1,859 research outputs found
Oscillations of the purity in the repeated-measurement-based generation of quantum states
Repeated observations of a quantum system interacting with another one can
drive the latter toward a particular quantum state, irrespectively of its
initial condition, because of an {\em effective non-unitary evolution}. If the
target state is a pure one, the degree of purity of the system approaches
unity, even when the initial condition of the system is a mixed state. In this
paper we study the behavior of the purity from the initial value to the final
one, that is unity. Depending on the parameters, after a finite number of
measurements, the purity exhibits oscillations, that brings about a lower
purity than that of the initial state, which is a point to be taken care of in
concrete applications.Comment: 5 pages, 3 figure
Injection and detection of spin in a semiconductor by tunneling via interface states
Injection and detection of spin accumulation in a semiconductor having
localized states at the interface is evaluated. Spin transport from a
ferromagnetic contact by sequential, two-step tunneling via interface states is
treated not in itself, but in parallel with direct tunneling. The spin
accumulation induced in the semiconductor channel is not suppressed, as
previously argued, but genuinely enhanced by the additional spin current via
interface states. Spin detection with a ferromagnetic contact yields a weighted
average of the spin accumulation in the channel and in the localized states. In
the regime where the spin accumulation in the localized states is much larger
than that in the channel, the detected spin signal is insensitive to the spin
accumulation in the localized states and the ferromagnet probes the spin
accumulation in the semiconductor channel.Comment: 7 pages, 2 figures. Theory onl
Thermal spin current and magnetothermopower by Seebeck spin tunneling
The recently observed Seebeck spin tunneling, the thermoelectric analog of
spin-polarized tunneling, is described. The fundamental origin is the spin
dependence of the Seebeck coefficient of a tunnel junction with at least one
ferromagnetic electrode. Seebeck spin tunneling creates a thermal flow of
spin-angular momentum across a tunnel barrier without a charge tunnel current.
In ferromagnet/insulator/semiconductor tunnel junctions this can be used to
induce a spin accumulation (\Delta \mu) in the semiconductor in response to a
temperature difference (\Delta T) between the electrodes. A phenomenological
framework is presented to describe the thermal spin transport in terms of
parameters that can be obtained from experiment or theory. Key ingredients are
a spin-polarized thermoelectric tunnel conductance and a tunnel spin
polarization with non-zero energy derivative, resulting in different Seebeck
tunnel coefficients for majority and minority spin electrons. We evaluate the
thermal spin current, the induced spin accumulation and \Delta\mu/\Delta T,
discuss limiting regimes, and compare thermal and electrical flow of spin
across a tunnel barrier. A salient feature is that the thermally-induced spin
accumulation is maximal for smaller tunnel resistance, in contrast to the
electrically-induced spin accumulation that suffers from the impedance mismatch
between a ferromagnetic metal and a semiconductor. The thermally-induced spin
accumulation produces an additional thermovoltage proportional to \Delta\mu,
which can significantly enhance the conventional charge thermopower. Owing to
the Hanle effect, the thermopower can also be manipulated with a magnetic
field, producing a Hanle magnetothermopower.Comment: 10 pages, 3 figures, 1 tabl
Entanglement Purification through Zeno-like Measurements
We present a novel method to purify quantum states, i.e. purification through
Zeno-like measurements, and show an application to entanglement purification.Comment: 5 pages, 1 figure; Contribution to the Proceedings of "Mysteries,
Puzzles and Paradoxes in Quantum Mechanics", Gargnano, Italy, 2003 (to be
published in J. Mod. Opt.
Distillation of Entanglement between Distant Systems by Repeated Measurements on Entanglement Mediator
A recently proposed purification method, in which the Zeno-like measurements
of a subsystem can bring about a distillation of another subsystem in
interaction with the former, is utilized to yield entangled states between
distant systems. It is shown that the measurements of a two-level system
locally interacting with other two spatially separated not coupled subsystems,
can distill entangled states from the latter irrespectively of the initial
states of the two subsystems.Comment: 11 pages, 2 figures; the version accepted for publication in Phys.
Rev.
Resonant Scattering Can Enhance the Degree of Entanglement
Generation of entanglement between two qubits by scattering an entanglement
mediator is discussed. The mediator bounces between the two qubits and exhibits
a resonant scattering. It is clarified how the degree of the entanglement is
enhanced by the constructive interference of such bouncing processes. Maximally
entangled states are available via adjusting the incident momentum of the
mediator or the distance between the two qubits, but their fine tunings are not
necessarily required to gain highly entangled states and a robust generation of
entanglement is possible.Comment: 7 pages, 13 figure
Enhancement of the transverse non-reciprocal magneto-optical effect
The origin and properties of the transverse non-reciprocal magneto-optical
(nMO) effect were studied. The transverse nMO effect occurs in the case when
light propagates perpendicularly to the magnetic field. It was demonstrated
that light can experience the transverse nMO effect only when it propagates in
the vicinity of a boundary between two materials and the optical field at least
in one material is evanescent. The transverse nMO effect is pronounced in the
cases of surface plasmons and waveguiding modes. The magnitude of the
transverse nMO effect is comparable to or greater than the magnitude of the
longitudinal nMO effect. In the case of surface plasmons propagating at a
boundary between the transition metal and the dielectric it is possible to
magnify the transverse nMO effect and the magneto-optical figure-of-merit may
increase from a few percents to above 100%. The scalar dispersion relation,
which describes the transverse MO effect in cases of waveguide modes and
surface plasmons propagating in a multilayer MO slab, was derived
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