41 research outputs found
The Significance of the -Numerical Range and the Local -Numerical Range in Quantum Control and Quantum Information
This paper shows how C-numerical-range related new strucures may arise from
practical problems in quantum control--and vice versa, how an understanding of
these structures helps to tackle hot topics in quantum information.
We start out with an overview on the role of C-numerical ranges in current
research problems in quantum theory: the quantum mechanical task of maximising
the projection of a point on the unitary orbit of an initial state onto a
target state C relates to the C-numerical radius of A via maximising the trace
function |\tr \{C^\dagger UAU^\dagger\}|. In quantum control of n qubits one
may be interested (i) in having U\in SU(2^n) for the entire dynamics, or (ii)
in restricting the dynamics to {\em local} operations on each qubit, i.e. to
the n-fold tensor product SU(2)\otimes SU(2)\otimes >...\otimes SU(2).
Interestingly, the latter then leads to a novel entity, the {\em local}
C-numerical range W_{\rm loc}(C,A), whose intricate geometry is neither
star-shaped nor simply connected in contrast to the conventional C-numerical
range. This is shown in the accompanying paper (math-ph/0702005).
We present novel applications of the C-numerical range in quantum control
assisted by gradient flows on the local unitary group: (1) they serve as
powerful tools for deciding whether a quantum interaction can be inverted in
time (in a sense generalising Hahn's famous spin echo); (2) they allow for
optimising witnesses of quantum entanglement. We conclude by relating the
relative C-numerical range to problems of constrained quantum optimisation, for
which we also give Lagrange-type gradient flow algorithms.Comment: update relating to math-ph/070200
Optimal control of circuit quantum electrodynamics in one and two dimensions
Optimal control can be used to significantly improve multi-qubit gates in
quantum information processing hardware architectures based on superconducting
circuit quantum electrodynamics. We apply this approach not only to dispersive
gates of two qubits inside a cavity, but, more generally, to architectures
based on two-dimensional arrays of cavities and qubits. For high-fidelity gate
operations, simultaneous evolutions of controls and couplings in the two
coupling dimensions of cavity grids are shown to be significantly faster than
conventional sequential implementations. Even under experimentally realistic
conditions speedups by a factor of three can be gained. The methods immediately
scale to large grids and indirect gates between arbitrary pairs of qubits on
the grid. They are anticipated to be paradigmatic for 2D arrays and lattices of
controllable qubits.Comment: Published version
Hamiltonian statistical mechanics
A framework for statistical-mechanical analysis of quantum Hamiltonians is
introduced. The approach is based upon a gradient flow equation in the space of
Hamiltonians such that the eigenvectors of the initial Hamiltonian evolve
toward those of the reference Hamiltonian. The nonlinear double-bracket
equation governing the flow is such that the eigenvalues of the initial
Hamiltonian remain unperturbed. The space of Hamiltonians is foliated by
compact invariant subspaces, which permits the construction of statistical
distributions over the Hamiltonians. In two dimensions, an explicit dynamical
model is introduced, wherein the density function on the space of Hamiltonians
approaches an equilibrium state characterised by the canonical ensemble. This
is used to compute quenched and annealed averages of quantum observables.Comment: 8 pages, 2 figures, references adde
Experimentally Realizable C-NOT Gate in a Flux Qubit/Resonator System
In this paper we present an experimentally realizable microwave pulse
sequence that effects a Controlled NOT (C-NOT) gate operation on a Josephson
junction-based flux-qubit/resonator system with high fidelity in the end state.
We obtained a C-NOT gate process fidelity of 0.988 (0.980) for a two (three)
qubit/resonator system under ideal conditions, and a fidelity of 0.903 for a
two qubit/resonator system under the best, currently achieved, experimental
conditions. In both cases, we found that "qubit leakage" to higher levels of
the resonator causes a majority of the loss of fidelity, and that such leakage
becomes more pronounced as decoherence effects increase.Comment: 7 pages, 4 figures, submitted to Phys. Rev.
Control aspects of quantum computing using pure and mixed states
Steering quantum dynamics such that the target states solve classically hard problems is paramount to quantum simulation and computation. And beyond, quantum control is also essential to pave the way to quantum technologies. Here, important control techniques are reviewed and presented in a unified frame covering quantum computational gate synthesis and spectroscopic state transfer alike. We emphasize that it does not matter whether the quantum states of interest are pure or not. While pure states underly the design of quantum circuits, ensemble mixtures of quantum states can be exploited in a more recent class of algorithms: it is illustrated by characterizing the Jones polynomial in order to distinguish between different (classes of) knots. Further applications include Josephson elements, cavity grids, ion traps and nitrogen vacancy centres in scenarios of closed as well as open quantum systems