533 research outputs found
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Phase Transitions in Dissipative Quantum Transport and Mesoscopic Nuclear Spin Pumping
Topological phase transitions can occur in the dissipative dynamics of a
quantum system when the ratio of matrix elements for competing transport
channels is varied. Here we establish a relation between such behavior in a
class of non-Hermitian quantum walk problems [M. S. Rudner and L. S. Levitov,
Phys. Rev. Lett. 102, 065703 (2009)] and nuclear spin pumping in double quantum
dots, which is mediated by the decay of a spin-blockaded electron triplet state
in the presence of spin-orbit and hyperfine interactions. The transition occurs
when the strength of spin-orbit coupling exceeds the strength of the net
hyperfine coupling, and results in the complete suppression of nuclear spin
pumping. Below the transition point, nuclear pumping is accompanied by a strong
reduction in current due to the presence of non-decaying "dark states" in this
regime. Due to its topological character, the transition is expected to be
robust against dephasing of the electronic degrees of freedom
Nuclear Spin Dynamics in Double Quantum Dots: Fixed Points, Transients, and Intermittency
Transport through spin-blockaded quantum dots provides a means for electrical
control and detection of nuclear spin dynamics in the host material. Although
such experiments have become increasingly popular in recent years,
interpretation of their results in terms of the underlying nuclear spin
dynamics remains challenging. Here we point out a fundamental process in which
nuclear spin dynamics can be driven by electron shot noise; fast electric
current fluctuations generate much slower nuclear polarization dynamics, which
in turn affect electron dynamics via the Overhauser field. The resulting
extremely slow intermittent current fluctuations account for a variety of
observed phenomena that were not previously understood.Comment: version accepted for publication in Physical Review B, figure
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Meta-Analysis in Educational Research
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Generating Entanglement and Squeezed States of Nuclear Spins in Quantum Dots
Entanglement generation and detection are two of the most sought-after goals
in the field of quantum control. Besides offering a means to probe some of the
most peculiar and fundamental aspects of quantum mechanics, entanglement in
many-body systems can be used as a tool to reduce fluctuations below the
standard quantum limit. For spins, or spin-like systems, such a reduction of
fluctuations can be realized with so-called squeezed states. Here we present a
scheme for achieving coherent spin squeezing of nuclear spin states in
few-electron quantum dots. This work represents a major shift from earlier
studies in quantum dots, which have explored classical "narrowing" of the
nuclear polarization distribution through feedback involving stochastic spin
flips. In contrast, we use the nuclear-polarization-dependence of the electron
spin resonance (ESR) to provide a non-linearity which generates a non-trivial,
area-preserving, "twisting" dynamics that squeezes and stretches the nuclear
spin Wigner distribution without the need for nuclear spin flips.Comment: 8 pgs, 3 fgs. References added, text update
Klein Backscattering and Fabry-Perot Interference in Graphene Heterojunctions
We present a theory of quantum-coherent transport through a lateral p-n-p
structure in graphene, which fully accounts for the interference of forward and
backward scattering on the p-n interfaces. The backreflection amplitude changes
sign at zero incidence angle because of the Klein phenomenon, adding a phase
to the interference fringes. The contributions of the two p-n interfaces
to the phase of the interference cancel with each other at zero magnetic field,
but become imbalanced at a finite field. The resulting half a period shift in
the Fabry-Perot fringe pattern, induced by a relatively weak magnetic field,
can provide a clear signature of Klein scattering in graphene. This effect is
shown to be robust in the presence of spatially inhomogeneous potential of
moderate strength.Comment: 5 pgs, 4 fg
Quantum Phase Tomography of a Strongly Driven Qubit
The interference between repeated Landau-Zener transitions in a qubit swept
through an avoided level crossing results in Stueckelberg oscillations in qubit
magnetization. The resulting oscillatory patterns are a hallmark of the
coherent strongly-driven regime in qubits, quantum dots and other two-level
systems. The two-dimensional Fourier transforms of these patterns are found to
exhibit a family of one-dimensional curves in Fourier space, in agreement with
recent observations in a superconducting qubit. We interpret these images in
terms of time evolution of the quantum phase of qubit state and show that they
can be used to probe dephasing mechanisms in the qubit.Comment: 5 pgs, 4 fg
The Organization of Working Memory Networks is Shaped by Early Sensory Experience
Early deafness results in crossmodal reorganization of the superior temporal cortex (STC). Here, we investigated the effect of deafness on cognitive processing. Specifically, we studied the reorganization, due to deafness and sign language (SL) knowledge, of linguistic and nonlinguistic visual working memory (WM). We conducted an fMRI experiment in groups that differed in their hearing status and SL knowledge: deaf native signers, and hearing native signers, hearing nonsigners. Participants performed a 2-back WM task and a control task. Stimuli were signs from British Sign Language (BSL) or moving nonsense objects in the form of point-light displays. We found characteristic WM activations in fronto-parietal regions in all groups. However, deaf participants also recruited bilateral posterior STC during the WM task, independently of the linguistic content of the stimuli, and showed less activation in fronto-parietal regions. Resting-state connectivity analysis showed increased connectivity between frontal regions and STC in deaf compared to hearing individuals. WM for signs did not elicit differential activations, suggesting that SL WM does not rely on modality-specific linguistic processing. These findings suggest that WM networks are reorganized due to early deafness, and that the organization of cognitive networks is shaped by the nature of the sensory inputs available during development
Self-Polarization and Dynamical Cooling of Nuclear Spins in Double Quantum Dots
Spontaneous nuclear polarization is predicted in double quantum dots in the
spin-blocked electron transport regime. The polarization results from an
instability of the zero-polarization state when singlet and triplet electron
energy levels are brought into resonance by the effective hyperfine field of
the nuclei on the electrons. The nuclear spins, once polarized, serve as a cold
bath for cooling electrons below the lattice (phonon) temperature. We estimate
the relevant time scales and discuss the conditions necessary to observe these
phenomena.Comment: 4 pages, 3 figures, updated journal versio
Neural Networks Supporting Phoneme Monitoring Are Modulated by Phonology but Not Lexicality or Iconicity: Evidence From British and Swedish Sign Language
Sign languages are natural languages in the visual domain. Because they lack a written
form, they provide a sharper tool than spoken languages for investigating lexicality effects
which may be confounded by orthographic processing. In a previous study, we showed
that the neural networks supporting phoneme monitoring in deaf British Sign Language
(BSL) users are modulated by phonology but not lexicality or iconicity. In the present
study, we investigated whether this pattern generalizes to deaf Swedish Sign Language
(SSL) users. British and SSLs have a largely overlapping phoneme inventory but are
mutually unintelligible because lexical overlap is small. This is important because it means
that even when signs lexicalized in BSL are unintelligible to users of SSL they are usually
still phonologically acceptable. During fMRI scanning, deaf users of the two different sign
languages monitored signs that were lexicalized in either one or both of those languages
for phonologically contrastive elements. Neural activation patterns relating to different
linguistic levels of processing were similar across SLs; in particular, we found no effect of
lexicality, supporting the notion that apparent lexicality effects on sublexical processing
of speech may be driven by orthographic strategies. As expected, we found an effect of
phonology but not iconicity. Further, there was a difference in neural activation between
the two groups in a motion-processing region of the left occipital cortex, possibly driven
by cultural differences, such as education. Importantly, this difference was not modulated
by the linguistic characteristics of the material, underscoring the robustness of the neural
activation patterns relating to different linguistic levels of processing
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