24,467 research outputs found
Universal Quantum Computation through Control of Spin-Orbit Coupling
We propose a method for quantum computation which uses control of spin-orbit
coupling in a linear array of single electron quantum dots. Quantum gates are
carried out by pulsing the exchange interaction between neighboring electron
spins, including the anisotropic corrections due to spin-orbit coupling.
Control over these corrections, even if limited, is sufficient for universal
quantum computation over qubits encoded into pairs of electron spins. The
number of voltage pulses required to carry out either single qubit rotations or
controlled-Not gates scales as the inverse of a dimensionless measure of the
degree of control of spin-orbit coupling.Comment: 4 pages, 3 figures (minor revision, references added
High purity bright single photon source
Using cavity-enhanced non-degenerate parametric downconversion, we have built
a frequency tunable source of heralded single photons with a narrow bandwidth
of 8 MHz, making it compatible with atomic quantum memories. The photon state
is 70% pure single photon as characterized by a tomographic measurement and
reconstruction of the quantum state, revealing a clearly negative Wigner
function. Furthermore, it has a spectral brightness of ~1,500 photons/s per MHz
bandwidth, making it one of the brightest single photon sources available. We
also investigate the correlation function of the down-converted fields using a
combination of two very distinct detection methods; photon counting and
homodyne measurement.Comment: 9 pages, 4 figures; minor changes, added referenc
Hydrogenic Spin Quantum Computing in Silicon: A Digital Approach
We suggest an architecture for quantum computing with spin-pair encoded
qubits in silicon. Electron-nuclear spin-pairs are controlled by a dc magnetic
field and electrode-switched on and off hyperfine interaction. This digital
processing is insensitive to tuning errors and easy to model. Electron
shuttling between donors enables multi-qubit logic. These hydrogenic spin
qubits are transferable to nuclear spin-pairs, which have long coherence times,
and electron spin-pairs, which are ideally suited for measurement and
initialization. The architecture is scalable to highly parallel operation.Comment: 4 pages, 5 figures; refereed and published version with improved
introductio
Ensemble Quantum Computation with atoms in periodic potentials
We show how to perform universal quantum computation with atoms confined in
optical lattices which works both in the presence of defects and without
individual addressing. The method is based on using the defects in the lattice,
wherever they are, both to ``mark'' different copies on which ensemble quantum
computation is carried out and to define pointer atoms which perform the
quantum gates. We also show how to overcome the problem of scalability on this
system
Quantum Phase Transitions and Bipartite Entanglement
We develop a general theory of the relation between quantum phase transitions
(QPTs) characterized by nonanalyticities in the energy and bipartite
entanglement. We derive a functional relation between the matrix elements of
two-particle reduced density matrices and the eigenvalues of general two-body
Hamiltonians of -level systems. The ground state energy eigenvalue and its
derivatives, whose non-analyticity characterizes a QPT, are directly tied to
bipartite entanglement measures. We show that first-order QPTs are signalled by
density matrix elements themselves and second-order QPTs by the first
derivative of density matrix elements. Our general conclusions are illustrated
via several quantum spin models.Comment: 5 pages, incl. 2 figures. v3: The version published in PRL, including
a few extra comments and clarifications for which there was no space in the
PR
Mesoporous matrices for quantum computation with improved response through redundance
We present a solid state implementation of quantum computation, which improves previously proposed optically driven schemes. Our proposal is based on vertical arrays of quantum dots embedded in a mesoporous material which can be fabricated with present technology. The redundant encoding typical of the chosen hardware protects the computation against gate errors and the effects of measurement induced noise. The system parameters required for quantum computation applications are calculated for II-VI and III-V materials and found to be within the experimental range. The proposed hardware may help minimize errors due to polydispersity of dot sizes, which is at present one of the main problems in relation to quantum dot-based quantum computation. (c) 2007 American Institute of Physics
Reconceptualising Personas Across Cultures: Archetypes, Stereotypes & Collective Personas in Pastoral Namibia
The paucity of projects where persona is the research foci and a lack of consensus on this artefact keep many reticent about its purpose and value. Besides crafting personas is expected to differ across cultures, which contrasts the advancements in Western theory with studies and progress in other sites. We postulate User-Created Personas reveal specific characteristics of situated contexts by allowing laypeople to design persona artefacts in their own terms. Hence analysing four persona sessions with an ethnic group in pastoral Namibia –ovaHerero– brought up a set of fundamental questions around the persona artefact regarding stereotypes, archetypes, and collective persona representations: (1) to what extent user depictions are stereotypical or archetypal? If stereotypes prime (2) to what degree are current personas a useful method to represent end-users in technology design? And, (3) how can we ultimately read accounts not conforming to mainstream individual persona descriptions but to collectives
Kondo Quantum Dots and the Novel Kondo-doublet interaction
We analyze the interactions between two Kondo Quantum Dots connected to a
Rashba-active Quantum Wire. We find that the Kondo-doublet interaction, at an
inter-dot distance of the order of the wire Fermi length, is over an order of
magnitude greater than the RKKY interaction. The effects induced on the
Kondo-doublet interaction by the wire spin-orbit coupling can be used to
control the Quantum Dots spin-spin correlation. These results imply that the
widely used assumption that the RKKY is the dominant interaction between
Anderson impurities must be revised.Comment: 4 pages, 4 figs, accepted for publication in PRL. title changed and
text polishe
Valence Bond Solids for Quantum Computation
Cluster states are entangled multipartite states which enable to do universal
quantum computation with local measurements only. We show that these states
have a very simple interpretation in terms of valence bond solids, which allows
to understand their entanglement properties in a transparent way. This allows
to bridge the gap between the differences of the measurement-based proposals
for quantum computing, and we will discuss several features and possible
extensions
Quantum spin models with electrons in Penning traps
We propose a scheme to engineer an effective spin Hamiltonian starting from a
system of electrons confined in micro-Penning traps. By means of appropriate
sequences of electromagnetic pulses, alternated to periods of free evolution,
we control the shape and strength of the spin-spin interaction. Moreover, we
can modify the effective magnetic field experienced by the particle spin. This
procedure enables us to reproduce notable quantum spin systems, such as Ising
and XY models. Thanks to its scalability, our scheme can be applied to a fairly
large number of trapped particles within the reach of near future technology.Comment: 22 pages, 1 figure, added minor changes and typos, accepted for
publication in PR
- …