371 research outputs found
Generalised models for torsional spine and fan magnetic reconnection
Three-dimensional null points are present in abundance in the solar corona,
and the same is likely to be true in other astrophysical environments. Recent
studies suggest that reconnection at such 3D nulls may play an important role
in the coronal dynamics. In this paper the properties of the torsional spine
and torsional fan modes of magnetic reconnection at 3D nulls are investigated.
New analytical models are developed, which for the first time include a current
layer that is spatially localised around the null, extending along either the
spine or the fan of the null. These are complemented with numerical
simulations. The principal aim is to investigate the effect of varying the
degree of asymmetry of the null point magnetic field on the resulting
reconnection process - where previous studies always considered a non-generic
radially symmetric null. The geometry of the current layers within which
torsional spine and torsional fan reconnection occur is found to be strongly
dependent on the symmetry of the magnetic field. Torsional spine reconnection
still occurs in a narrow tube around the spine, but with elliptical
cross-section when the fan eigenvalues are different, and with the short axis
of the ellipse being along the strong field direction. The spatiotemporal peak
current, and the peak reconnection rate attained, are found not to depend
strongly on the degree of asymmetry. For torsional fan reconnection, the
reconnection occurs in a planar disk in the fan surface, which is again
elliptical when the symmetry of the magnetic field is broken. The short axis of
the ellipse is along the weak field direction, with the current being peaked in
these weak field regions. The peak current and peak reconnection rate in this
case are clearly dependent on the asymmetry, with the peak current increasing
but the reconnection rate decreasing as the degree of asymmetry is increased
Super-resolution imaging of a low frequency levitated oscillator
We describe the measurement of the secular motion of a levitated nanoparticle
in a Paul trap with a CMOS camera. This simple method enables us to reach
signal-to-noise ratios as good as 10 with a displacement sensitivity
better than 10/Hz. This method can be used to extract trap
parameters as well as the properties of the levitated particles. We demonstrate
continuous monitoring of the particle dynamics on timescales of the order of
weeks. We show that by using the improvement given by super-resolution imaging,
a significant reduction in the noise floor can be attained, with an increase in
the bandwidth of the force sensitivity. This approach represents a competitive
alternative to standard optical detection for a range of low frequency
oscillators where low optical powers are require
Testing collapse models with levitated nanoparticles: the detection challenge
We consider a nanoparticle levitated in a Paul trap in ultrahigh cryogenic
vacuum, and look for the conditions which allow for a stringent
noninterferometric test of spontaneous collapse models. In particular we
compare different possible techniques to detect the particle motion. Key
conditions which need to be achieved are extremely low residual pressure and
the ability to detect the particle at ultralow power. We compare three
different detection approaches based respectively on a optical cavity, optical
tweezer and a electrical readout, and for each one we assess advantages,
drawbacks and technical challenges
Detection of weak stochastic force in a parametrically stabilized micro opto-mechanical system
Measuring a weak force is an important task for micro-mechanical systems,
both when using devices as sensitive detectors and, particularly, in
experiments of quantum mechanics. The optimal strategy for resolving a weak
stochastic signal force on a huge background (typically given by thermal noise)
is a crucial and debated topic, and the stability of the mechanical resonance
is a further, related critical issue. We introduce and analyze the parametric
control of the optical spring, that allows to stabilize the resonance and
provides a phase reference for the oscillator motion, yet conserving a free
evolution in one quadrature of the phase space. We also study quantitatively
the characteristics of our micro opto-mechanical system as detector of
stochastic force for short measurement times (for quick, high resolution
monitoring) as well as for the longer term observations that optimize the
sensitivity. We compare a simple, naive strategy based on the evaluation of the
variance of the displacement (that is a widely used technique) with an optimal
Wiener-Kolmogorov data analysis. We show that, thanks to the parametric
stabilization of the effective susceptibility, we can more efficiently
implement Wiener filtering, and we investigate how this strategy improves the
performance of our system. We finally demonstrate the possibility to resolve
stochastic force variations well below 1% of the thermal noise
Inhomogeneous mechanical losses in micro-oscillators with high reflectivity coating
We characterize the mechanical quality factor of micro-oscillators covered by
a highly reflective coating. We test an approach to the reduction of mechanical
losses, that consists in limiting the size of the coated area to reduce the
strain and the consequent energy loss in this highly dissipative component.
Moreover, a mechanical isolation stage is incorporated in the device. The
results are discussed on the basis of an analysis of homogeneous and
non-homogeneous losses in the device and validated by a set of Finite-Element
models. The contributions of thermoelastic dissipation and coating losses are
separated and the measured quality factors are found in agreement with the
calculated values, while the absence of unmodeled losses confirms that the
isolation element integrated in the device efficiently uncouples the dynamics
of the mirror from the support system. Also the resonant frequencies evaluated
by Finite-Element models are in good agreement with the experimental data, and
allow the estimation of the Young modulus of the coating. The models that we
have developed and validated are important for the design of oscillating
micro-mirrors with high quality factor and, consequently, low thermal noise.
Such devices are useful in general for high sensitivity sensors, and in
particular for experiments of quantum opto-mechanics
Quadratic optomechanical cooling of a cavity-levitated nanosphere
We report on cooling the center-of-mass motion of a nanoparticle due to a purely quadratic coupling between its motion and the optical field of a high finesse cavity. The resulting interaction gives rise to a Van der Pol nonlinear damping, which is analogous to conventional parametric feedback where the cavity provides passive feedback without measurement. We show experimentally that like feedback cooling the resulting energy distribution is strongly nonthermal and can be controlled by the nonlinear damping of the cavity. As quadratic coupling has a prominent role in proposed protocols to generate deeply nonclassical states, our work represents a first step for producing such states in a levitated system
Sympathetic cooling and squeezing of two colevitated nanoparticles
Levitated particles are an ideal tool for measuring weak forces and investigating quantum mechanics in macroscopic objects. Arrays of two or more of these particles have been suggested for improving force sensitivity and entangling macroscopic objects. In this article, two charged, silica nanoparticles, that are coupled through their mutual Coulomb repulsion, are trapped in a Paul trap, and the individual masses and charges of both particles are characterized. We demonstrate sympathetic cooling of one nanoparticle coupled via the Coulomb interaction to the second nanoparticle to which feedback cooling is directly applied. We also implement sympathetic squeezing through a similar process showing nonthermal motional states can be transferred by the Coulomb interaction. This work establishes protocols to cool and manipulate arrays of nanoparticles for sensing and minimizing the effect of optical heating in future experiments
An ultra-low dissipation micro-oscillator for quantum opto-mechanics
Generating non-classical states of light by opto-mechanical coupling depends
critically on the mechanical and optical properties of micro-oscillators and on
the minimization of thermal noise. We present an oscillating micro-mirror with
a mechanical quality factor Q = 2.6x10^6 at cryogenic temperature and a Finesse
of 65000, obtained thanks to an innovative approach to the design and the
control of mechanical dissipation. Already at 4 K with an input laser power of
2 mW, the radiation-pressure quantum fluctuations become the main noise source,
overcoming thermal noise. This feature makes our devices particularly suitable
for the production of pondero-motive squeezing.Comment: 21 pages including Supplementary Informatio
Characterisation of a charged particle levitated nano-oscillator
We describe the construction and characterisation of a nano-oscillator formed by a Paul trap. The frequency and temperature stability of the nano-oscillator was measured over several days allowing us to identify the major sources of trap and environmental fluctuations. We measure an overall frequency stability of 2 ppm/hr and a temperature stability of more than 5 hours via the Allan deviation. Importantly, we find that the charge on the nanoscillator is stable over a timescale of at least two weeks and that the mass of the oscillator, can be measured with a 3 % uncertainty. This allows us to distinguish between the trapping of a single nanosphere and a nano-dumbbell formed by a cluster of two nanospheres
On the periodicity of linear and nonlinear oscillatory reconnection
(Abridged for ArXiv) An injection of energy towards a magnetic null point can
drive reversals of current sheet polarity leading to time-dependent Oscillatory
Reconnection, which is a possible explanation of how periodic phenomena can be
generated when reconnection occurs in the solar atmosphere. However, the
details of what controls the period of these oscillations is poorly understood,
despite being crucial in assessing whether OR can account for observed periodic
behaviour. This paper aims to highlight that different types of reconnection
reversal are supported about null points, and that these are distinct from the
oscillation on the closed-boundary, linear systems considered in the 1990s. In
particular, we explore the features of a nonlinear oscillation local to the
null point, and examine the effect of resistivity and perturbation energy on
the period, contrasting it to the linear case. It is found that in the linear
systems, the inverse Lundquist number dictates the period, provided the
perturbation energy is small relative to the inverse Lundquist number defined
on the boundary, regardless of the broadband structure of the initial
perturbation. However, when the perturbation energy exceeds the threshold
required for 'nonlinear' null collapse to occur, a complex oscillation of the
magnetic field is produced which is, at best, only weakly-dependent on the
resistivity. The resultant periodicity is strongly influenced by the amount of
free energy, with more energetic perturbations producing higher-frequency
oscillations. Crucially, with regards to typical solar-based and
astrophysical-based input energies, we demonstrate that the majority far exceed
the threshold for nonlinearity to develop. This substantially alters the
properties and periodicity of both null collapse and subsequent OR. Therefore,
nonlinear regimes of OR should be considered in solar and astrophysical
contexts.Comment: Accepted in A&A, 5 Figures (1 animated
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