66 research outputs found
Characterization and Control of Quantum Spin Chains and Rings
Information flow in quantum spin networks is considered. Two types of control
-- temporal bang-bang switching control and control by varying spatial degrees
of freedom -- are explored and shown to be effective in speeding up information
transfer and increasing transfer fidelities. The control is model-based and
therefore relies on accurate knowledge of the system parameters. An efficient
protocol for simultaneous identification of the coupling strength and the exact
number of spins in a chain is presented.Comment: to appear in ISCCSP 201
Stabilizing Quantum States by Constructive Design of Open Quantum Dynamics
Based on recent work on the asymptotic behavior of controlled quantum
Markovian dynamics, we show that any generic quantum state can be stabilized by
devising constructively a simple Lindblad-GKS generator that can achieve global
asymptotic stability at the desired state. The applications of such result is
demonstrated by designing a direct feedback strategy that achieves global
stabilization of a qubit state encoded in a noise-protected subspace.Comment: Revised version with stronger proofs showing uniqueness can be
achieved in all cases by using the freedom to the choose diagonal elements of
both the Hamiltonian and Lindblad operator, and exploiting the fact that the
non-existence of two orthogonal eigenvectors of the Lindblad operator is
sufficient but not necessary for global asymptotic stability of the target
stat
Symmetry & Controllability for Spin Networks with a Single-Node Control
We consider the relation of symmetries and subspace controllability for spin
networks with XXZ coupling subject to control of a single node by a local
potential (Z-control). Such networks decompose into excitation subspaces.
Focusing on the single excitation subspace it is shown that for single-node
Z-controls external symmetries are characterized by eigenstates of the system
Hamiltonian that have zero overlap with the control node, and there are no
internal symmetries. It is further shown that there are symmetries that persist
even in the presence of random perturbations. For uniformly coupled XXZ chains
a characterization of all possible symmetries is given, which shows a strong
dependence on the position of the node we control. Finally, it is shown
rigorously for uniform Heisenberg and XX chains subject to single-node
Z-control that the lack of symmetry is not only necessary but sufficient for
subspace controllability. The latter approach is then generalized to establish
controllability results for simple branched networks.Comment: 11 pages, some figures. 3 tables, minor revisio
Global Control Methods for GHZ State Generation on 1-D Ising Chain
We discuss how to prepare an Ising chain in a GHZ state using a single global
control field only. This model does not require the spins to be individually
addressable and is applicable to quantum systems such as cold atoms in optical
lattices, some liquid- or solid-state NMR experiments, and many nano-scale
quantum structures. We show that GHZ states can always be reached
asymptotically from certain easy-to-prepare initial states using adiabatic
passage, and under certain conditions finite-time reachability can be ensured.
To provide a reference useful for future experimental implementations three
different control strategies to achieve the objective, adiabatic passage,
Lyapunov control and optimal control are compared, and their advantages and
disadvantages discussed, in particular in the presence of realistic
imperfections such as imperfect initial state preparation, system inhomogeneity
and dephasing.Comment: 13 pages, 11 figure
Quantum Control of Two-Qubit Entanglement Dissipation
We investigate quantum control of the dissipation of entanglement under
environmental decoherence. We show by means of a simple two-qubit model that
standard control methods - coherent or open-loop control - will not in general
prevent entanglement loss. However, we propose a control method utilising a
Wiseman-Milburn feedback/measurement control scheme which will effectively
negate environmental entanglement dissipation.Comment: 11 pages,4 figures, minor correctio
Fundamental Speed Limits on Quantum Coherence and Correlation Decay
The study and control of coherence in quantum systems is one of the most
exciting recent developments in physics. Quantum coherence plays a crucial role
in emerging quantum technologies as well as fundamental experiments. A major
obstacle to the utilization of quantum effects is decoherence, primarily in the
form of dephasing that destroys quantum coherence, and leads to effective
classical behaviour. We show that there are universal relationships governing
dephasing, which constrain the relative rates at which quantum correlations can
disappear. These effectively lead to speed limits which become especially
important in multi-partite systems
Quantum Control Theory for State Transformations: Dark States and their Enlightenment
For many quantum information protocols such as state transfer, entanglement
transfer and entanglement generation, standard notions of controllability for
quantum systems are too strong. We introduce the weaker notion of accessible
pairs, and prove an upper bound on the achievable fidelity of a transformation
between a pair of states based on the symmetries of the system. A large class
of spin networks is presented for which this bound can be saturated. In this
context, we show how the inaccessible dark states for a given
excitation-preserving evolution can be calculated, and illustrate how some of
these can be accessed using extra catalytic excitations. This emphasises that
it is not sufficient for analyses of state transfer in spin networks to
restrict to the single excitation subspace. One class of symmetries in these
spin networks is exactly characterised in terms of the underlying graph
properties.Comment: 14 pages, 3 figures v3: rewritten for increased clarit
Investigation on vanadium chemistry in basic-oxygen-furnace (BOF) slags: a first approach
Basic oxygen furnace (BOF) slag accounts for the majority of all residual materials produced during steelmaking and may typically contain certain transition metals. Vanadium, in particular, came
into focus in recent years because of its potential environmental toxicity as well as its economic value.
This study addresses the vanadium chemistry in BOF slags to better understand its recovery and save
handling of the waste stream. The experimental results from the electron probe microanalysis (EPMA)
study show that vanadium is preferably incorporated in calcium orthosilicate-like compounds (COS), with two variations occurring, a low vanadium COS (COS-Si) (approx. 1 wt.%), and a high vanadium COS (COS-V) (up to 18 wt.%). Additionally, vanadium is incorporated in dicalcium ferrite-like compounds (DFS) with an average amount of 3 wt.%. Using powder x-ray diffraction analysis (PXRD), EPMA, and virtual component models, stoichiometric formulas of the main vanadium-bearing phases were postulated. The stoichiometries give an estimate of the oxidation states of vanadium in the respective hosts. According to these results, trivalent vanadium is incorporated on the Fe-position in dicalcium ferrite solid solution (DFS), and V4+ and V5+ are incorporated on the
Si-position of the COS
Structured Singular Value Analysis for Spintronics Network Information Transfer Control
Control laws for selective transfer of information encoded in excitations of a quantum network, based on shaping the energy landscape using time-invariant, spatially-varying bias fields, can be successfully designed using numerical optimization. Such control laws, already departing from classicality by replacing closed-loop asymptotic stability with alternative notions of localization, have the intriguing property that for all practical purposes they achieve the upper bound on the fidelity, yet the (logarithmic) sensitivity of the fidelity to such structured perturbation as spin coupling errors and bias field leakages is nearly vanishing. Here, these differential sensitivity results are extended to large structured variations using -design tools to reveal a crossover region in the space of controllers where objectives usually thought to be conflicting are actually concordant
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