237,182 research outputs found
Real-time Information, Uncertainty and Quantum Feedback Control
Feedback is the core concept in cybernetics and its effective use has made
great success in but not limited to the fields of engineering, biology, and
computer science. When feedback is used to quantum systems, two major types of
feedback control protocols including coherent feedback control (CFC) and
measurement-based feedback control (MFC) have been developed. In this paper, we
compare the two types of quantum feedback control protocols by focusing on the
real-time information used in the feedback loop and the capability in dealing
with parameter uncertainty. An equivalent relationship is established between
quantum CFC and non-selective quantum MFC in the form of operator-sum
representation. Using several examples of quantum feedback control, we show
that quantum MFC can theoretically achieve better performance than quantum CFC
in stabilizing a quantum state and dealing with Hamiltonian parameter
uncertainty. The results enrich understanding of the relative advantages
between quantum MFC and quantum CFC, and can provide useful information in
choosing suitable feedback protocols for quantum systems.Comment: 24 page
Variational Optimal-Control Problems with Delayed Arguments on Time Scales
This article deals with variational optimal-control problems on time scales
in the presence of delay in the state variables. The problem is considered on a
time scale unifying the discrete, the continuous and the quantum cases. Two
examples in the discrete and quantum cases are analyzed to illustrate our
results.Comment: To apear in Advances in Difference Equation
Sampling-based Learning Control for Quantum Systems with Uncertainties
Robust control design for quantum systems has been recognized as a key task
in the development of practical quantum technology. In this paper, we present a
systematic numerical methodology of sampling-based learning control (SLC) for
control design of quantum systems with uncertainties. The SLC method includes
two steps of "training" and "testing". In the training step, an augmented
system is constructed using artificial samples generated by sampling
uncertainty parameters according to a given distribution. A gradient flow based
learning algorithm is developed to find the control for the augmented system.
In the process of testing, a number of additional samples are tested to
evaluate the control performance where these samples are obtained through
sampling the uncertainty parameters according to a possible distribution. The
SLC method is applied to three significant examples of quantum robust control
including state preparation in a three-level quantum system, robust
entanglement generation in a two-qubit superconducting circuit and quantum
entanglement control in a two-atom system interacting with a quantized field in
a cavity. Numerical results demonstrate the effectiveness of the SLC approach
even when uncertainties are quite large, and show its potential for robust
control design of quantum systems.Comment: 11 pages, 9 figures, in press, IEEE Transactions on Control Systems
Technology, 201
Coupling Superconducting Qubits via a Cavity Bus
Superconducting circuits are promising candidates for constructing quantum
bits (qubits) in a quantum computer; single-qubit operations are now routine,
and several examples of two qubit interactions and gates having been
demonstrated. These experiments show that two nearby qubits can be readily
coupled with local interactions. Performing gates between an arbitrary pair of
distant qubits is highly desirable for any quantum computer architecture, but
has not yet been demonstrated. An efficient way to achieve this goal is to
couple the qubits to a quantum bus, which distributes quantum information among
the qubits. Here we show the implementation of such a quantum bus, using
microwave photons confined in a transmission line cavity, to couple two
superconducting qubits on opposite sides of a chip. The interaction is mediated
by the exchange of virtual rather than real photons, avoiding cavity induced
loss. Using fast control of the qubits to switch the coupling effectively on
and off, we demonstrate coherent transfer of quantum states between the qubits.
The cavity is also used to perform multiplexed control and measurement of the
qubit states. This approach can be expanded to more than two qubits, and is an
attractive architecture for quantum information processing on a chip.Comment: 6 pages, 4 figures, to be published in Natur
Optimal path for a quantum teleportation protocol in entangled networks
Bellman's optimality principle has been of enormous importance in the
development of whole branches of applied mathematics, computer science, optimal
control theory, economics, decision making, and classical physics. Examples are
numerous: dynamic programming, Markov chains, stochastic dynamics, calculus of
variations, and the brachistochrone problem. Here we show that Bellman's
optimality principle is violated in a teleportation problem on a quantum
network. This implies that finding the optimal fidelity route for teleporting a
quantum state between two distant nodes on a quantum network with bi-partite
entanglement will be a tough problem and will require further investigation.Comment: 4 pages, 1 figure, RevTeX
Quantum interference in optical fields and atomic radiation
We discuss the connection between quantum interference effects in optical
beams and radiation fields emitted from atomic systems. We illustrate this
connection by a study of the first- and second-order correlation functions of
optical fields and atomic dipole moments. We explore the role of correlations
between the emitting systems and present examples of practical methods to
implement two systems with non-orthogonal dipole moments. We also derive
general conditions for quantum interference in a two-atom system and for a
control of spontaneous emission. The relation between population trapping and
dark states is also discussed. Moreover, we present quantum dressed-atom models
of cancellation of spontaneous emission, amplification on dark transitions,
fluorescence quenching and coherent population trapping.Comment: To be published in Journal of Modern Optics Special Issue on Quantum
Interferenc
Effects of uncertainties and errors on Lyapunov control
Lyapunov control (open-loop) is often confronted with uncertainties and
errors in practical applications. In this paper, we analyze the robustness of
Lyapunov control against the uncertainties and errors in quantum control
systems. The analysis is carried out through examinations of uncertainties and
errors, calculations of the control fidelity under influences of the
certainties and errors, as well as discussions on the caused effects. Two
examples, a closed control system and an open control system, are presented to
illustrate the general formulism.Comment: 4 pages, 5 figure
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