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Liquid State NMR as a Test-bed for Developing Quantum Control Methods



C. A. Ryan, C. Negrevergne, M. Laforest, Emanuel Knill, R. Laflamme


Quantum mechanics has been successfully describing experimental phenomena for close to a century. However, it has only been much more recently that the idea of using quantum mechanical evolution to process information was realized. This has revolutionized the way we view the complexity of information processing, and quantum information processing (QIP) devices may be able to handle certain computational tasks, such as factoring large integers or simulating quantum systems, exponentially faster than classical computers. One of the reasons for the long lag time between the development of quantum mechanics and its application to information processing is that quantum effects are extremely delicate and difficult to detect. The system must be well isolated from the environment or decoherence will quickly destroy the quantum superpositions and entanglement that the QIP's exploit, and reduce the system to a probabilistic classical state. The ideally unitary evolution can be seen as a quantum algorithm and the deviations due to decoherence or inaccurate control fields are errors in the computation. This new perspective lead to the realization that in the same way it is possible to encode classical information against noise, it is possible to efficiently encode against and correct the noise affecting the quantum information. Provided that the error rate is low enough it is possible to compute accurately in the presence of errors. Thus the control and isolation does not have to be arbitrarily good: it only has to reach some fault-tolerant threshold which is reasonably expected to be 1/10000 computationally relevant errors per gate. In practice meeting the accuracy threshold requires balancing the competing goals of both a controlled interaction and a coherently evolving quantum system. Moreover, in the context of building a large-scale QIP, the classical resources required to apply such control should scale efficiently with the size of the quantum system. This is an extraordinary challenge and the prospect of a quantum computer has provided the impetus to explore many technologies in the pursuit of a scalable quantum computer.
Physical Review A (Atomic, Molecular and Optical Physics)


nuclear magnetic resonance, quantum computing, quantum control


Ryan, C. , Negrevergne, C. , Laforest, M. , Knill, E. and Laflamme, R. (2008), Liquid State NMR as a Test-bed for Developing Quantum Control Methods, Physical Review A (Atomic, Molecular and Optical Physics), [online], (Accessed December 5, 2023)
Created July 14, 2008, Updated October 12, 2021