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Science 28 September 2007:
Vol. 317. no. 5846, pp. 1893 - 1896
DOI: 10.1126/science.1145699
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Reports

Symmetrized Characterization of Noisy Quantum Processes

Joseph Emerson,1,2 Marcus Silva,2,3 Osama Moussa,2,3 Colm Ryan,2,3 Martin Laforest,2,3 Jonathan Baugh,2 David G. Cory,4 Raymond Laflamme2,3,5

A major goal of developing high-precision control of many-body quantum systems is to realize their potential as quantum computers. A substantial obstacle to this is the extreme fragility of quantum systems to "decoherence" from environmental noise and other control limitations. Although quantum computation is possible if the noise affecting the quantum system satisfies certain conditions, existing methods for noise characterization are intractable for present multibody systems. We introduce a technique based on symmetrization that enables direct experimental measurement of some key properties of the decoherence affecting a quantum system. Our method reduces the number of experiments required from exponential to polynomial in the number of subsystems. The technique is demonstrated for the optimization of control over nuclear spins in the solid state.

1 Department of Applied Math, University of Waterloo, Waterloo, ON N2L 3G1, Canada.
2 Institute for Quantum Computing, University of Waterloo, Waterloo, ON N2L 3G1, Canada.
3 Department of Physics and Astronomy, University of Waterloo, Waterloo, ON N2L 3G1, Canada.
4 Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
5 Perimeter Institute for Theoretical Physics, Waterloo, ON N2L 2Y5, Canada.

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