55 research outputs found
Symmetrization and Entanglement of Arbitrary States of Qubits
Given two arbitrary pure states and of qubits or higher
level states, we provide arguments in favor of states of the form instead of symmetric or
anti-symmetric states, as natural candidates for optimally entangled states
constructed from these states. We show that such states firstly have on the
average a high value of concurrence, secondly can be constructed by a universal
unitary operator independent of the input states. We also show that these
states are the only ones which can be produced with perfect fidelity by any
quantum operation designed for intertwining two pure states with a relative
phase. A probabilistic method is proposed for producing any pre-determined
relative phase into the combination of any two arbitrary states.Comment: 6 pages, 1 figur
Quantum phase transitions in the Kondo-necklace model: Perturbative continuous unitary transformation approach
The Kondo-necklace model can describe magnetic low-energy limit of strongly
correlated heavy fermion materials. There exist multiple energy scales in this
model corresponding to each phase of the system. Here, we study quantum phase
transition between the Kondo-singlet phase and the antiferromagnetic long-range
ordered phase, and show the effect of anisotropies in terms of quantum
information properties and vanishing energy gap. We employ the "perturbative
continuous unitary transformations" approach to calculate the energy gap and
spin-spin correlations for the model in the thermodynamic limit of one, two,
and three spatial dimensions as well as for spin ladders. In particular, we
show that the method, although being perturbative, can predict the expected
quantum critical point, where the gap of low-energy spectrum vanishes, which is
in good agreement with results of other numerical and Green's function
analyses. In addition, we employ concurrence, a bipartite entanglement measure,
to study the criticality of the model. Absence of singularities in the
derivative of concurrence in two and three dimensions in the Kondo-necklace
model shows that this model features multipartite entanglement. We also discuss
crossover from the one-dimensional to the two-dimensional model via the ladder
structure.Comment: 12 pages, 6 figure
Quantum control theory for coupled 2-electron dynamics in quantum dots
We investigate optimal control strategies for state to state transitions in a
model of a quantum dot molecule containing two active strongly interacting
electrons. The Schrodinger equation is solved nonperturbatively in conjunction
with several quantum control strategies. This results in optimized electric
pulses in the THz regime which can populate combinations of states with very
short transition times. The speedup compared to intuitively constructed pulses
is an order of magnitude. We furthermore make use of optimized pulse control in
the simulation of an experimental preparation of the molecular quantum dot
system. It is shown that exclusive population of certain excited states leads
to a complete suppression of spin dephasing, as was indicated in Nepstad et al.
[Phys. Rev. B 77, 125315 (2008)].Comment: 24 pages, 9 figure
Quantum Renormalization Group for Ground-State Fidelity
Ground-state fidelity (GSF) and quantum renormalization group theory (QRG)
have proven useful tools in the study of quantum critical systems. Here we lay
out a general, unified formalism of GSF and QRG; specifically, we propose a
method to calculate GSF through QRG, obviating the need for calculating or
approximating ground states. This method thus enhances characterization of
quantum criticality as well as scaling analysis of relevant properties with
system size. We illustrate the formalism in the one-dimensional Ising model in
a transverse field and the anisotropic spin-1/2 Heisenberg model.Comment: 5 pages and 5 figures (revised
On a suggestion relating topological and quantum mechanical entanglements
We analyze a recent suggestion \cite{kauffman1,kauffman2} on a possible
relation between topological and quantum mechanical entanglements. We show that
a one to one correspondence does not exist, neither between topologically
linked diagrams and entangled states, nor between braid operators and quantum
entanglers. We also add a new dimension to the question of entangling
properties of unitary operators in general.Comment: RevTex, 7 eps figures, to be published in Phys. Lett. A (2004
Bound entanglement in quantum phase transitions
We investigate quantum phase transitions in which a change in the type of
entanglement from bound entanglement to either free entanglement or
separability may occur. In particular, we present a theoretical method to
construct a class of quantum spin-chain Hamiltonians that exhibit this type of
quantum criticality. Given parameter-dependent two-site reduced density
matrices (with prescribed entanglement properties), we lay out a reverse
construction for a compatible pure state for the whole system, as well as a
class of Hamiltonians for which this pure state is a ground state. This
construction is illustrated through several examples.Comment: 9 pages, 8 figure
Quantum Process Tomography: Resource Analysis of Different Strategies
Characterization of quantum dynamics is a fundamental problem in quantum
physics and quantum information science. Several methods are known which
achieve this goal, namely Standard Quantum Process Tomography (SQPT),
Ancilla-Assisted Process Tomography (AAPT), and the recently proposed scheme of
Direct Characterization of Quantum Dynamics (DCQD). Here, we review these
schemes and analyze them with respect to some of the physical resources they
require. Although a reliable figure-of-merit for process characterization is
not yet available, our analysis can provide a benchmark which is necessary for
choosing the scheme that is the most appropriate in a given situation, with
given resources. As a result, we conclude that for quantum systems where
two-body interactions are not naturally available, SQPT is the most efficient
scheme. However, for quantum systems with controllable two-body interactions,
the DCQD scheme is more efficient than other known QPT schemes in terms of the
total number of required elementary quantum operations.Comment: 15 pages, 5 figures, published versio
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A functional genetic toolbox for human tissue-derived organoids.
Funder: Alzheimers Research UK Stem Cell Research CentreHuman organoid systems recapitulate key features of organs offering platforms for modelling developmental biology and disease. Tissue-derived organoids have been widely used to study the impact of extrinsic niche factors on stem cells. However, they are rarely used to study endogenous gene function due to the lack of efficient gene manipulation tools. Previously, we established a human foetal lung organoid system (Nikolić et al., 2017). Here, using this organoid system as an example we have systematically developed and optimised a complete genetic toolbox for use in tissue-derived organoids. This includes 'Organoid Easytag' our efficient workflow for targeting all types of gene loci through CRISPR-mediated homologous recombination followed by flow cytometry for enriching correctly-targeted cells. Our toolbox also incorporates conditional gene knock-down or overexpression using tightly-inducible CRISPR interference and CRISPR activation which is the first efficient application of these techniques to tissue-derived organoids. These tools will facilitate gene perturbation studies in tissue-derived organoids facilitating human disease modelling and providing a functional counterpart to many on-going descriptive studies, such as the Human Cell Atlas Project
Quantum adiabatic machine learning
We develop an approach to machine learning and anomaly detection via quantum
adiabatic evolution. In the training phase we identify an optimal set of weak
classifiers, to form a single strong classifier. In the testing phase we
adiabatically evolve one or more strong classifiers on a superposition of
inputs in order to find certain anomalous elements in the classification space.
Both the training and testing phases are executed via quantum adiabatic
evolution. We apply and illustrate this approach in detail to the problem of
software verification and validation.Comment: 21 pages, 9 figure
Distributed phase-covariant cloning with atomic ensembles via quantum Zeno dynamics
We propose an interesting scheme for distributed orbital state quantum
cloning with atomic ensembles based on the quantum Zeno dynamics. These atomic
ensembles which consist of identical three-level atoms are trapped in distant
cavities connected by a single-mode integrated optical star coupler. These
qubits can be manipulated through appropriate modulation of the coupling
constants between atomic ensemble and classical field, and the cavity decay can
be largely suppressed as the number of atoms in the ensemble qubits increases.
The fidelity of each cloned qubit can be obtained with analytic result. The
present scheme provides a new way to construct the quantum communication
network.Comment: 5 pages, 4 figure
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