35 research outputs found
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
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
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
Robust Control Performance for Open Quantum Systems
The robustness of quantum control in the presence of uncertainties is
important for practical applications but their quantum nature poses many
challenges for traditional robust control. In addition to uncertainties in the
system and control Hamiltonians and initial state preparation, there is
uncertainty about interactions with the environment leading to decoherence.
This paper investigates the robust performance of control schemes for open
quantum systems subject to such uncertainties. A general formalism is
developed, where performance is measured based on the transmission of a dynamic
perturbation or initial state preparation error to a final density operator
error. This formulation makes it possible to apply tools from classical robust
control, especially structured singular value analysis, to assess robust
performance of controlled, open quantum systems. However, there are additional
difficulties that must be overcome, especially at low frequency ().
For example, at , the Bloch equations for the density operator are
singular, and this causes lack of continuity of the structured singular value.
We address this issue by analyzing the dynamics on invariant subspaces and
defining a pseudo-inverse that enables us to formulate a specialized version of
the matrix inversion lemma. The concepts are demonstrated with an example of
two qubits in a leaky cavity under laser driving fields and spontaneous
emission. In addition, a new performance index is introduced for this system.
Instead of the tracking or transfer fidelity error, performance is measured by
the steady-steady entanglement generated, which is quantified by a non-linear
function of the system state called concurrence. Simulations show that there is
no conflict between this performance index, its log-sensitivity and stability
margin under decoherence, unlike for conventional control problems [...].Comment: 12 pages, 5 figures, 2 table
Design of Feedback Control Laws for Information Transfer in Spintronics Networks
Information encoded in networks of stationary, interacting spin-1/2 particles is central for many applications ranging from quantum spintronics to quantum information processing. Without control, however, information transfer through such networks is generally inefficient. \new{Currently available control methods to maximize the transfer fidelities and speeds mainly rely on dynamic control using time-varying fields and often assume instantaneous readout. We present an alternative approach to achieving} efficient, high-fidelity transfer of excitations by shaping the energy landscape via the design of time-invariant feedback control laws without recourse to dynamic control. \new{Both instantaneous readout and the more realistic case of finite readout windows are considered. The technique can also be used to freeze information by designing energy landscapes that achieve Anderson localization.} Perfect state or super-optimal transfer and localization are enabled by conditions on the eigenstructure of the system and signature properties for the eigenvectors. Given the eigenstructure enabled by super-optimality, it is shown that feedback controllers that achieve perfect state transfer are, surprisingly, also the most robust with regard to uncertainties in the system and control parameters
Applying classical control techniques to quantum systems: entanglement versus stability margin and other limitations
Development of robust quantum control has been challenging and there are numerous obstacles to applying classical robust control to quantum system including bilinearity, marginal stability, state preparation errors, nonlinear figures of merit. The requirement of marginal stability, while not satisfied for closed quantum systems, can be satisfied for open quantum systems where Lindbladian behavior leads to non-unitary evolution, and allows for nonzero classical stability margins, but it remains difficult to extract physical insight when classical robust control tools are applied to these systems. We consider a straightforward example of the entanglement between two qubits dissipatively coupled to a lossy cavity and analyze it using the classical stability margin and structured perturbations. We attempt, where possible, to extract physical insight from these analyses. Our aim is to highlight where classical robust control can assist in the analysis of quantum systems and identify areas where more work needs to be done to develop specific methods for quantum robust control