398 research outputs found
Quantum Computing with Electron Spins in Quantum Dots
Several topics on the implementation of spin qubits in quantum dots are
reviewed. We first provide an introduction to the standard model of quantum
computing and the basic criteria for its realization. Other alternative
formulations such as measurement-based and adiabatic quantum computing are
briefly discussed. We then focus on spin qubits in single and double GaAs
electron quantum dots and review recent experimental achievements with respect
to initialization, coherent manipulation and readout of the spin states. We
extensively discuss the problem of decoherence in this system, with particular
emphasis on its theoretical treatment and possible ways to overcome it.Comment: Lecture notes for Course CLXXI "Quantum Coherence in Solid State
Systems" Int. School of Physics "Enrico Fermi", Varenna, July 2008, 61 pages,
20 figure
Estimation of Spin-Spin Interaction by Weak Measurement Scheme
Precisely knowing an interaction Hamiltonian is crucial to realize quantum
information tasks, especially to experimentally demonstrate a quantum computer
and a quantum memory. We propose a scheme to experimentally evaluate the
spin-spin interaction for a two-qubit system by the weak measurement technique
initiated by Yakir Aharonov and his colleagues. Furthermore, we numerically
confirm our proposed scheme in a specific system of a nitrogen vacancy center
in diamond. This means that the weak measurement can also be taken as a
concrete example of the quantum process tomography.Comment: 4 pages, 1 table, 2 figures, to appear in Europhysics Letter
Universal Control of Nuclear Spins Via Anisotropic Hyperfine Interactions
We show that nuclear spin subsystems can be completely controlled via
microwave irradiation of resolved anisotropic hyperfine interactions with a
nearby electron spin. Such indirect addressing of the nuclear spins via
coupling to an electron allows us to create nuclear spin gates whose
operational time is significantly faster than conventional direct addressing
methods. We experimentally demonstrate the feasibility of this method on a
solid-state ensemble system consisting of one electron and one nuclear spin.Comment: RevTeX4, 8 pages, 8 figure
Quantum cellular automata quantum computing with endohedral fullerenes
We present a scheme to perform universal quantum computation using global
addressing techniques as applied to a physical system of endohedrally doped
fullerenes. The system consists of an ABAB linear array of Group V endohedrally
doped fullerenes. Each molecule spin site consists of a nuclear spin coupled
via a Hyperfine interaction to an electron spin. The electron spin of each
molecule is in a quartet ground state . Neighboring molecular electron
spins are coupled via a magnetic dipole interaction. We find that an
all-electron construction of a quantum cellular automata is frustrated due to
the degeneracy of the electronic transitions. However, we can construct a
quantum celluar automata quantum computing architecture using these molecules
by encoding the quantum information on the nuclear spins while using the
electron spins as a local bus. We deduce the NMR and ESR pulses required to
execute the basic cellular automata operation and obtain a rough figure of
merit for the the number of gate operations per decoherence time. We find that
this figure of merit compares well with other physical quantum computer
proposals. We argue that the proposed architecture meets well the first four
DiVincenzo criteria and we outline various routes towards meeting the fifth
criteria: qubit readout.Comment: 16 pages, Latex, 5 figures, See http://planck.thphys.may.ie/QIPDDF/
submitted to Phys. Rev.
Application of Optimal Control to CPMG Refocusing Pulse Design
We apply optimal control theory (OCT) to the design of refocusing pulses
suitable for the CPMG sequence that are robust over a wide range of B0 and B1
offsets. We also introduce a model, based on recent progress in the analysis of
unitary dynamics in the field of quantum information processing (QIP), that
describes the multiple refocusing dynamics of the CPMG sequence as a dephasing
Pauli channel. This model provides a compact characterization of the
consequences and severity of residual pulse errors. We illustrate the methods
by considering a specific example of designing and analyzing broadband OCT
refocusing pulses of length 10 t180 that are constrained by the maximum
instantaneous pulse power. We show that with this refocusing pulse, the CPMG
sequence can refocus over 98% of magnetization for resonance offsets up to 3.2
times the maximum RF amplitude, even in the presence of +/- 10% RF
inhomogeneity.Comment: 23 pages, 10 figures; Revised and reformatted version with new title
and significant changes to Introduction and Conclusions section
Bose-Einstein condensation of semi-hard bosons in S=1 dimerized organic compound F2PNNNO
An analysis of the energy spectrum and the magnetization curve of
two-dimensional organic antiferromagnet F2PNNNO with a spin-one dimerized
structure shows that a behavior of the compound in an external magnetic field
can be explained within a lattice boson model with an extended Pauli's
exclusion principle, i.e. no more than two bosons per a dimer. The unusual
magnetization curve observed experimentally in the compound reflects a sequence
of phase transitions intrinsic for a lattice boson system with strong on-site
and inter-site repulsions due to a tuning of magnon density by the applied
magnetic field
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