1,904 research outputs found
Quantum measurement of a mesoscopic spin ensemble
We describe a method for precise estimation of the polarization of a
mesoscopic spin ensemble by using its coupling to a single two-level system.
Our approach requires a minimal number of measurements on the two-level system
for a given measurement precision. We consider the application of this method
to the case of nuclear spin ensemble defined by a single electron-charged
quantum dot: we show that decreasing the electron spin dephasing due to nuclei
and increasing the fidelity of nuclear-spin-based quantum memory could be
within the reach of present day experiments.Comment: 8 pages, 2 figures; minor changes, published versio
Mental, Social and Visual Alienation in D’Alessandro’s Photography
This chapter analyzes the first of several photobooks that illustrated the reform of psychiatric health care in Italy in the 1960s and 1970s: Luciano D’Alessandro’s 1969 Gli esclusi. In 1967, D’Alessandro was invited by the director of the asylum of Nocera Superiore, Sergio Piro, to document through photography the abysmal conditions of the “total institution” that was the pre-reform mental hospital. D’Alessandro first published a small selection of photos, in Popular Photography Italiana (1967), which he then expanded in Gli esclusi. This chapter claims that, in the evolution between the two publications, we can read the complex and multilayered notion of alienation that informed the work of reform, especially that of one of the most famous figures associated with it, Franco Basaglia. By analyzing D’Alessandro’s Gli esclusi through the notion of alienation, this chapter lets what Sekula calls the conditions of “readability” of the photographic message emerge
Time-optimal rotation of a spin 1/2: application to the NV center spin in diamond
We study the applicability of the time optimal bang-bang control designed for
spin-1/2 [U. Boscain and P. Mason, J. Math. Phys. {\bf 47}, 062101 (2006)] to
the rotation of the electron spin of a nitrogen-vacancy (NV) center in diamond.
The spin of the NV center is a three-level system, with two levels forming a
relevant qubit subspace where the time-varying magnetic control field performs
rotation, and the third level being idle. We find that the bang-bang control
protocol decreases the rotation time by 20--25% in comparison with the
traditional oscillating sinusoidal driving. We also find that for most values
of the bias field the leakage to the idle level is very small, so that the NV
center is a suitable testbed for experimental study of the time-optimal
protocols. For some special values of the bias field, however, the unwanted
leakage is greatly increased. We demonstrate that this is caused by the
resonance with higher-order Fourier harmonics of the bang-bang driving field.Comment: 6 pages, 4 figure
Quantum-to-Classical Correspondence and Hubbard-Stratonovich Dynamical Systems, a Lie-Algebraic Approach
We propose a Lie-algebraic duality approach to analyze non-equilibrium
evolution of closed dynamical systems and thermodynamics of interacting quantum
lattice models (formulated in terms of Hubbard-Stratonovich dynamical systems).
The first part of the paper utilizes a geometric Hilbert-space-invariant
formulation of unitary time-evolution, where a quantum Hamiltonian is viewed as
a trajectory in an abstract Lie algebra, while the sought-after evolution
operator is a trajectory in a dynamic group, generated by the algebra via
exponentiation. The evolution operator is uniquely determined by the
time-dependent dual generators that satisfy a system of differential equations,
dubbed here dual Schrodinger-Bloch equations, which represent a viable
alternative to the conventional Schrodinger formulation. These dual
Schrodinger-Bloch equations are derived and analyzed on a number of specific
examples. It is shown that deterministic dynamics of a closed classical
dynamical system occurs as action of a symmetry group on a classical manifold
and is driven by the same dual generators as in the corresponding quantum
problem. This represents quantum-to-classical correspondence. In the second
part of the paper, we further extend the Lie algebraic approach to a wide class
of interacting many-particle lattice models. A generalized Hubbard-Stratonovich
transform is proposed and it is used to show that the thermodynamic partition
function of a generic many-body quantum lattice model can be expressed in terms
of traces of single-particle evolution operators governed by the dynamic
Hubbard-Stratonovich fields. Finally, we derive Hubbard-Stratonovich dynamical
systems for the Bose-Hubbard model and a quantum spin model and use the
Lie-algebraic approach to obtain new non-perturbative dual descriptions of
these theories.Comment: 25 pages, 1 figure; v2: citations adde
Constructing General Unitary Maps from State Preparations
We present an efficient algorithm for generating unitary maps on a
-dimensional Hilbert space from a time-dependent Hamiltonian through a
combination of stochastic searches and geometric construction. The protocol is
based on the eigen-decomposition of the map. A unitary matrix can be
implemented by sequentially mapping each eigenvector to a fiducial state,
imprinting the eigenphase on that state, and mapping it back to the
eigenvector. This requires the design of only state-to-state maps generated
by control waveforms that are efficiently found by a gradient search with
computational resources that scale polynomially in . In contrast, the
complexity of a stochastic search for a single waveform that simultaneously
acts as desired on all eigenvectors scales exponentially in . We extend this
construction to design maps on an -dimensional subspace of the Hilbert space
using only stochastic searches. Additionally, we show how these techniques
can be used to control atomic spins in the ground electronic hyperfine manifold
of alkali atoms in order to implement general qudit logic gates as well to
perform a simple form of error correction on an embedded qubit.Comment: 9 pages, 3 figure
Dynamical Quantum Error Correction of Unitary Operations with Bounded Controls
Dynamically corrected gates were recently introduced [Khodjasteh and Viola,
Phys. Rev. Lett. 102, 080501 (2009)] as a tool to achieve decoherence-protected
quantum gates based on open-loop Hamiltonian engineering. Here, we further
expand the framework of dynamical quantum error correction, with emphasis on
elucidating under what conditions decoherence suppression can be ensured while
performing a generic target quantum gate, using only available bounded-strength
control resources. Explicit constructions for physically relevant error models
are detailed, including arbitrary linear decoherence and pure dephasing on
qubits. The effectiveness of dynamically corrected gates in an illustrative
non-Markovian spin-bath setting is investigated numerically, confirming the
expected fidelity performance in a wide parameter range. Robutness against a
class of systematic control errors is automatically incorporated in the
perturbative error regime.Comment: 21 pages, 7 figures (errors fixed, figures added, text updated
Quantum control theory for coupled 2-electron dynamics in quantum dots
We investigate optimal control strategies for state to state transitions in a
model of a quantum dot molecule containing two active strongly interacting
electrons. The Schrodinger equation is solved nonperturbatively in conjunction
with several quantum control strategies. This results in optimized electric
pulses in the THz regime which can populate combinations of states with very
short transition times. The speedup compared to intuitively constructed pulses
is an order of magnitude. We furthermore make use of optimized pulse control in
the simulation of an experimental preparation of the molecular quantum dot
system. It is shown that exclusive population of certain excited states leads
to a complete suppression of spin dephasing, as was indicated in Nepstad et al.
[Phys. Rev. B 77, 125315 (2008)].Comment: 24 pages, 9 figure
Investigation of interactions between poly-l-lysine-coated boron nitride nanotubes and C2C12 cells: up-take, cytocompatibility, and differentiation
Boron nitride nanotubes (BNNTs) have generated considerable interest within the scientific community by virtue of their unique physical properties, which can be exploited in the biomedical field. In the present in vitro study, we investigated the interactions of poly-l-lysine-coated BNNTs with C2C12 cells, as a model of muscle cells, in terms of cytocompatibility and BNNT internalization. The latter was performed using both confocal and transmission electron microscopy. Finally, we investigated myoblast differentiation in the presence of BNNTs, evaluating the protein synthesis of differentiating cells, myotube formation, and expression of some constitutive myoblastic markers, such as MyoD and Cx43, by reverse transcription – polymerase chain reaction and Western blot analysis. We demonstrated that BNNTs are highly internalized by C2C12 cells, with neither adversely affecting C2C12 myoblast viability nor significantly interfering with myotube formation
Urban public health, a multidisciplinary approach
Urban environment is a highly complex interactive socio-physical system, with competing expectations and priorities. Public health interventions have always had a fundamental role in the control of diseases in cities. WHO considers urbanization as one of the key challenges for public health in the twenty-first century, since cities offer significant opportunities to improve public health if health-enhancing policies and actions are promoted. A multidisciplinary approach is required, but the basic differences existing between technical and health disciplines make the interaction difficult. The multidisciplinary collaboration is still at a very early stage of development, and needs to be further understood and planned. The author concludes stressing the need for a transversal training, but also for sharing knowledge, instruments and methods, involving all the actors in the planning process, to develop a real multidisciplinary approach
Hamiltonian purification
The problem of Hamiltonian purification introduced by Burgarth et al. [D. K.
Burgarth et al., Nat. Commun. 5, 5173 (2014)] is formalized and discussed.
Specifically, given a set of non-commuting Hamiltonians {h1, . . ., hm}
operating on a d-dimensional quantum system Hd, the problem consists in
identifying a set of commuting Hamiltonians {H1,...,Hm} operating on a larger
dE-dimensional system H_{dE} which embeds H_d as a proper subspace, such that
hj = PHjP with P being the projection which allows one to recover Hd from HdE .
The notions of spanning-set purification and generator purification of an
algebra are also introduced and optimal solutions for u(d) are provided.Comment: 13 pages, 3 figure
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