1,238 research outputs found
Quantum computing with mixed states
We discuss a model for quantum computing with initially mixed states.
Although such a computer is known to be less powerful than a quantum computer
operating with pure (entangled) states, it may efficiently solve some problems
for which no efficient classical algorithms are known. We suggest a new
implementation of quantum computation with initially mixed states in which an
algorithm realization is achieved by means of optimal basis independent
transformations of qubits.Comment: 2 figures, 52 reference
Fidelity of optimally controlled quantum gates with randomly coupled multiparticle environments
This work studies the feasibility of optimal control of high-fidelity quantum
gates in a model of interacting two-level particles. One particle (the qubit)
serves as the quantum information processor, whose evolution is controlled by a
time-dependent external field. The other particles are not directly controlled
and serve as an effective environment, coupling to which is the source of
decoherence. The control objective is to generate target one-qubit gates in the
presence of strong environmentally-induced decoherence and under physically
motivated restrictions on the control field. It is found that interactions
among the environmental particles have a negligible effect on the gate fidelity
and require no additional adjustment of the control field. Another interesting
result is that optimally controlled quantum gates are remarkably robust to
random variations in qubit-environment and inter-environment coupling
strengths. These findings demonstrate the utility of optimal control for
management of quantum-information systems in a very precise and specific
manner, especially when the dynamics complexity is exacerbated by inherently
uncertain environmental coupling.Comment: tMOP LaTeX, 9 pages, 3 figures; Special issue of the Journal of
Modern Optics: 37th Winter Colloquium on the Physics of Quantum Electronics,
2-6 January 200
Optimal two-qubit quantum circuits using exchange interactions
The Heisenberg exchange interaction is a natural method to implement
non-local (i.e., multi-qubit) quantum gates in quantum information processing.
We consider quantum circuits comprising of gates, which are
realized through the exchange interaction, and single-qubit gates. A universal
two-qubit quantum circuit is constructed from only three gates
and six single-qubit gates. We further show that three gates
are not only sufficient, but necessary. Since six single-qubit gates are known
to be necessary, our universal two-qubit circuit is optimal in terms of the
number of {\em both} and single-qubit gates.Comment: 4 page
Implementing Shor's algorithm on Josephson Charge Qubits
We investigate the physical implementation of Shor's factorization algorithm
on a Josephson charge qubit register. While we pursue a universal method to
factor a composite integer of any size, the scheme is demonstrated for the
number 21. We consider both the physical and algorithmic requirements for an
optimal implementation when only a small number of qubits is available. These
aspects of quantum computation are usually the topics of separate research
communities; we present a unifying discussion of both of these fundamental
features bridging Shor's algorithm to its physical realization using Josephson
junction qubits. In order to meet the stringent requirements set by a short
decoherence time, we accelerate the algorithm by decomposing the quantum
circuit into tailored two- and three-qubit gates and we find their physical
realizations through numerical optimization.Comment: 12 pages, submitted to Phys. Rev.
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