696 research outputs found
Spin-orbit-induced strong coupling of a single spin to a nanomechanical resonator
We theoretically investigate the deflection-induced coupling of an electron
spin to vibrational motion due to spin-orbit coupling in suspended carbon
nanotube quantum dots. Our estimates indicate that, with current capabilities,
a quantum dot with an odd number of electrons can serve as a realization of the
Jaynes-Cummings model of quantum electrodynamics in the strong-coupling regime.
A quantized flexural mode of the suspended tube plays the role of the optical
mode and we identify two distinct two-level subspaces, at small and large
magnetic field, which can be used as qubits in this setup. The strong intrinsic
spin-mechanical coupling allows for detection, as well as manipulation of the
spin qubit, and may yield enhanced performance of nanotubes in sensing
applications.Comment: 5 pages, 3 figures + appendix; published versio
Anomalous random multipolar driven insulators
It is by now well established that periodically driven quantum many-body systems can realize topological nonequilibrium phases without any equilibrium counterpart. Here we show that, even in the absence of time translation symmetry, nonequilibrium topological phases of matter can exist in aperiodically driven systems for tunably parametrically long prethermal lifetimes. As a prerequisite, we first demonstrate the existence of longlived prethermal Anderson localization in two dimensions under random multipolar driving. We then show that the localization may be topologically nontrivial with a quantized bulk orbital magnetization even though there are no well-defined Floquet operators. We further confirm the existence of this anomalous random multipolar driven insulator by detecting quantized charge pumping at the boundaries, which renders it experimentally observable
Explicit Processing Demands Reveal Language Modality-Specific Organization of Working Memory
The working memory model for Ease of Language Understanding (ELU) predicts that processing differences between language modalities emerge when cognitive demands are explicit. This prediction was tested in three working memory experiments with participants who were Deaf Signers (DS), Hearing Signers (HS), or Hearing Nonsigners (HN). Easily nameable pictures were used as stimuli to avoid confounds relating to sensory modality. Performance was largely similar for DS, HS, and HN, suggesting that previously identified intermodal differences may be due to differences in retention of sensory information. When explicit processing demands were high, differences emerged between DS and HN, suggesting that although working memory storage in both groups is sensitive to temporal organization, retrieval is not sensitive to temporal organization in DS. A general effect of semantic similarity was also found. These findings are discussed in relation to the ELU model
A Gibbs approach to Chargaff's second parity rule
Chargaff's second parity rule (CSPR) asserts that the frequencies of short
polynucleotide chains are the same as those of the complementary reversed
chains. Up to now, this hypothesis has only been observed empirically and there
is currently no explanation for its presence in DNA strands. Here we argue that
CSPR is a probabilistic consequence of the reverse complementarity between
paired strands, because the Gibbs distribution associated with the chemical
energy between the bonds satisfies CSPR. We develop a statistical test to study
the validity of CSPR under the Gibbsian assumption and we apply it to a large
set of bacterial genomes taken from the GenBank repository.Comment: 16 page
Hot Carrier Transport and Photocurrent Response in Graphene
Strong electron-electron interactions in graphene are expected to result in
multiple-excitation generation by the absorption of a single photon. We show
that the impact of carrier multiplication on photocurrent response is enhanced
by very inefficient electron cooling, resulting in an abundance of hot
carriers. The hot-carrier-mediated energy transport dominates the photoresponse
and manifests itself in quantum efficiencies that can exceed unity, as well as
in a characteristic dependence of the photocurrent on gate voltages. The
pattern of multiple photocurrent sign changes as a function of gate voltage
provides a fingerprint of hot-carrier-dominated transport and carrier
multiplication.Comment: 4 pgs, 2 fg
A valley-spin qubit in a carbon nanotube
Although electron spins in III-V semiconductor quantum dots have shown great
promise as qubits, a major challenge is the unavoidable hyperfine decoherence
in these materials. In group IV semiconductors, the dominant nuclear species
are spinless, allowing for qubit coherence times that have been extended up to
seconds in diamond and silicon. Carbon nanotubes are a particularly attractive
host material, because the spin-orbit interaction with the valley degree of
freedom allows for electrical manipulation of the qubit. In this work, we
realise such a qubit in a nanotube double quantum dot. The qubit is encoded in
two valley-spin states, with coherent manipulation via electrically driven spin
resonance (EDSR) mediated by a bend in the nanotube. Readout is performed by
measuring the current in Pauli blockade. Arbitrary qubit rotations are
demonstrated, and the coherence time is measured via Hahn echo. Although the
measured decoherence time is only 65 ns in our current device, this work offers
the possibility of creating a qubit for which hyperfine interaction can be
virtually eliminated
Bub1-Mediated Adaptation of the Spindle Checkpoint
During cell division, the spindle checkpoint ensures accurate chromosome segregation by monitoring the kinetochore–microtubule interaction and delaying the onset of anaphase until each pair of sister chromosomes is properly attached to microtubules. The spindle checkpoint is deactivated as chromosomes start moving toward the spindles in anaphase, but the mechanisms by which this deactivation and adaptation to prolonged mitotic arrest occur remain obscure. Our results strongly suggest that Cdc28-mediated phosphorylation of Bub1 at T566 plays an important role for the degradation of Bub1 in anaphase, and the phosphorylation is required for adaptation of the spindle checkpoint to prolonged mitotic arrest
Revealing the conduction band and pseudovector potential in 2D moir\'e semiconductors
Stacking monolayer semiconductors results in moir\'e patterns that host many
correlated and topological electronic phenomena, but measurements of the basic
electronic structure underpinning these phenomena are scarce. Here, we
investigate the properties of the conduction band in moir\'e heterobilayers
using submicron angle-resolved photoemission spectroscopy with electrostatic
gating, focusing on the example of WS2/WSe2. We find that at all twist angles
the conduction band edge is the K-point valley of the WS2, with a band gap of
1.58 +- 0.03 eV. By resolving the conduction band dispersion, we observe an
unexpectedly small effective mass of 0.15 +- 0.02 m_e. In addition, we observe
replicas of the conduction band displaced by reciprocal lattice vectors of the
moir\'e superlattice. We present arguments and evidence that the replicas are
due to modification of the conduction band states by the moir\'e potential
rather than to final-state diffraction. Interestingly, the replicas display an
intensity pattern with reduced, 3-fold symmetry, which we show implicates the
pseudo vector potential associated with in-plane strain in moir\'e band
formation.Comment: Main text: 12 pages, 4 figures. Appended Supporting Information: 10
pages, 11 figure
Helical distribution of the bacterial chemoreceptor via colocalization with the Sec protein translocation machinery
In Escherichia coli, chemoreceptor clustering at a cell pole seems critical for signal amplification and adaptation. However, little is known about the mechanism of localization itself. Here we examined whether the aspartate chemoreceptor (Tar) is inserted directly into the polar membrane by using its fusion to green fluorescent protein (GFP). After induction of Tar–GFP, fluorescent spots first appeared in lateral membrane regions, and later cell poles became predominantly fluorescent. Unexpectedly, Tar–GFP showed a helical arrangement in lateral regions, which was more apparent when a Tar–GFP derivative with two cysteine residues in the periplasmic domain was cross-linked to form higher oligomers. Moreover, similar distribution was observed even when the cytoplasmic domain of the double cysteine Tar–GFP mutant was replaced by that of the kinase EnvZ, which does not localize to a pole. Observation of GFP–SecE and a translocation-defective MalE–GFP mutant, as well as indirect immunofluorescence microscopy on SecG, suggested that the general protein translocation machinery (Sec) itself is arranged into a helical array, with which Tar is transiently associated. The Sec coil appeared distinct from the MreB coil, an actin-like cytoskeleton. These findings will shed new light on the mechanisms underlying spatial organization of membrane proteins in E. coli
Can One Trust Quantum Simulators?
Various fundamental phenomena of strongly-correlated quantum systems such as
high- superconductivity, the fractional quantum-Hall effect, and quark
confinement are still awaiting a universally accepted explanation. The main
obstacle is the computational complexity of solving even the most simplified
theoretical models that are designed to capture the relevant quantum
correlations of the many-body system of interest. In his seminal 1982 paper
[Int. J. Theor. Phys. 21, 467], Richard Feynman suggested that such models
might be solved by "simulation" with a new type of computer whose constituent
parts are effectively governed by a desired quantum many-body dynamics.
Measurements on this engineered machine, now known as a "quantum simulator,"
would reveal some unknown or difficult to compute properties of a model of
interest. We argue that a useful quantum simulator must satisfy four
conditions: relevance, controllability, reliability, and efficiency. We review
the current state of the art of digital and analog quantum simulators. Whereas
so far the majority of the focus, both theoretically and experimentally, has
been on controllability of relevant models, we emphasize here the need for a
careful analysis of reliability and efficiency in the presence of
imperfections. We discuss how disorder and noise can impact these conditions,
and illustrate our concerns with novel numerical simulations of a paradigmatic
example: a disordered quantum spin chain governed by the Ising model in a
transverse magnetic field. We find that disorder can decrease the reliability
of an analog quantum simulator of this model, although large errors in local
observables are introduced only for strong levels of disorder. We conclude that
the answer to the question "Can we trust quantum simulators?" is... to some
extent.Comment: 20 pages. Minor changes with respect to version 2 (some additional
explanations, added references...
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