13,068 research outputs found

### Spin swap vs. double occupancy in quantum gates

We propose an approach to realize quantum gates with electron spins localized
in a semiconductor that uses double occupancy to advantage. With a fast
(non-adiabatic) time control of the tunnelling, the probability of double
occupancy is first increased and then brought back exactly to zero. The quantum
phase built in this process can be exploited to realize fast quantum
operations. We illustrate the idea focusing on the half-swap operation, which
is the key two-qubit operation needed to build a CNOT gate.Comment: 5 pages, 2 figure

### Treatments of the exchange energy in density-functional theory

Following a recent work [Gal, Phys. Rev. A 64, 062503 (2001)], a simple
derivation of the density-functional correction of the Hartree-Fock equations,
the Hartree-Fock-Kohn-Sham equations, is presented, completing an integrated
view of quantum mechanical theories, in which the Kohn-Sham equations, the
Hartree-Fock-Kohn-Sham equations and the ground-state Schrodinger equation
formally stem from a common ground: density-functional theory, through its
Euler equation for the ground-state density. Along similar lines, the Kohn-Sham
formulation of the Hartree-Fock approach is also considered. Further, it is
pointed out that the exchange energy of density-functional theory built from
the Kohn-Sham orbitals can be given by degree-two homogeneous N-particle
density functionals (N=1,2,...), forming a sequence of degree-two homogeneous
exchange-energy density functionals, the first element of which is minus the
classical Coulomb-repulsion energy functional.Comment: 19 pages; original manuscript from 2001 (v1) revised for publication,
with presentation substantially improved, some errors corrected, plus an
additional summarizing figure (Appendix B) include

### Spin state transition in LaCoO3 by variational cluster approximation

The variational cluster approximation is applied to the calculation of
thermodynamical quantities and single-particle spectra of LaCoO3. Trial
self-energies and the numerical value of the Luttinger-Ward functional are
obtained by exact diagonalization of a CoO6 cluster. The VCA correctly predicts
LaCoO3 as a paramagnetic insulator and a gradual and relatively smooth increase
of the occupation of high-spin Co3+ ions causes the temperature dependence of
entropy and magnetic susceptibility. The single particle spectral function
agrees well with experiment, the experimentally observed temperature dependence
of photoelectron spectra is reproduced satisfactorily. Remaining discrepancies
with experiment highlight the importance of spin orbit coupling and local
lattice relaxation.Comment: Revtex file with 10 eps figure

### A priori probability that a qubit-qutrit pair is separable

We extend to arbitrarily coupled pairs of qubits (two-state quantum systems)
and qutrits (three-state quantum systems) our earlier study (quant-ph/0207181),
which was concerned with the simplest instance of entangled quantum systems,
pairs of qubits. As in that analysis -- again on the basis of numerical
(quasi-Monte Carlo) integration results, but now in a still higher-dimensional
space (35-d vs. 15-d) -- we examine a conjecture that the Bures/SD (statistical
distinguishability) probability that arbitrarily paired qubits and qutrits are
separable (unentangled) has a simple exact value, u/(v Pi^3)= >.00124706, where
u = 2^20 3^3 5 7 and v = 19 23 29 31 37 41 43 (the product of consecutive
primes). This is considerably less than the conjectured value of the Bures/SD
probability, 8/(11 Pi^2) = 0736881, in the qubit-qubit case. Both of these
conjectures, in turn, rely upon ones to the effect that the SD volumes of
separable states assume certain remarkable forms, involving "primorial"
numbers. We also estimate the SD area of the boundary of separable qubit-qutrit
states, and provide preliminary calculations of the Bures/SD probability of
separability in the general qubit-qubit-qubit and qutrit-qutrit cases.Comment: 9 pages, 3 figures, 2 tables, LaTeX, we utilize recent exact
computations of Sommers and Zyczkowski (quant-ph/0304041) of "the Bures
volume of mixed quantum states" to refine our conjecture

### Correlated band structure of NiO, CoO and MnO by variational cluster approximation

The variational cluster approximation proposed by Potthoff is applied to the
calculation of the single-particle spectral function of the transition metal
oxides MnO, CoO and NiO. Trial self-energies and the numerical value of the
Luttinger-Ward functional are obtained by exact diagonalization of a
TMO6-cluster. The single-particle parameters of this cluster serve as
variational parameters to construct a stationary point of the grand potential
of the lattice system. The stationary point is found by a crossover procedure
which allows to go continuously from an array of disconnected clusters to the
lattice system. The self-energy is found to contain irrelevant degrees of
freedom which have marginal impact on the grand potential and which need to be
excluded to obtain meaningful results. The obtained spectral functions are in
good agreement with experimental data.Comment: 14 pages, 17 figure

### Joint effect of lattice interaction and potential fluctuation in colossal magnetoresistive manganites

Taking into account both the Jahn-Teller lattice distortion and the on-site
electronic potential fluctuations in the orbital-degenerated double-exchange
model, in which both the core-spin and the lattice distortion are treated
classically, we investigate theoretically the metal-insulator transition (MIT)
in manganites by considering the electronic localization effect. An inverse
matrix method is developed for calculation in which we use the inverse of the
transfer matrix to obtain the localization length. We find that within
reasonable range of parameters, both the lattice effect and the potential
fluctuation are responsible to the occurrence of the MIT. The role of the
orbital configuration is also discussed.Comment: 4 figure

### Traveling sealer for contoured table Patent

Sealing apparatus for joining two pieces of frangible material

### On the nucleon-nucleon interaction leading to a standing wave instability in symmetric nuclear matter

We examine a recently proposed nucleon-nucleon interaction, claimed by its
authors both realistic and leading to a standing wave instability in symmetric
nuclear matter. Contrary to these claims, we find that this interaction leads
to a serious overbinding of 4He, 16O and 40Ca nuclei when the Hartree-Fock
method is properly applied. The resulting nuclear densities contradict the
experimental data and all realistic Hartree-Fock results.Comment: 4 pages, 1 figur

### Perspective: Treating electron over-delocalization with the DFT+U method

Many people in the materials science and solid-state community are familiar with the acronym âDFT+U.â For those less familiar, this technique uses ideas from model Hamiltonians that permit the description of both metals and insulators to address problems of electron over-delocalization in practical implementations of density functional theory (DFT). Exchange-correlation functionals in DFT are often described as belonging to a hierarchical âJacobâs ladderâ of increasing accuracy in moving from local to non-local descriptions of exchange and correlation. DFT+U is not on this âladderâ but rather acts as an âelevatorâ because it systematically tunes relative energetics, typically on a localized subshell (e.g., d or f electrons), regardless of the underlying functional employed. However, this tuning is based on a metric of the local electron density of the subshells being addressed, thus necessitating physical or chemical or intuition about the system of interest. I will provide a brief overview of the history of how DFT+U came to be starting from the origin of the Hubbard and Anderson model Hamiltonians. This history lesson is necessary because it permits us to make the connections between the âHubbard Uâ and fundamental outstanding challenges in electronic structure theory, and it helps to explain why this method is so widely applied to transition-metal oxides and organometallic complexes alike.Burroughs Wellcome Fund (Career Award at the Scientific Interface)Massachusetts Institute of Technology. Research Support CorporationNational Science Foundation (U.S.) (ECCS-1449291)MIT Energy InitiativeMassachusetts Institute of Technology. Department of Chemical Engineering (Startup Funds

### Quantum-Dot Cellular Automata using Buried Dopants

The use of buried dopants to construct quantum-dot cellular automata is
investigated as an alternative to conventional electronic devices for
information transport and elementary computation. This provides a limit in
terms of miniaturisation for this type of system as each potential well is
formed by a single dopant atom. As an example, phosphorous donors in silicon
are found to have good energy level separation with incoherent switching times
of the order of microseconds. However, we also illustrate the possibility of
ultra-fast quantum coherent switching via adiabatic evolution. The switching
speeds are numerically calculated and found to be 10's of picoseconds or less
for a single cell. The effect of decoherence is also simulated in the form of a
dephasing process and limits are estimated for operation with finite dephasing.
The advantages and limitations of this scheme over the more conventional
quantum-dot based scheme are discussed. The use of a buried donor cellular
automata system is also discussed as an architecture for testing several
aspects of buried donor based quantum computing schemes.Comment: Minor changes in response to referees comments. Improved section on
scaling and added plot of incoherent switching time

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