Quantum Information Processing, Vol. 6, No. 6, December 2007 (© 2007)
Signatures of Incoherence in a Quantum Information
Michael K. Henry,
Alexey V. Gorshkov,
Yaakov S. Weinstein,
Jonathan S. Hodges,
Timothy F. Havel,
David G. Cory
Received May 24, 2007; accepted August 19, 2007; Published online: November 8, 2007
Incoherent noise is manifest in measurements of expectation values when the
underlying ensemble evolves under a classical distribution of unitary processes.
While many incoherent processes appear decoherent, there are important differ-
ences. The distribution functions underlying incoherent processes are either static
or slowly varying with respect to control operations and so the errors introduced
by these distributions are refocusable. The observation and control of incoherence
in small Hilbert spaces is well known. Here we explore incoherence during an
entangling operation, such as is relevant in quantum information processing. As
expected, it is more difﬁcult to separate incoherence and decoherence over such
processes. However, by studying the ﬁdelity decay under a cyclic entangling map
we are able to identify distinctive experimental signatures of incoherence. This is
demonstrated both through numerical simulations and experimentally in a three
qubit nuclear magnetic resonance implementation.
KEY WORDS: decoherence; incoherence; open quantum system; ﬁdelity
decay; quantum error correction.
PACS: 03·65·yz; 03·67-a; 03.67·Ac; 03·67·Pp.
Department of Nuclear Science and Engineering, Massachusetts Institute of Technology,
Cambridge, MA 02139, USA.
Physics Department, Harvard University, Cambridge, MA 02138, USA.
Quantum Information Science Group, The Mitre Corporation, Eatontown, NJ 07724, USA.
Department of Applied Mathematics and Institute for Quantum Computing, University of
Waterloo, Waterloo, ON, Canada N2L 3G1.
Department of Natural Sciences, New Mexico Highlands University, Las Vegas, NM 87701,
ITAMP, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA.
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