Quantum Information Processing, Vol. 5, No. 6, December 2006 (© 2006)
Efﬁcient Read-out of Flux Qubits at Flux
and Frank K. Wilhelm
Received January 6, 2006; accepted April 4, 2006; Published online October 31, 2006
We discuss a novel method to read out a superconducting ﬂux qubit at the ﬂux
degeneracy point, using a charge measurement through a single charge transistor
SCT. The interaction is based on the Aharonov-Casher geometric phase. The
read-out cycle indicates one of the states by a ﬂuorescence-like cycle. We discuss
how to efﬁciently implement this scheme with the SCT in the RF mode.
KEY WORDS: Quantum information; ﬂux qubits.
PACS: 03.67.Lx; 74.40.+k; 85.25.Cp.
There have been many suggestions on quantum computers built on super-
conducting technology, making use of the protection from unwanted exci-
tations provided by the superconducting gap.
One of the most promising qubits is the ﬂux qubit,
a superconducting ring in which the circulating current can only take on
two discrete values, forming the qubit states. The ring is made up by three
superconducting islands connected in series via Josephson junctions. Any
solid state qubit suffers from decoherence, not only due to ﬂuctuation in
the control ﬁelds but also from ﬂuctuations in the other degrees of free-
dom in the solid. For a ﬂux qubit, the main noise contribution comes
Dept. of Microtechnology and Nanoscience (MC2), 41296 Chalmers University of Technol-
ogy, Gothenburg, Sweden.
Sektion Physik and CeNS, Ludwig-Maximilians-Universit
at, Theresienstr. 37, 80333
Present address: Department of Physics and Astronomy and IQC, University of Waterloo,
Waterloo, Ontario, Canada.
To whom correspondence should be addressed. E-mail: firstname.lastname@example.org
1570-0755/06/1200-0563/0 © 2006 Springer Science+Business Media, Inc.