12,835 research outputs found
Quantum Computing in Arrays Coupled by 'Always On' Interactions
It has recently been shown that one can perform quantum computation in a
Heisenberg chain in which the interactions are 'always on', provided that one
can abruptly tune the Zeeman energies of the individual (pseudo-)spins. Here we
provide a more complete analysis of this scheme, including several
generalizations. We generalize the interaction to an anisotropic form
(incorporating the XY, or Forster, interaction as a limit), providing a proof
that a chain coupled in this fashion tends to an effective Ising chain in the
limit of far off-resonant spins. We derive the primitive two-qubit gate that
results from exploiting abrupt Zeeman tuning with such an interaction. We also
demonstrate, via numerical simulation, that the same basic scheme functions in
the case of smoothly shifted Zeeman energies. We conclude with some remarks
regarding generalisations to two- and three-dimensional arrays.Comment: 16 pages (preprint format) inc. 3 figure
Multi-Qubit Gates in Arrays Coupled by 'Always On' Interactions
Recently there has been interest in the idea of quantum computing without
control of the physical interactions between component qubits. This is highly
appealing since the 'switching' of such interactions is a principal difficulty
in creating real devices. It has been established that one can employ 'always
on' interactions in a one-dimensional Heisenberg chain, provided that one can
tune the Zeeman energies of the individual (pseudo-)spins. It is important to
generalize this scheme to higher dimensional networks, since a real device
would probably be of that kind. Such generalisations have been proposed, but
only at the severe cost that the efficiency of qubit storage must *fall*. Here
we propose the use of multi-qubit gates within such higher-dimensional arrays,
finding a novel three-qubit gate that can in fact increase the efficiency
beyond the linear model. Thus we are able to propose higher dimensional
networks that can constitute a better embodiment of the 'always on' concept - a
substantial step toward bringing this novel concept to full fruition.Comment: 20 pages in preprint format, inc. 3 figures. This version has fixed
typos and printer-friendly figures, and is to appear in NJ
Optoelectronics of Inverted Type-I CdS/CdSe Core/Crown Quantum Ring
Inverted type-I heterostructure core/crown quantum rings (QRs) are
quantum-efficient luminophores, whose spectral characteristics are highly
tunable. Here, we study the optoelectronic properties of type-I core/crown
CdS/CdSe QRs in the zincblende phase - over contrasting lateral size and crown
width. For this we inspect their strain profiles, transition energies,
transition matrix elements, spatial charge densities, electronic bandstructure,
band-mixing probabilities, optical gain spectra, maximum optical gains and
differential optical gains. Our framework uses an effective-mass envelope
function theory based on the 8-band kp method employing the valence
force field model for calculating the atomic strain distributions. The gain
calculations are based on the density-matrix equation and take into
consideration the excitonic effects with intraband scattering. Variations in
the QR lateral size and relative widths of core and crown (ergo the
composition) affect their energy levels, band-mixing probabilities, optical
transition matrix elements, emission wavelengths/intensity, etc. The optical
gain of QRs is also strongly dimension and composition dependent with further
dependency on the injection carrier density causing band-filling effect. They
also affect the maximum and differential gain at varying dimensions and
compositions.Comment: Published in AIP Journal of Applied Physics (11 pages, 7 figures
Polaritonic characteristics of insulator and superfluid phases in a coupled-cavity array
Recent studies of quantum phase transitions in coupled atom-cavity arrays
have focused on the similarities between such systems and the Bose-Hubbard
model. However, the bipartite nature of the atom-cavity systems that make up
the array introduces some differences. In order to examine the unique features
of the coupled-cavity system, the behavior of a simple two-site model is
studied over a wide range of parameters. Four regions are identified, in which
the ground state of the system may be classified as either a polaritonic
insulator, a photonic superfluid, an atomic insulator, or a polaritonic
superfluid.Comment: 7 pages, 9 figures, 1 table, REVTeX 4; published versio
Quantum Energy Teleportation in Spin Chain Systems
We propose a protocol for quantum energy teleportation which transports
energy in spin chains to distant sites only by local operations and classical
communication. By utilizing ground-state entanglement and notion of negative
energy density region, energy is teleported without breaking any physical laws
including causality and local energy conservation. Because not excited physical
entity but classical information is transported in the protocol, the
dissipation rate of energy in transport is expected to be strongly suppressed.Comment: 22 pages, 4 figure, to be published in JPS
Upward Point-Set Embeddability
We study the problem of Upward Point-Set Embeddability, that is the problem
of deciding whether a given upward planar digraph has an upward planar
embedding into a point set . We show that any switch tree admits an upward
planar straight-line embedding into any convex point set. For the class of
-switch trees, that is a generalization of switch trees (according to this
definition a switch tree is a -switch tree), we show that not every
-switch tree admits an upward planar straight-line embedding into any convex
point set, for any . Finally we show that the problem of Upward
Point-Set Embeddability is NP-complete
Entanglement of Two Impurities through Electron Scattering
We study how two magnetic impurities embedded in a solid can be entangled by
an injected electron scattering between them and by subsequent measurement of
the electron's state. We start by investigating an ideal case where only the
electronic spin interacts successively through the same unitary operation with
the spins of the two impurities. In this case, high (but not maximal)
entanglement can be generated with a significant success probability. We then
consider a more realistic description which includes both the forward and back
scattering amplitudes. In this scenario, we obtain the entanglement between the
impurities as a function of the interaction strength of the electron-impurity
coupling. We find that our scheme allows us to entangle the impurities
maximally with a significant probability
Quantum Computing with an 'Always On' Heisenberg Interaction
Many promising ideas for quantum computing demand the experimental ability to
directly switch 'on' and 'off' a physical coupling between the component
qubits. This is typically the key difficulty in implementation, and precludes
quantum computation in generic solid state systems, where interactions between
the constituents are 'always on'. Here we show that quantum computation is
possible in strongly coupled (Heisenberg) systems even when the interaction
cannot be controlled. The modest ability of 'tuning' the transition energies of
individual qubits proves to be sufficient, with a suitable encoding of the
logical qubits, to generate universal quantum gates. Furthermore, by tuning the
qubits collectively we provide a scheme with exceptional experimental
simplicity: computations are controlled via a single 'switch' of only six
settings. Our schemes are applicable to a wide range of physical
implementations, from excitons and spins in quantum dots through to bulk
magnets.Comment: 4 pages, 3 figs, 2 column format. To appear in PR
Spin systems with dimerized ground states
In view of the numerous examples in the literature it is attempted to outline
a theory of Heisenberg spin systems possessing dimerized ground states (``DGS
systems") which comprises all known examples. Whereas classical DGS systems can
be completely characterized, it was only possible to provide necessary or
sufficient conditions for the quantum case. First, for all DGS systems the
interaction between the dimers must be balanced in a certain sense. Moreover,
one can identify four special classes of DGS systems: (i) Uniform pyramids,
(ii) systems close to isolated dimer systems, (iii) classical DGS systems, and
(iv), in the case of , systems of two dimers satisfying four
inequalities. Geometrically, the set of all DGS systems may be visualized as a
convex cone in the linear space of all exchange constants. Hence one can
generate new examples of DGS systems by positive linear combinations of
examples from the above four classes.Comment: With corrections of proposition 4 and other minor change
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