates by known systematic tools for design of pulse sequences in NMR spectroscopy. This is in contrast to the current trend of developing NMR pulse sequences for ensemble quantum computing independent of established NMR spectroscopic technology, although many of the pertinent spin manipulations are already known in NMR spectroscopy. The SWAP operation is discussed in detail and one version is only (2J)−1long, which compares favorably with 3(2J)−1 in a recent implementation (Linden N, Barjat H, Kupce E, Freeman R. Chem Phys Lett 1999; 307:198–204). Alternatively, the number of π pulses can be reduced in that sequence. © 2000 John Wiley & Sons, Inc. Concepts Magn Reson 12: 389–395, 2000
New concepts are described for nuclear magnetic resonance (NMR) implementations of spin ensemble quantum computing in one and two dimensions. Similarities and differences between ensemble and pure state quantum computing are discussed by using a Liouville space formalism based on polarization and single transition operators. The introduction of an observer spin, that is coupled to the spins carrying the quantum bits, allows a mapping of the states of a quantum computer on a set of transitions between energy levels. This is spectroscopically favorable compared to a mapping on the energy levels themselves. Two complementary parallelization schemes for quantum computing are presented: one exploits the parallel processing feature inherent in multidimensional NMR, while the other employs mixed superposition states represented by operators in Liouville space. The spin swap operation, introduced in this paper, allows a convenient extension of quantum computing to spin systems where not all spin–spin couplings are resolved. The concepts are illustrated by implementations of logic operations and identities consisting of a sequence of basic logic gate operations.
Quantum information processing is a multi-disciplinary science involving physics, mathematics, computer science, and even quantum chemistry. It is centred around the idea of manipulating physical systems at the quantum level, either for simulation of physical systems, or numerical computation. Although it has been known for almost a decade that a quantum computer would enable the solution of problems deemed infeasible classically, constructing one has been beyond today's capabilities. In this work we explore one proposed implementation of a quantum computer: Nuclear Magnetic Resonance (NMR) spectroscopy. We also develop a numerical software tool, a pulse sequence compiler, for use in the implementation of quantum computer programs on an NMR quantum computer. Our pulse sequence compiler takes as input the specifications of the molecule used as a quantum register, the desired quantum gate, and experimental data on the actual effects of RF pulses on a sample of the molecule, and outputs an optimum set of pre and post 'virtual' gates that minimize the error induced.
Nuclear magnetic resonance spectroscopy has demonstrated significant experimental progress toward the development of quantum computations. The developments so far have taken place mainly through the use of spin 12 nuclei. In this paper we describe the use of a spin 32 nucleus, oriented in a liquid crystal matrix for the creation of pseudopure states and the implementation of a complete set of two-qubit reversible logic gates using single-quantum transition-selective pulses, extending the range of practice of NMR toward quantum computation.
Quantum computing using two-dimensional NMR has recently been described using scalar coupling evolution technique [J. Chem. Phys.,109,10603 (1998)]. In the present paper, we describe two-dimensional NMR quantum computing with the help of selective pulses. A number of logic gates are implemented using two and three qubits with one extra observer spin. Some many-in-one gates (or Portmanteau gates) are implemented. Toffoli gate (or AND/NAND gate) and OR/NOR gates are implemented on three qubits. Deutsch-Jozsa quantum algorithm for one and two qubits, using one extra work qubit, has also been implemented using selective pulses after creating a coherent superposition state, in the two-dimensional methodology.