148 research outputs found
Nanoelectromechanical Systems
Nanoelectromechanical systems (NEMS) are nano-to-micrometer scale mechanical
resonators coupled to electronic devices of similar dimensions. NEMS show
promise for fast, ultrasensitive force microscopy and for deepening our
understanding of how classical dynamics arises by approximation to quantum
dynamics. This article begins with a survey of NEMS and then describes certain
aspects of their classical dynamics. In particular, we show that for weak
coupling the action of the electronic device on the mechanical resonator can be
effectively that of a thermal bath, this despite the device being a driven,
far-from-equilibrium system.Comment: Submitted to Contemporary Physics (invited review); 33 pages, 11
figure
Resolved Sideband Cooling of a Micromechanical Oscillator
Micro- and nanoscale opto-mechanical systems provide radiation pressure
coupling of optical and mechanical degree of freedom and are actively pursued
for their ability to explore quantum mechanical phenomena of macroscopic
objects. Many of these investigations require preparation of the mechanical
system in or close to its quantum ground state. Remarkable progress in ground
state cooling has been achieved for trapped ions and atoms confined in optical
lattices. Imperative to this progress has been the technique of resolved
sideband cooling, which allows overcoming the inherent temperature limit of
Doppler cooling and necessitates a harmonic trapping frequency which exceeds
the atomic species' transition rate. The recent advent of cavity back-action
cooling of mechanical oscillators by radiation pressure has followed a similar
path with Doppler-type cooling being demonstrated, but lacking inherently the
ability to attain ground state cooling as recently predicted. Here we
demonstrate for the first time resolved sideband cooling of a mechanical
oscillator. By pumping the first lower sideband of an optical microcavity,
whose decay rate is more than twenty times smaller than the eigen-frequency of
the associated mechanical oscillator, cooling rates above 1.5 MHz are attained.
Direct spectroscopy of the motional sidebands reveals 40-fold suppression of
motional increasing processes, which could enable reaching phonon occupancies
well below unity (<0.03). Elemental demonstration of resolved sideband cooling
as reported here should find widespread use in opto-mechanical cooling
experiments. Apart from ground state cooling, this regime allows realization of
motion measurement with an accuracy exceeding the standard quantum limit.Comment: 13 pages, 5 figure
Quantum-squeezing effects of strained multilayer graphene NEMS
Quantum squeezing can improve the ultimate measurement precision by squeezing one desired fluctuation of the two physical quantities in Heisenberg relation. We propose a scheme to obtain squeezed states through graphene nanoelectromechanical system (NEMS) taking advantage of their thin thickness in principle. Two key criteria of achieving squeezing states, zero-point displacement uncertainty and squeezing factor of strained multilayer graphene NEMS, are studied. Our research promotes the measured precision limit of graphene-based nano-transducers by reducing quantum noises through squeezed states
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
Higher Spin Black Holes from CFT
Higher spin gravity in three dimensions has explicit black holes solutions,
carrying higher spin charge. We compute the free energy of a charged black hole
from the holographic dual, a 2d CFT with extended conformal symmetry, and find
exact agreement with the bulk thermodynamics. In the CFT, higher spin
corrections to the free energy can be calculated at high temperature from
correlation functions of W-algebra currents.Comment: 24 pages; v2 reference adde
Quantum W-symmetry in AdS_3
It has recently been argued that, classically, massless higher spin theories
in AdS_3 have an enlarged W_N-symmetry as the algebra of asymptotic isometries.
In this note we provide evidence that this symmetry is realised
(perturbatively) in the quantum theory. We perform a one loop computation of
the fluctuations for a massless spin field around a thermal AdS_3
background. The resulting determinants are evaluated using the heat kernel
techniques of arXiv:0911.5085. The answer factorises holomorphically, and the
contributions from the various spin fields organise themselves into vacuum
characters of the W_N symmetry. For the case of the hs(1,1) theory consisting
of an infinite tower of massless higher spin particles, the resulting answer
can be simply expressed in terms of (two copies of) the MacMahon function.Comment: 23 pages; v2: References adde
Sisyphus cooling and amplification by a superconducting qubit
Laser cooling of the atomic motion paved the way for remarkable achievements
in the fields of quantum optics and atomic physics, including Bose-Einstein
condensation and the trapping of atoms in optical lattices. More recently
superconducting qubits were shown to act as artificial two-level atoms,
displaying Rabi oscillations, Ramsey fringes, and further quantum effects.
Coupling such qubits to resonators brought the superconducting circuits into
the realm of quantum electrodynamics (circuit QED). It opened the perspective
to use superconducting qubits as micro-coolers or to create a population
inversion in the qubit to induce lasing behavior of the resonator. Furthering
these analogies between quantum optical and superconducting systems we
demonstrate here Sisyphus cooling of a low frequency LC oscillator coupled to a
near-resonantly driven superconducting qubit. In the quantum optics setup the
mechanical degrees of freedom of an atom are cooled by laser driving the atom's
electronic degrees of freedom. Here the roles of the two degrees of freedom are
played by the LC circuit and the qubit's levels, respectively. We also
demonstrate the counterpart of the Sisyphus cooling, namely Sisyphus
amplification. Parallel to the experimental demonstration we analyze the system
theoretically and find quantitative agreement, which supports the
interpretation and allows us to estimate system parameters.Comment: 7 pages, 4 figure
Quantizing higher-spin gravity in free-field variables
We study the formulation of massless higher-spin gravity on AdS in a
gauge in which the fundamental variables satisfy free field Poisson brackets.
This gauge choice leaves a small portion of the gauge freedom unfixed, which
should be further quotiented out. We show that doing so leads to a bulk version
of the Coulomb gas formalism for CFT's: the generators of the residual
gauge symmetries are the classical limits of screening charges, while the
gauge-invariant observables are classical charges. Quantization in these
variables can be carried out using standard techniques and makes manifest a
remnant of the triality symmetry of . This symmetry can be
used to argue that the theory should be supplemented with additional matter
content which is precisely that of the Prokushkin-Vasiliev theory. As a further
application, we use our formulation to quantize a class of conical surplus
solutions and confirm the conjecture that these are dual to specific degenerate
primaries, to all orders in the large central charge expansion.Comment: 31 pages + appendices. V2: typos corrected, reference adde
Signal-Regulated Pre-mRNA Occupancy by the General Splicing Factor U2AF
Alternative splicing of transcripts in a signal-dependent manner has emerged as an important concept to ensure appropriate expression of splice variants under different conditions. Binding of the general splicing factor U2AF to splice sites preceding alternatively spliced exons has been suggested to be an important step for splice site recognition. For splicing to proceed, U2AF has to be replaced by other factors. We show here that U2AF interacts with the signal-dependent splice regulator Sam68 and that forced expression of Sam68 results in enhanced binding of the U2AF65 subunit to an alternatively spliced pre-mRNA sequence in vivo. Conversely, the rapid signal-induced and phosphorylation-dependent interference with Sam68 binding to RNA was accompanied by reduced pre-mRNA occupancy of U2AF in vivo. Our data suggest that Sam68 can affect splice site occupancy by U2AF in signal-dependent splicing. We propose that the induced release of U2AF from pre-mRNA provides a regulatory step to control alternative splicing
Higher spin AdS_3 supergravity and its dual CFT
Vasiliev's higher spin supergravity theory on three dimensional anti-de
Sitter space is studied and, in particular, the partition function is computed
at one loop level. The dual conformal field theory is proposed to be the
N=(2,2) CP^N Kazama-Suzuki model in two dimensions. The proposal is based on
symmetry considerations and comparison of the bulk partition function with the
conformal field theory. Our findings suggest that the theory is strong-weak
self-dual.Comment: 36 page
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