273 research outputs found
Laser-like Instabilities in Quantum Nano-electromechanical Systems
We discuss negative damping regimes in quantum nano-electromechanical systems
formed by coupling a mechanical oscillator to a single-electron transistor
(normal or superconducting). Using an analogy to a laser with a tunable
atom-field coupling, we demonstrate how these effects scale with system
parameters. We also discuss the fluctuation physics of both the oscillator and
the single-electron transistor in this regime, and the degree to which the
oscillator motion is coherent.Comment: 4+ pages, 1 figure; reference to the work of Dykman and Krivoglaz
adde
Using a qubit to measure photon number statistics of a driven, thermal oscillator
We demonstrate theoretically how photon number statistics of a driven, damped
oscillator at finite temperature can be extracted by measuring the dephasing
spectrum of a two-level system dispersively coupled to the oscillator; we thus
extend the work of Dykman (1987) and Gambetta et al. (2006). We carefully
consider the fidelity of this scheme-- to what extent does the measurement
reflect the initial number statistics of the mode? We also derive analytic
results for the dephasing of a qubit by a driven, thermal mode, and compare
results obtained at different levels of approximation. Our results have
relevance both to experiments in circuit cavity QED, as well as to
nano-electromechanical systems.Comment: 11 pages; 2 figures adde
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ERK1/2 signaling dominates over RhoA signaling in regulating early changes in RNA expression induced by endothelin-1 in neonatal rat cardiomyocytes
Cardiomyocyte hypertrophy is associated with changes in gene expression. Extracellular signal-regulated kinases 1/2 (ERK1/2) and RhoA [activated by hypertrophic agonists (e.g. endothelin-1)] regulate gene expression and are implicated in the response, but their relative significance in regulating the cardiomyocyte transcriptome is unknown. Our aim was to establish the significance of ERK1/2 and/or RhoA in the early cardiomyocyte transcriptomic response to endothelin-1.Cardiomyocytes were exposed to endothelin-1 (1 h) with/without PD184352 (to inhibit ERK1/2) or C3 transferase (C3T, to inhibit RhoA). RNA expression was analyzed using microarrays and qPCR. ERK1/2 signaling positively regulated approximately 65% of the early gene expression response to ET-1 with a small (approximately 2%) negative effect, whereas RhoA signaling positively regulated approximately 10% of the early gene expression response to ET-1 with a greater (approximately 14%) negative contribution. Of RNAs non-responsive to endothelin-1, 66 or 448 were regulated by PD184352 or C3T, respectively, indicating that RhoA had a more significant effect on baseline RNA expression. mRNAs upregulated by endothelin-1 encoded a number of receptor ligands (e.g. Ereg, Areg, Hbegf) and transcription factors (e.g. Abra/Srf) that potentially propagate the response.ERK1/2 dominates over RhoA in the early transcriptomic response to endothelin-1. RhoA plays a major role in maintaining baseline RNA expression but, with upregulation of Abra/Srf by endothelin-1, RhoA may regulate changes in RNA expression over longer times. Our data identify ERK1/2 as a more significant node than RhoA in regulating the early stages of cardiomyocyte hypertrophy
Cooling a nanomechanical resonator with quantum back-action
Quantum mechanics demands that the act of measurement must affect the
measured object. When a linear amplifier is used to continuously monitor the
position of an object, the Heisenberg uncertainty relationship requires that
the object be driven by force impulses, called back-action. Here we measure the
back-action of a superconducting single-electron transistor (SSET) on a
radiofrequency nanomechanical resonator. The conductance of the SSET, which is
capacitively coupled to the resonator, provides a sensitive probe of the
latter's position;back-action effects manifest themselves as an effective
thermal bath, the properties of which depend sensitively on SSET bias
conditions. Surprisingly, when the SSET is biased near a transport resonance,
we observe cooling of the nanomechanical mode from 550mK to 300mK-- an effect
that is analogous to laser cooling in atomic physics. Our measurements have
implications for nanomechanical readout of quantum information devices and the
limits of ultrasensitive force microscopy (such as single-nuclear-spin magnetic
resonance force microscopy). Furthermore, we anticipate the use of these
backaction effects to prepare ultracold and quantum states of mechanical
structures, which would not be accessible with existing technology.Comment: 28 pages, 7 figures; accepted for publication in Natur
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Is prosodic production driven by lexical development ? Longitudinal evidence from babble and words
This study investigated the relation between lexical development and the production of prosodic prominence in disyllabic babble and words. Monthly recordings from nine typically developing Belgian-Dutch-speaking infants were analysed from the onset of babbling until a cumulative vocabulary of 200 words was reached. The differentiation between the two syllables of isolated disyllabic utterances was computed for f0, intensity and duration measurements. Results showed that the ambient trochaic pattern emerged in babble, but became enhanced in words. Words showed more prosodic differentiation in terms of f0 and intensity and a more even duration ratio. Age or vocabulary size did not predict the expansion of f0 or intensity in words, whereas vocabulary size was related to the production of more even-timed syllables. The findings are discussed in terms of lexicalist accounts of phonetic development and a potential phonetic highlighting function of first words
Quantum Theory of Cavity-Assisted Sideband Cooling of Mechanical Motion
We present a fully quantum theory describing the cooling of a cantilever
coupled via radiation pressure to an illuminated optical cavity. Applying the
quantum noise approach to the fluctuations of the radiation pressure force, we
derive the opto-mechanical cooling rate and the minimum achievable phonon
number. We find that reaching the quantum limit of arbitrarily small phonon
numbers requires going into the good cavity (resolved phonon sideband) regime
where the cavity linewidth is much smaller than the mechanical frequency and
the corresponding cavity detuning. This is in contrast to the common assumption
that the mechanical frequency and the cavity detuning should be comparable to
the cavity damping.Comment: 5 pages, 2 figure
Andreev bound states and -junction transition in a superconductor / quantum-dot / superconductor system
We study Andreev bound states and -junction transition in a
superconductor / quantum-dot / superconductor (S-QD-S) system by Green function
method. We derive an equation to describe the Andreev bound states in S-QD-S
system, and provide a unified understanding of the -junction transition
caused by three different mechanisms: (1) {\it Zeeman splitting.} For QD with
two spin levels and , we find that the surface
of the Josephson current vs the configuration of
exhibits interesting profile: a sharp peak
around ; a positive ridge in the region of
; and a {\em % negative}, flat, shallow
plain in the region of . (2){\it \
Intra-dot interaction.} We deal with the intra-dot Coulomb interaction by
Hartree-Fock approximation, and find that the system behaves as a -junction when QD becomes a magnetic dot due to the interaction. The
conditions for -junction transition are also discussed. (3) {\it \
Non-equilibrium distribution.} We replace the Fermi distribution by
a non-equilibrium one , and allow
Zeeman splitting in QD where The curves of
vs show the novel effect of interplay of
non-equilibrium distribution with magnetization in QD.Comment: 18 pages, 8 figures, Late
Quantum Noise Interference and Back-action Cooling in Cavity Nanomechanics
We present a theoretical analysis of a novel cavity electromechanical system
where a mechanical resonator directly modulates the damping rate kappa of a
driven electromagnetic cavity. We show that via a destructive interference of
quantum noise, the driven cavity can effectively act like a zero-temperature
bath irrespective of the ratio kappa / omega_M, where omega_M is the mechanical
frequency. This scheme thus allows one to cool the mechanical resonator to its
ground state without requiring the cavity to be in the so-called `good cavity'
limit kappa << omega_M.Comment: 4+ pages, 2 figures. Error in second last paragraph correcte
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