49 research outputs found
Self-consistent theory of molecular switching
We study the model of a molecular switch comprised of a molecule with a soft
vibrational degree of freedom coupled to metallic leads. In the presence of
strong electron-ion interaction, different charge states of the molecule
correspond to substantially different ionic configurations, which can lead to
very slow switching between energetically close configurations (Franck-Condon
blockade). Application of transport voltage, however, can drive the molecule
far out of thermal equilibrium and thus dramatically accelerate the switching.
The tunneling electrons play the role of a heat bath with an effective
temperature dependent on the applied transport voltage. Including the
transport-induced "heating" selfconsistently, we determine the stationary
current-voltage characteristics of the device, and the switching dynamics for
symmetric and asymmetric devices. We also study the effects of an extra
dissipative environment and demonstrate that it can lead to enhanced
non-linearities in the transport properties of the device and dramatically
suppress the switching dynamics
Light scattering by magnons in whispering gallery mode cavities
Brillouin light scattering is an established technique to study magnons, the
elementary excitations of a magnet. Its efficiency can be enhanced by cavities
that concentrate the light intensity. Here, we theoretically study inelastic
scattering of photons by a magnetic sphere that supports optical whispering
gallery modes in a plane normal to the magnetization. Magnons with low angular
momenta scatter the light in the forward direction with a pronounced asymmetry
in the Stokes and the anti-Stokes scattering strength, consistent with earlier
studies. Magnons with large angular momenta constitute Damon Eschbach modes are
shown to inelastically reflect light. The reflection spectrum contains either a
Stokes or anti-Stokes peak, depending on the direction of the magnetization, a
selection rule that can be explained by the chirality of the Damon Eshbach
magnons. The controllable energy transfer can be used to manage the
thermodynamics of the magnet by light
Weak values of electron spin in a double quantum dot
We propose a protocol for a controlled experiment to measure a weak value of
the electron's spin in a solid state device. The weak value is obtained by a
two step procedure -- weak measurement followed by a strong one
(post-selection), where the outcome of the first measurement is kept provided a
second post-selected outcome occurs. The set-up consists of a double quantum
dot and a weakly coupled quantum point contact to be used as a detector.
Anomalously large values of the spin of a two electron system are predicted, as
well as negative values of the total spin. We also show how to incorporate the
adverse effect of decoherence into this procedure.Comment: 4+ pages, 3 figures, final published versio
Optimal mode matching in cavity optomagnonics
Inelastic scattering of photons is a promising technique to manipulate
magnons but it suffers from weak intrinsic coupling. We theoretically discuss
an idea to increase optomagnonic coupling in optical whispering gallery mode
cavities, by generalizing previous analysis to include the exchange
interaction. We predict that the optomagnonic coupling constant to surface
magnons in yttrium iron garnet (YIG) spheres with radius m
can be up to times larger than that to the macrospin Kittel mode. Whereas
this enhancement falls short of the requirements for magnon manipulation in
YIG, nanostructuring and/or materials with larger magneto-optical constants can
bridge this gap.Comment: Comments welcom
Engineering Entangled Coherent States of Magnons and Phonons via a Transmon Qubit
We propose a scheme for generating and controlling entangled coherent states
(ECS) of magnons, i.e. the quanta of the collective spin excitations in
magnetic systems, or phonons in mechanical resonators. The proposed hybrid
circuit architecture comprises a superconducting transmon qubit coupled to a
pair of magnonic Yttrium Iron Garnet (YIG) spherical resonators or mechanical
beam resonators via flux-mediated interactions. Specifically, the coupling
results from the magnetic/mechanical quantum fluctuations modulating the qubit
inductor, formed by a superconducting quantum interference device (SQUID). We
show that the resulting radiation-pressure interaction of the qubit with each
mode, can be employed to generate maximally-entangled states of magnons or
phonons. In addition, we numerically demonstrate a protocol for the preparation
of magnonic and mechanical Bell states with high fidelity including realistic
dissipation mechanisms. Furthermore, we have devised a scheme for reading out
the prepared states using standard qubit control and resonator field
displacements. Our work demonstrates an alternative platform for quantum
information using ECS in hybrid magnonic and mechanical quantum networks