2,128 research outputs found
Issues pertaining to D'yakonov-Perel' spin relaxation in quantum wire channels
We elucidate the origin and nature of the D'yakonov-Perel' spin relaxation in
a quantum wire structure, showing (analytically) that there are three necessary
conditions for it to exist: (i) transport must be multi-channeled, (ii) there
must be a Rashba spin orbit interaction in the wire, and (iii) there must also
be a Dresselhaus spin orbit interaction. Therefore, the only effective way to
completely eliminate the D'yakonov-Perel' relaxation in compound semiconductor
channels with structural and bulk inversion asymmetry is to ensure strictly
single channeled transport. In view of that, recent proposals in the literature
that advocate using multi-channeled quantum wires for spin transistors appear
ill-advised
Classical light steering leading to quantum-like security
We show how single system steering can be exhibited by classical light, a
feature originating from superposition in classical optics that also enables
entanglement and Bell-violation by classical light beams. Single system
steering is the temporal analogue of Einstein-Podolsky-Rosen (EPR) steering in
the quantum domain, enabling control of the state of a remote system, and can
hence be connected to the security of secret key generation between two remote
parties. We derive the steering criterion for a single mode coherent state when
displaced parity measurements are performed at two different times. The
security bound of the Bennett-Brassard 1984 (BB84) protocol under the gaussian
cloning attack is calculated to yield an, in principle, ideal and quantum-like
key rate using a fine-grained uncertainty relation corresponding to the
classical phase space.Comment: 6 pages, 1 figure, comments are welcom
Spin transport in nanowires
We study high-field spin transport of electrons in a quasi one-dimensional
channel of a gate controlled spin interferometer (SPINFET) using a
semiclassical formalism (spin density matrix evolution coupled with Boltzmann
transport equation). Spin dephasing (or depolarization) is predominantly caused
by D'yakonov-Perel' relaxation associated with momentum dependent spin orbit
coupling effects that arise due to bulk inversion asymmetry (Dresselhaus spin
orbit coupling) and structural inversion asymmetry (Rashba spin orbit
coupling). Spin dephasing length in a one dimensional channel has been found to
be an order of magnitude higher than that in a two dimensional channel. This
study confirms that the ideal configuration for a SPINFET is one where the
ferromagnetic source and drain contacts are magnetized along the axis of the
channel. The spin dephasing length in this case is about 22.5 microns at
lattice temperature of 30K and 10 microns at lattice temperature of 77 K for an
electric field of 2 kV/cm. Spin dephasing length has been found to be weakly
dependent on the driving electric field and strongly dependent on the lattice
temperature
Decay of spin polarized hot carrier current in a quasi one-dimensional spin valve structure
We study the spatial decay of spin polarized hot carrier current in a
spin-valve structure consisting of a semiconductor quantum wire flanked by
half-metallic ferromagnetic contacts. The current decays because of
D'yakonov-Perel' spin relaxation in the semiconductor caused by Rashba spin
orbit interaction. The associated relaxation length is found to decrease with
increasing lattice temperature (in the range 30-77 K) and exhibit a
non-monotonic dependence on the electric field driving the current. The
relaxation lengths are several tens of microns which are at least an order of
magnitude larger than what has been theoretically calculated for
two-dimensional structures at comparable temperatures, Rashba interaction
strengths and electric fields. This improvement is a consequence of
one-dimensional carrier confinement that does not necessarily suppress carrier
scattering, but nevertheless suppresses D'yakonov-Perel' spin relaxation.Comment: 2 figures. Submitted to Appl. Phys. Let
Some applications of uncertainty relations in quantum information
We discuss some applications of various versions of uncertainty relations for
both discrete and continuous variables in the context of quantum information
theory. The Heisenberg uncertainty relation enables demonstration of the EPR
paradox. Entropic uncertainty relations are used to reveal quantum steering for
non-Gaussian continuous variable states. Entropic uncertainty relations for
discrete variables are studied in the context of quantum memory where
fine-graining yields the optimum lower bound of uncertainty. The fine-grained
uncertainty relation is used to obtain connections between uncertainty and the
nonlocality of retrieval games for bipartite and tipartite systems. The
Robertson-Schrodinger uncertainty relation is applied for distinguishing pure
and mixed states of discrete variables.Comment: Based on review talk given at the International Programme on Quantum
Information, Institute of Physics, Bhubaneswar, February 2014. To be
published as Conference Proceedings in IJQI styl
Confinement induced control of similarity solutions in premelting dynamics and other thin film problems
We study the combined effects of nonlocal elasticity and confinement induced
ordering on the dynamics of thermomolecular pressure gradient driven premelted
films bound by an elastic membrane. The confinement induced ordering is modeled
using a film thickness dependent viscosity. When there is no confinement
induced ordering, we recover the similarity solution for the evolution of the
elastic membrane, which exhibits an infinite sequence of oscillations. However,
when the confinement induced viscosity is comparable to the bulk viscosity, the
numerical solutions of the full system reveal the conditions under which the
oscillations and similarity solutions vanish. Implications of our results for
general thermomechanical dynamics, frost heave observations and cryogenic cell
preservation are discussed. Finally, through its influence on the viscosity,
the confinement effect implicitly introduces a new universal length scale into
the volume flux. Thus, there are a host of thin film problems, from droplet
breakup to wetting/dewetting dynamics, whose properties (similarity solutions,
regularization, and compact support) will change under the action of the
confinement effect. Therefore, our study suggests revisiting the mathematical
structure and experimental implications of a wide range of problems within the
framework of the confinement effect.Comment: 18 Pages, 12 Figure
Confinement effects in premelting dynamics
We examine the effects of confinement on the dynamics of premelted films
driven by thermomolecular pressure gradients. Our approach is to modify a
well-studied setting in which the thermomolecular pressure gradient is driven
by a temperature gradient parallel to an interfacially premelted elastic wall.
The modification treats the increase in viscosity associated with the thinning
of films, studied in a wide variety of materials, using a power law and we
examine the consequent evolution of the confining elastic wall. We treat (1) a
range of interactions that are known to underlie interfacial premelting and (2)
a constant temperature gradient wherein the thermomolecular pressure gradient
is a constant. The difference between the cases with and without the proximity
effect arises in the volume flux of premelted liquid. The proximity effect
increases the viscosity as the film thickness decreases thereby requiring the
thermomolecular pressure driven flux to be accommodated at higher temperatures
where the premelted film thickness is the largest. Implications for experiment
and observations of frost heave are discussed.Comment: 20 pages (including 8 pages of supplemental materials), 8 figures.
Degenerate reference calls and some alignment problems from the earlier
version are corrected in this versio
Improving the fidelity of teleportation through noisy channels using weak measurement
We employ the technique of weak measurement in order to enable preservation
of teleportation fidelity for two-qubit noisy channels. We consider one or both
qubits of a maximally entangled state to undergo amplitude damping, and show
that the application of weak measurement and a subsequent reverse operation
could lead to a fidelity greater than for any value of the decoherence
parameter. The success probability of the protocol decreases with the strength
of weak measurement, and is lower when both the qubits are affected by
decoherence. Finally, our protocol is shown to work for the Werner state too.Comment: 9 pages, 4 figures, new section adde
Transverse spin relaxation time in organic molecules: A possible platform for fault tolerant room temperature quantum computing
We report measurement of the ensemble averaged transverse spin relaxation
time (T2*) in bulk and few molecules of the organic semiconductor
tris(8-hydroxyquinolinolato aluminum) or Alq3. This system exhibits two
characteristic T2* times, the longer of which is temperature-independent and
the shorter is temperature-dependent, indicating that the latter is most likely
limited by spin-phonon interaction. Based on the measured data, we infer that
the single particle T2 time is long enough to meet Knill's criterion for fault
tolerant quantum computing, even at room temperature. Alq3 is also an optically
active organic and we propose a simple optical scheme for spin qubit read out.
Moreover, we found that the temperature-dependent T2* time is considerably
shorter in bulk Alq3 powder than in few molecules confined in 1-2 nm sized
cavities, which is suggestive of a new type of ``phonon bottleneck effect''.
This is very intriguing for organic molecules where carriers are always
localized over individual molecules but the phonons are delocalized
Fine-grained uncertainty relation and biased non-local games in bipartite and tripartite systems
The fine-grained uncertainty relation can be used to discriminate among
classical, quantum and super-quantum correlations based on their strength of
nonlocality, as has been shown for bipartite and tripartite systems with
unbiased measurement settings. Here we consider the situation when two and
three parties, respectively, choose settings with bias for playing certain
non-local games. We show analytically that while the fine-grained uncertainty
principle is still able to distinguish classical, quantum and super-quantum
correlations for biased settings corresponding to certain ranges of the biasing
parameters, the above-mentioned discrimination is not manifested for all
biasing.Comment: 6 pages, Accepted for publication in Physical Review
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