9,876 research outputs found
Enhancement of entanglement in one-dimensional disordered systems
The pairwise quantum entanglement of sites in disordered electronic
one-dimensional systems (rings) is studied. We focus on the effect of diagonal
and off diagonal disorder on the concurrence between electrons on
neighbor and non neighbor sites as a function of band filling. In the
case of diagonal disorder, increasing the degree of disorder leads to a
decrease of the concurrence with respect to the ordered case. However,
off-diagonal disorder produces a surprisingly strong enhancement of
entanglement. This remarkable effect occurs near half filling, where the
concurrence becomes up to 15% larger than in the ordered system.Comment: 21 pages, 9 figure
Thresholds for Linear Optics Quantum Computing with Photon Loss at the Detectors
We calculate the error threshold for the linear optics quantum computing
proposal by Knill, Laflamme and Milburn [Nature 409, pp. 46--52 (2001)] under
an error model where photon detectors have efficiency <100% but all other
components -- such as single photon sources, beam splitters and phase shifters
-- are perfect and introduce no errors. We make use of the fact that the error
model induced by the lossy hardware is that of an erasure channel, i.e., the
error locations are always known. Using a method based on a Markov chain
description of the error correction procedure, our calculations show that, with
the 7 qubit CSS quantum code, the gate error threshold for fault tolerant
quantum computation is bounded below by a value between 1.78% and 11.5%
depending on the construction of the entangling gates.Comment: 7 pages, 6 figure
The Bravyi-Kitaev transformation for quantum computation of electronic structure
Quantum simulation is an important application of future quantum computers
with applications in quantum chemistry, condensed matter, and beyond. Quantum
simulation of fermionic systems presents a specific challenge. The
Jordan-Wigner transformation allows for representation of a fermionic operator
by O(n) qubit operations. Here we develop an alternative method of simulating
fermions with qubits, first proposed by Bravyi and Kitaev [S. B. Bravyi, A.Yu.
Kitaev, Annals of Physics 298, 210-226 (2002)], that reduces the simulation
cost to O(log n) qubit operations for one fermionic operation. We apply this
new Bravyi-Kitaev transformation to the task of simulating quantum chemical
Hamiltonians, and give a detailed example for the simplest possible case of
molecular hydrogen in a minimal basis. We show that the quantum circuit for
simulating a single Trotter time-step of the Bravyi-Kitaev derived Hamiltonian
for H2 requires fewer gate applications than the equivalent circuit derived
from the Jordan-Wigner transformation. Since the scaling of the Bravyi-Kitaev
method is asymptotically better than the Jordan-Wigner method, this result for
molecular hydrogen in a minimal basis demonstrates the superior efficiency of
the Bravyi-Kitaev method for all quantum computations of electronic structure
Entanglement of two qubits mediated by one-dimensional plasmonic waveguides
We investigate qubit-qubit entanglement mediated by plasmons supported by
one-dimensional waveguides. We explore both the situation of spontaneous
formation of entanglement from an unentangled state and the emergence of driven
steady-state entanglement under continuous pumping. In both cases, we show that
large values for the concurrence are attainable for qubit-qubit distances
larger than the operating wavelength by using plasmonic waveguides that are
currently available.Comment: 4 pages, 4 figures. Minor Changes. Journal Reference added.
Highlighted in Physic
Spin noise and Bell inequalities in a realistic superconductor-quantum dot entangler
Charge and spin current correlations are analyzed in a source of
spin-entangled electrons built from a superconductor and two quantum dots in
parallel. In addition to the ideal (crossed Andreev) channel, parasitic
channels (direct Andreev and cotunneling) and spin flip processes are fully
described in a density matrix framework. The way they reduce both the
efficiency and the fidelity of the entangler is quantitatively described by
analyzing the zero-frequency noise correlations of charge current as well as
spin current in the two output branches. Spin current noise is characterized by
a spin Fano factor, equal to 0 (total current noise) and -1 (crossed
correlations) for an ideal entangler. The violation of the Bell inequalities,
as a test of non-locality (entanglement) of split pairs, is formulated in terms
of the correlations of electron charge and spin numbers counted in a specific
time window . The efficiency of the test is analyzed, comparing to
the various time scales in the entangler operation.Comment: 8 pages, 5 figures, references added, to appear in Phys. Rev.
N II 5668-5712, a New Class of Spectral Features in Eta Carinae
We report on the N II 5668-5712 emission and absorption lines in the spectrum
of Eta Carinae. Spectral lines of the stellar wind regions can be classified
into four physically distinct categories: 1) low-excitation emission such as H
I and Fe II, 2) higher excitation He I features, 3) the N II lines discussed in
this paper, and 4) He II emission. These categories have different combinations
of radial velocity behavior, excitation processes, and dependences on the
secondary star. The N II lines are the only known features that originate in
"normal" undisturbed zones of the primary wind but depend primarily on the
location of the hot secondary star. N II probably excludes some proposed
models, such as those where He I lines originate in the secondary star's wind
or in an accretion disk.Comment: 4 figures, 1 tabl
Two electron entanglement enhancement by an inelastic scattering process
In order to assess inelastic effects on two fermion entanglement production,
we address an exactly solvable two-particle scattering problem where the target
is an excitable scatterer. Useful entanglement, as measured by the two particle
concurrence, is obtained from post-selection of oppositely scattered particle
states. The matrix formalism is generalized in order to address non-unitary
evolution in the propagating channels. We find the striking result that
inelasticity can actually increase concurrence as compared to the elastic case
by increasing the uncertainty of the single particle subspace. Concurrence
zeros are controlled by either single particle resonance energies or total
reflection conditions that ascertain precisely one of the electron states.
Concurrence minima also occur and are controlled by entangled resonance
situations were the electron becomes entangled with the scatterer, and thus
does not give up full information of its state. In this model, exciting the
scatterer can never fully destroy phase coherence due to an intrinsic limit to
the probability of inelastic events.Comment: 8 pages, to appear in Phys. Rev
Simulating Hamiltonians in Quantum Networks: Efficient Schemes and Complexity Bounds
We address the problem of simulating pair-interaction Hamiltonians in n node
quantum networks where the subsystems have arbitrary, possibly different,
dimensions. We show that any pair-interaction can be used to simulate any other
by applying sequences of appropriate local control sequences. Efficient schemes
for decoupling and time reversal can be constructed from orthogonal arrays.
Conditions on time optimal simulation are formulated in terms of spectral
majorization of matrices characterizing the coupling parameters. Moreover, we
consider a specific system of n harmonic oscillators with bilinear interaction.
In this case, decoupling can efficiently be achieved using the combinatorial
concept of difference schemes. For this type of interactions we present optimal
schemes for inversion.Comment: 19 pages, LaTeX2
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