2,350 research outputs found
Complete controllability of finite-level quantum systems
Complete controllability is a fundamental issue in the field of control of
quantum systems, not least because of its implications for dynamical
realizability of the kinematical bounds on the optimization of observables. In
this paper we investigate the question of complete controllability for
finite-level quantum systems subject to a single control field, for which the
interaction is of dipole form. Sufficient criteria for complete controllability
of a wide range of finite-level quantum systems are established and the
question of limits of complete controllability is addressed. Finally, the
results are applied to give a classification of complete controllability for
four-level systems.Comment: 14 pages, IoP-LaTe
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
Degrees of controllability for quantum systems and applications to atomic systems
Precise definitions for different degrees of controllability for quantum
systems are given, and necessary and sufficient conditions are discussed. The
results are applied to determine the degree of controllability for various
atomic systems with degenerate energy levels and transition frequencies.Comment: 20 pages, IoP LaTeX, revised and expanded versio
On the Contractivity of Hilbert-Schmidt distance under open system dynamics
We show that the Hilbert-Schmidt distance, unlike the trace distance, between
quantum states is generally not monotonic for open quantum systems subject to
Lindblad semigroup dynamics. Sufficient conditions for contractivity of the
Hilbert-Schmidt norm in terms of the dissipation generators are given. Although
these conditions are not necessary, simulations suggest that non-contractivity
is the typical case, i.e., that systems for which the Hilbert-Schmidt distance
between quantum states is monotonically decreasing form only a small set of all
possible dissipative systems for N>2, in contrast to the case N=2 where the
Hilbert-Schmidt distance is always monotonically decreasing.Comment: Major revision. We would particularly like to thank D Perez-Garcia
for constructive feedbac
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
NuSTAR hard X-ray data and Gemini 3D spectra reveal powerful AGN and outflow histories in two low-redshift Lyman- blobs
We have shown that Lyman- blobs (LABs) may still exist even at
, about 7 billion years later than most other LABs known (Schirmer et
al. 2016). Their luminous Ly and [OIII] emitters at offer
new insights into the ionization mechanism. This paper focuses on the two X-ray
brightest LABs at , SDSS J01130106 (J0113) and SDSS J11550147
(J1155), comparable in size and luminosity to `B1', one of the best-studied
LABs at 2. Our NuSTAR hard X-ray (3--30 keV) observations reveal
powerful active galactic nuclei (AGN) with -- erg cm s. J0113 also faded by a
factor of between 2014 and 2016, emphasizing that variable AGN may
cause apparent ionization deficits in LABs. Joint spectral analyses including
Chandra data constrain column densities of cm (J0113) and cm (J1155). J0113 is likely buried in
a torus with a narrow ionization cone, but ionizing radiation is also leaking
in other directions as revealed by our Gemini/GMOS 3D spectroscopy. The latter
shows a bipolar outflow over kpc, with a peculiar velocity profile that is
best explained by AGN flickering. X-ray analysis of J1155 reveals a weakly
absorbed AGN that may ionize over a wide solid angle, consistent with our 3D
spectra. Extinction corrected [OIII] log-luminosities are high, . The
velocity dispersions are low, -- km s, even at the AGN
positions. We argue that this is a combination of high extinction hiding the
turbulent gas, and previous outflows that have cleared the escape paths for
their successors.Comment: 15 pages, 17 Figures, accepted for publication in Ap
Subspace confinement : how good is your qubit?
The basic operating element of standard quantum computation is the qubit, an isolated two-level system that can be accurately controlled, initialized and measured. However, the majority of proposed physical architectures for quantum computation are built from systems that contain much more complicated Hilbert space structures. Hence, defining a qubit requires the identification of an appropriate controllable two-dimensional sub-system. This prompts the obvious question of how well a qubit, thus defined, is confined to this subspace, and whether we can experimentally quantify the potential leakage into states outside the qubit subspace. We demonstrate how subspace leakage can be characterized using minimal theoretical assumptions by examining the Fourier spectrum of the oscillation experiment
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