2,621 research outputs found
Experimental Hamiltonian identification for controlled two-level systems
We present a strategy to empirically determine the internal and control Hamiltonians for an unknown two-level system (black box) subject to various (piecewise constant) control fields when direct readout by measurement is limited to a single, fixed observable
Orbits of quantum states and geometry of Bloch vectors for -level systems
Physical constraints such as positivity endow the set of quantum states with
a rich geometry if the system dimension is greater than two. To shed some light
on the complicated structure of the set of quantum states, we consider a
stratification with strata given by unitary orbit manifolds, which can be
identified with flag manifolds. The results are applied to study the geometry
of the coherence vector for n-level quantum systems. It is shown that the
unitary orbits can be naturally identified with spheres in R^{n^2-1} only for
n=2. In higher dimensions the coherence vector only defines a non-surjective
embedding into a closed ball. A detailed analysis of the three-level case is
presented. Finally, a refined stratification in terms of symplectic orbits is
considered.Comment: 15 pages LaTeX, 3 figures, reformatted, slightly modified version,
corrected eq.(3), to appear in J. Physics
Control of non-controllable quantum systems: A quantum control algorithm based on Grover iteration
A new notion of controllability, eigenstate controllability, is defined for
finite-dimensional bilinear quantum mechanical systems which are neither
strongly completely controllably nor completely controllable. And a quantum
control algorithm based on Grover iteration is designed to perform a quantum
control task of steering a system, which is eigenstate controllable but may not
be (strongly) completely controllable, from an arbitrary state to a target
state.Comment: 7 pages, no figures, submitte
Controlled phase gate for solid-state charge qubits
We describe a mechanism for realizing a controlled phase gate for solid-state
charge qubits. By augmenting the positionally defined qubit with an auxiliary
state, and changing the charge distribution in the three-dot system, we are
able to effectively switch the Coulombic interaction, effecting an entangling
gate. We consider two architectures, and numerically investigate their
robustness to gate noise.Comment: 14 pages, 11 figures, 2 tables, RevTeX
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
Experimental Hamiltonian Identification for Qubits subject to Multiple Independent Control Mechanisms
We consider a qubit subject to various independent control mechanisms and
present a general strategy to identify both the internal Hamiltonian and the
interaction Hamiltonian for each control mechanism, relying only on a single,
fixed readout process such as measurements.Comment: submitted to Proceedings of the QCMC04 (4 pages RevTeX, 5 figures
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