30,039 research outputs found
Quantum parallelism of the controlled-NOT operation: an experimental criterion for the evaluation of device performance
It is shown that a quantum controlled-NOT gate simultaneously performs the
logical functions of three distinct conditional local operations. Each of these
local operations can be verified by measuring a corresponding truth table of
four local inputs and four local outputs. The quantum parallelism of the gate
can then be observed directly in a set of three simple experimental tests, each
of which has a clear intuitive interpretation in terms of classical logical
operations. Specifically, quantum parallelism is achieved if the average
fidelity of the three classical operations exceeds 2/3. It is thus possible to
evaluate the essential quantum parallelism of an experimental controlled-NOT
gate by testing only three characteristic classical operations performed by the
gate.Comment: 6 pages, no figures, added references and discussio
Looking for meson molecules in B decays
We discuss the possibility of observing a loosely bound molecular state in a
B three-body hadronic decay. In particular we use the QCD sum rule approach to
study a molecular current. We consider an isovector-scalar
molecular current and we use the two-point and
three-point functions to study the mass and decay width of such state. We
consider the contributions of condensates up to dimension six and we work at
leading order in . We obtain a mass around 1.1 GeV, consistent with a
loosely bound state, and a decay width
around 10 MeV.Comment: 7 pages, 8 figure
Distillation of local purity from quantum states
Recently Horodecki et al. [Phys. Rev. Lett. 90, 100402 (2003)] introduced an
important quantum information processing paradigm, in which two parties sharing
many copies of the same bipartite quantum state distill local pure states, by
means of local unitary operations assisted by a one-way (two-way) completely
dephasing channel. Local pure states are a valuable resource from a
thermodynamical point of view, since they allow thermal energy to be converted
into work by local quantum heat engines. We give a simple
information-theoretical characterization of the one-way distillable local
purity, which turns out to be closely related to a previously known operational
measure of classical correlations, the one-way distillable common randomness.Comment: 8 page
Frustration, interaction strength and ground-state entanglement in complex quantum systems
Entanglement in the ground state of a many-body quantum system may arise when
the local terms in the system Hamiltonian fail to commute with the interaction
terms in the Hamiltonian. We quantify this phenomenon, demonstrating an analogy
between ground-state entanglement and the phenomenon of frustration in spin
systems. In particular, we prove that the amount of ground-state entanglement
is bounded above by a measure of the extent to which interactions frustrate the
local terms in the Hamiltonian. As a corollary, we show that the amount of
ground-state entanglement is bounded above by a ratio between parameters
characterizing the strength of interactions in the system, and the local energy
scale. Finally, we prove a qualitatively similar result for other energy
eigenstates of the system.Comment: 11 pages, 3 figure
DMRG and periodic boundary conditions: a quantum information perspective
We introduce a picture to analyze the density matrix renormalization group
(DMRG) numerical method from a quantum information perspective. This leads us
to introduce some modifications for problems with periodic boundary conditions
in which the results are dramatically improved. The picture also explains some
features of the method in terms of entanglement and teleportation.Comment: 4 page
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
Collapse of the quantum correlation hierarchy links entropic uncertainty to entanglement creation
Quantum correlations have fundamental and technological interest, and hence
many measures have been introduced to quantify them. Some hierarchical
orderings of these measures have been established, e.g., discord is bigger than
entanglement, and we present a class of bipartite states, called premeasurement
states, for which several of these hierarchies collapse to a single value.
Because premeasurement states are the kind of states produced when a system
interacts with a measurement device, the hierarchy collapse implies that the
uncertainty of an observable is quantitatively connected to the quantum
correlations (entanglement, discord, etc.) produced when that observable is
measured. This fascinating connection between uncertainty and quantum
correlations leads to a reinterpretation of entropic formulations of the
uncertainty principle, so-called entropic uncertainty relations, including ones
that allow for quantum memory. These relations can be thought of as
lower-bounds on the entanglement created when incompatible observables are
measured. Hence, we find that entanglement creation exhibits complementarity, a
concept that should encourage exploration into "entanglement complementarity
relations".Comment: 19 pages, 2 figures. Added Figure 1 and various remarks to improve
clarity of presentatio
Distance measures to compare real and ideal quantum processes
With growing success in experimental implementations it is critical to
identify a "gold standard" for quantum information processing, a single measure
of distance that can be used to compare and contrast different experiments. We
enumerate a set of criteria such a distance measure must satisfy to be both
experimentally and theoretically meaningful. We then assess a wide range of
possible measures against these criteria, before making a recommendation as to
the best measures to use in characterizing quantum information processing.Comment: 15 pages; this version in line with published versio
Entanglement and Quantum Phase Transition Revisited
We show that, for an exactly solvable quantum spin model, a discontinuity in
the first derivative of the ground state concurrence appears in the absence of
quantum phase transition. It is opposed to the popular belief that the
non-analyticity property of entanglement (ground state concurrence) can be used
to determine quantum phase transitions. We further point out that the
analyticity property of the ground state concurrence in general can be more
intricate than that of the ground state energy. Thus there is no one-to-one
correspondence between quantum phase transitions and the non-analyticity
property of the concurrence. Moreover, we show that the von Neumann entropy, as
another measure of entanglement, can not reveal quantum phase transition in the
present model. Therefore, in order to link with quantum phase transitions, some
other measures of entanglement are needed.Comment: RevTeX 4, 4 pages, 1 EPS figures. some modifications in the text.
Submitted to Phys. Rev.
Quantum computation with unknown parameters
We show how it is possible to realize quantum computations on a system in
which most of the parameters are practically unknown. We illustrate our results
with a novel implementation of a quantum computer by means of bosonic atoms in
an optical lattice. In particular we show how a universal set of gates can be
carried out even if the number of atoms per site is uncertain.Comment: 3 figure
- …